D'Angelo Durán
Universidad de Chile, Ciencias Ecológicas, Graduate Student
- Microbiology, Environmental Science, Conservation Biology, Botany, Ecology, Forestry, and 19 moreCarbon Sequestration, CO2 emissions, GHG Emissions, Carbon Capture and Storage, Methane, Atacama, CO2 fluxes, Climate Change, Global Warming, Biology, Environmental Sustainability, Sustainable Development, Biotechnology, Biodiversity, Environmental Studies, Zoology, Remote Sensing, Landscape Ecology, and Forest Ecologyedit
This study focused on the biotic oxidation of methane in landfill covers as a technology for reducing greenhouse gas emissions from landfills, particularly those located in the boreal climatic zone. First, methane oxidation was studied in... more
This study focused on the biotic oxidation of methane in landfill covers as a technology for reducing greenhouse gas emissions from landfills, particularly those located in the boreal climatic zone. First, methane oxidation was studied in laboratory batch assays in a landfill cover soil consisting of a composted mixture of sewage sludge and chemical sludge which had been installed on the landfill surface 4-5 years earlier. Second, methane oxidation was studied using mechanically-biologically treated municipal solid waste (MBT residual) as a material for methane-oxidizing landfill covers both in continuously methane-sparged laboratory columns and in an outdoor pilot lysimeter. Finally, methane oxidation was studied at a closed full-scale landfill with a European Union landfill directive-compliant, multilayer final cover system containing a water impermeable layer, passive gas collection and distribution system, and a soil cover consisting of sludge compost and peat. In the four-year old landfill cover, the methane oxidation rates at moisture of >33% of water-holding capacity increased along with temperature (Q₁₀ values 6.5 - 8.4 at 1 - 19 °C) while methane oxidation was suppressed at moisture of 17% of water-holding capacity. Methane oxidation (0.2-4.3 μg CH4 gdw¯¹ h¯¹ at 1 - 6 °C) and increase in oxidation rate over time were observed even at 1 °C. In MBT residual, high methane oxidation rates were observed in laboratory columns (12.2-82.3 g CH4 m¯² d¯¹ at 2 - 25 °C) and in batch assays with samples from the columns (up to 104 μg CH4 gdw¯¹ h¯¹ at 5 °C and 581 μg CH4 gdw¯¹ h¯¹ at 25 °C). In an outdoor lysimeter filled with MBT residual and containing a cover layer made from the same MBT residual, >96% of the methane produced (<16 g CH4 m¯² d¯¹) was oxidized between April and October, while in January oxidation was lower (<0.6 g CH4 m¯² d¯¹; this was <22% of the methane produced). In the full-scale landfill, of the mean methane flux (2.92-27.3 g CH4 m¯² d¯¹) entering the cover layer at the measuring points at the four measuring times, >25% was oxidized in October and February, 0% in November and >46% in June. At each time, the high methane fluxes into the soil cover at a few points reduced the mean oxidation rate. To conclude, methane-oxidizing landfill biocovers appear feasible for reducing methane emissions in boreal climatic conditions while reduced oxidation rates are likely to occur in wintertime. To maximize the methane oxidation rate at low ambient temperature, the oxidation layer should have a spatially even gas influx, sufficient thickness and suitable characteristics, particularly those related to oxygen transport and thermal insulation.
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Interstitial methane profiles from six sediment cores taken on the slope and abyssal plain of the Gulf of Mexico can be explained by simple kinetic modeling. Methane is apparently produced at a constant rate and microbially consumed in... more
Interstitial methane profiles from six sediment cores taken on the slope and abyssal plain of the Gulf of Mexico can be explained by simple kinetic modeling. Methane is apparently produced at a constant rate and microbially consumed in the sulfate-reducing zone. Rates of production and consumption are estimated from best-fit solutions to a steady-state diagenetic equation. Production and consumption balance to form uniform concentrations of 5 to 10 μL¯¹ in the first few meters of slope and abyssal sediments. Effects of upward diffusion from large accumulations of methane in sulfate-free zones deeper than about 10m are not detectable.
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In estuaries, the gas transfer velocity (k) is driven by a combination of two major physical drivers, wind and water current. The k values for CO₂ in the macrotidal Gironde Estuary were obtained from 159 simultaneous pCO₂ and floating... more
In estuaries, the gas transfer velocity (k) is driven by a combination of two major physical drivers, wind and water current. The k values for CO₂ in the macrotidal Gironde Estuary were obtained from 159 simultaneous pCO₂ and floating chamber flux measurements. Values of k increased with wind speed and were significantly greater when water currents and wind were in opposing directions. At low wind speeds (<1 m s¯¹), k increased with water current velocities (0–1.5 m s¯¹) following an exponential trend. The latter was a good proxy for the Y-intercept in a generic equation for k versus wind speed in estuaries. We also found that, in this turbid estuary, k was significantly lower at high turbidity. The presence of suspended material in great concentrations (TSS > 0.2 g L¯¹) had a significant role in attenuating turbulence and therefore gas exchange. This result has important consequences for modeling water oxygenation in estuarine turbidity maxima. For seven low turbidity estuaries previously described in the literature, the slope of the linear regression between k and wind speed correlates very well with the estuary surface area due to a fetch effect. In the Gironde Estuary, this slope follows the same trend at low turbidity (TSS < 0.2 g L¯¹), but is on average significantly lower than in other large estuaries and decreases linearly with the TSS concentration. A new generic equation for estuaries is proposed that gives k as a function of water current velocity, wind speed, estuarine surface area and TSS concentration.
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Methane and suspended particulate matter (SPM) concentrations, monitored bimonthly during one hydrological year (2003–2004) along 70 km transects in the tidal regions of the Garonne and Dordogne rivers (SW France), showed a significant... more
Methane and suspended particulate matter (SPM) concentrations, monitored bimonthly during one hydrological year (2003–2004) along 70 km transects in the tidal regions of the Garonne and Dordogne rivers (SW France), showed a significant negative correlation, both spatially and temporally. During spring in clear waters (SPM , 50 mg L¯¹), methane production was first evidenced by a net increase in methane concentrations, in parallel with temperature and a decrease in river flow. In summer, as soon as the estuarine turbidity maximum (ETM) appeared and SPM concentrations exceeded 100 mg L¯¹, methane concentrations decreased from ,600 to ,30 nmol L¯¹ in one month. More downstream in the turbid Gironde estuary, methane concentrations were occasionally below atmospheric equilibrium. In dark microcosms, high methane consumption was observed in samples from the ETM with SPM concentrations .2,000 mg L¯¹, but not after removing the SPM by settling (SPM 5 16 mg L¯¹), nor in a sample collected few kilometers upstream, with SPM 5 3 mg L¯¹. Methane oxidation was also able to draw down methane concentrations below half the atmospheric equilibrium value in an ETM sample. Suspended clays in the ETM enhance methane oxidation and strongly reduce methane fluxes to the atmosphere.
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Methane concentrations, oxidation rates, exchanges at the sediment-water interface and emissions to the atmosphere were studied between February and December 2000 along an estuarine gradient in Randers Fjord (Denmark). Methane... more
Methane concentrations, oxidation rates, exchanges at the sediment-water interface and emissions to the atmosphere were studied between February and December 2000 along an estuarine gradient in Randers Fjord (Denmark). Methane concentrations measured at 3 stations, 1 in freshwater, 1 in brackish water (salinity 3 to 7) and 1 in saltwater (salinity 17 to 23), showed high supersaturation with respect to atmospheric equilibrium, with concentrations ranging from 186–420, 70–290 and 28–124 nM and median concentrations of 347, 125 and 41 nM respectively. Calculated median fluxes to the atmosphere were 355, 126 and 40 μmol m¯² d¯¹ at the 3 stations respectively. The contribution of water-column methane oxidation to the total methane sinks (oxidation and emission) was 22 to 42% in the river (depth 8 m), but fell to less than 3% at the brackish station, owing to lower rates and shallow depth (1.7 m). No oxidation could be detected in the saltwater. Methane fluxes through the sediment-water interface were directed downwards at the brackish station (from –19 to –353 μmol m¯² d¯¹ in December and July respectively) and upwards at the saltwater station (from 3 to 400 μmol m¯² d¯¹ in March and July respectively). At the brackish station, methane uptake by the sediment accounted for 16 to 55% of the total methane sink. Potential aerobic methane oxidation in surface sediments revealed the presence of a population of methanotrophs active at ambient methane concentrations at the brackish station, but not at the saltwater station. During summer, methane production at the saltwater station appeared to occur in the first 1 cm of the sediment and was 40 times higher than at the brackish station. The turnover time of methane relative to all sinks was 4 to 7 d in the freshwater, 0.5 to 1.8 d in the brackish water and 0.6 to 4.7 d in the saltwater. Our results confirm the important role of the estuarine zone in recycling methane. Most of the methane carried by the river is oxidised and released to the atmosphere in the upper estuary, and new production of methane occurs in the lower estuary, where, in addition, oxidation is inefficient.
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Previous research suggests that soil organic C pools may be a feature of semiarid regions that are particularly sensitive to climatic changes. We instituted an 18-mo experiment along an elevation gradient in northern Arizona to evaluate... more
Previous research suggests that soil organic C pools may be a feature of semiarid regions that are particularly sensitive to climatic changes. We instituted an 18-mo experiment along an elevation gradient in northern Arizona to evaluate the influence of temperature, moisture, and soil C pool size on soil respiration. Soils, from underneath different tree canopy types and interspaces of three semiarid ecosystems,
were moved upslope and/or downslope to modify soil climate. Soils moved downslope experienced increased temperature and decreased precipitation, resulting in decreased soil moisture and soil respiration (as much as 23 and 20%, respectively). Soils moved upslope to more mesic, cooler sites had greater soil water content and increased rates of soil respiration (as much as 40%), despite decreased temperature. Soil respiration rates normalized for total C were
not significantly different within any of the three incubation sites, indicating that under identical climatic conditions, soil respiration is directly related to soil C pool size for the incubated soils. Normalized soil respiration rates between sites differed significantly for all soil types and were always greater for soils incubated under more mesic, but cooler, conditions. Total soil C did not change significantly during the experiment, but estimates suggest that significant portions of the rapidly cycling C pool were lost. While long-term decreases in aboveground and belowground detrital inputs may ultimately be greater than decreased soil respiration, the initial response to increased temperature and decreased precipitation in these systems is a decrease in annual soil C efflux.
were moved upslope and/or downslope to modify soil climate. Soils moved downslope experienced increased temperature and decreased precipitation, resulting in decreased soil moisture and soil respiration (as much as 23 and 20%, respectively). Soils moved upslope to more mesic, cooler sites had greater soil water content and increased rates of soil respiration (as much as 40%), despite decreased temperature. Soil respiration rates normalized for total C were
not significantly different within any of the three incubation sites, indicating that under identical climatic conditions, soil respiration is directly related to soil C pool size for the incubated soils. Normalized soil respiration rates between sites differed significantly for all soil types and were always greater for soils incubated under more mesic, but cooler, conditions. Total soil C did not change significantly during the experiment, but estimates suggest that significant portions of the rapidly cycling C pool were lost. While long-term decreases in aboveground and belowground detrital inputs may ultimately be greater than decreased soil respiration, the initial response to increased temperature and decreased precipitation in these systems is a decrease in annual soil C efflux.
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A review is presented on trace gas exchange of CH₄, CO, N₂O, and NOx arising from agriculture and natural sources in the world’s semiarid and arid zones due to soil processes. These gases are important contributors to the radiative... more
A review is presented on trace gas exchange of CH₄, CO, N₂O, and NOx arising from agriculture and natural sources in the world’s semiarid and arid zones due to soil processes. These gases are important contributors to the radiative forcing and the chemistry of the atmosphere. Quantitative information is summarized from the available studies. Between 5 and 40% of the global soil–atmosphere exchange for these gases (CH₄, CO, N₂O, and NOx) may occur in semiarid and arid zones, but for each of these gases there are fewer than a dozen studies to support the individual estimates, and these are from a limited number of locations. Significant differences in the biophysical and chemical processes controlling these trace gas exchanges are identified through the comparison of semiarid and arid zones with the moist temperate or wet/dry savanna land regions. Therefore, there is a poorly quantified understanding of the contribution of these regions to the global trace gas cycles and atmospheric chemistry. More importantly, there is a poor understanding of the feedback between these exchanges, global change, and regional land use and air pollution issues. A set of research issues is presented.
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Carbon dioxide and methane emissions from estuaries are reviewed in relation with biogeochemical processes and carbon cycling. In estuaries, carbon dioxide and methane emissions show a large spatial and temporal variability, which results... more
Carbon dioxide and methane emissions from estuaries are reviewed in relation with biogeochemical processes and carbon cycling. In estuaries, carbon dioxide and methane emissions show a large spatial and temporal variability, which results from a complex interaction of river carbon inputs, sedimentation and resuspension processes, microbial processes in waters and sediments, tidal exchanges with marshes and flats and gas exchange with the atmosphere. The net mineralization of land- and marsh-derived organic carbon leads to high C0₂ atmospheric emissions (10 - 1000 mmol x m¯² x d¯¹ i.e. 44 - 44000 mg x m¯² x d¯¹) from inner estuarine wa¬ters and tidal flats and marsh sediments. Estuarine plumes at sea are sites of intense primary production and show large seasonal variations of pC0₂ from undersaturation to oversaturation; on an annual basis, some plumes behave as net sinks of atmospheric C0₂ and some others as net sources; CO₂ atmospheric fluxes in plumes are usually one order of magnitude lower than in inner estuaries. Methane emissions to the atmosphere are moderate in estuaries (0.02 - 0.5 mmol x m¯² x d¯¹ i.e. 0.32 - 8 mg x m¯² x d¯¹), except in vegetated tidal flats and marshes, particularly those at freshwater sites, where sediments may be CH₄-saturated. CH₄ emissions from subtidal estuarine waters are the result of lateral inputs from river and marshes fol¬lowed by physical ventilation, rather than intense in - situ production in the sediments, where oxic and suboxic conditions dominate. Microbial oxida¬tion significantly reduces the CH₄ emissions at low salinity (< 10) only.
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Continental-scale estimations of terrestrial methane (CH₄) and nitrous oxide (N₂O) fluxes over a long time period are crucial to accurately assess the global balance of greenhouse gases and enhance our understanding and prediction of... more
Continental-scale estimations of terrestrial methane (CH₄) and nitrous oxide (N₂O) fluxes over a long time period are crucial to accurately assess the global balance of greenhouse gases and enhance our understanding and prediction of global climate change and terrestrial ecosystem feedbacks. Using a process-based global biogeochemical model, the Dynamic Land Ecosystem Model (DLEM), we quantified simultaneously CH₄ and N₂O fluxes in North America’s terrestrial ecosystems from 1979 to 2008. During the past 30 years, approximately 14.69 ± 1.64 T g C a¯¹ (1 T g = 10¹² g) of CH₄, and 1.94 ± 0.1 T g N a¯¹ of N₂O were released from terrestrial ecosystems in North America. At the country level, both the US and Canada acted as CH₄ sources to the atmosphere, but Mexico mainly oxidized and consumed CH₄ from the atmosphere. Wetlands in North America contributed predominantly to the regional CH₄ source, while all other ecosystems acted as sinks for atmospheric CH₄, of which forests accounted for 36.8%. Regarding N₂O emission in North America, the US, Canada, and Mexico contributed 56.19%, 18.23%, and 25.58%, respectively, to the continental source over the past 30 years. Forests and croplands were the two ecosystems that contributed most to continental N₂O emission. The inter-annual variations of CH₄ and N₂O fluxes in North America were mainly attributed to year-to-year climatic variability. While only annual precipitation was found to have a significant effect on annual CH₄ flux, both mean annual temperature and annual precipitation were significantly correlated to annual N₂O flux. The regional estimates and spatiotemporal patterns of terrestrial ecosystem CH₄ and N₂O fluxes in North America generated in this study provide useful information for global change research and policy making.
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The hyperarid core of the Atacama Desert, Chile, is possibly the driest and most abiotic place on Earth, yet endolithic microorganisms thrive inside halite pinnacles that are part of ancient salt flats. The existence of this microbial... more
The hyperarid core of the Atacama Desert, Chile, is possibly the driest and most abiotic place on Earth, yet endolithic microorganisms thrive inside halite pinnacles that are part of ancient salt flats. The existence of this microbial community in an environment that excludes any other life forms suggests biological adaptation to high salinity and desiccation stress, and indicates an alternative source of water for life other than rainfall, fog or dew. Here we show that halite endoliths obtain liquid water through spontaneous capillary condensation at relative humidity (RH) much lower than the deliquescence RH of NaCl. We describe how this condensation occurs inside nano-pores smaller than 100 nm, in a newly identified halite phase that is intimately associated with the endolithic aggregates. This nano-porous phase helps retain liquid water for long periods of time by preventing its evaporation even in conditions of utmost dryness. Our results explain how life has colonized and adapted to one of the most extreme environments on our planet, expanding the water activity envelope for life on Earth, and broadening the spectrum of possible habitats for life beyond our planet.
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The Atacama Desert, one of the most arid landscapes on Earth, serves as an analog for the dry conditions on Mars and as a test bed in the search for life on other planets. During the Life in the Atacama (LITA) 2004 field experiment,... more
The Atacama Desert, one of the most arid landscapes on Earth, serves as an analog for the dry conditions on Mars and as a test bed in the search for life on other planets. During the Life in the Atacama (LITA) 2004 field experiment, satellite imagery and ground-based rover data were used in concert with a ‘follow-the-water’ exploration strategy to target regions of biological interest in two (1 coastal, 1 inland) desert study sites. Within these regions, environments were located, studied and mapped with spectroscopic and fluorescence imaging (FI) for habitats and microbial life. Habitats included aqueous sedimentary deposits (e.g., evaporites), igneous materials (e.g., basalt, ash deposits), rock outcrops, drainage channels and basins, and alluvial fans. Positive biological signatures (chlorophyll, DNA, protein) were detected at 81% of the 21 locales surveyed with the FI during the long-range, autonomous traverses totaling 30 km. FI sensitivity in detecting microbial life in extreme deserts explains the high percentage of positives despite the low actual abundance of heterotrophic soil bacteria in coastal (<1–104 CFU/g-soil) and interior (<1–102 CFU/g-soil) desert soils. Remote habitat, microbial and climate observations agreed well with ground-truth, indicating a drier and less microbially rich interior compared to the relatively wetter and abundant biology of the coastal site where rover sensors detected the presence of fog and abundant surface lichens. LITA project results underscore the importance of an explicit focus by all engineering and science disciplines on microbially relevant scales (mm to nm), and highlight the success of satellite-based and ‘follow-the-water’ strategies for locating diverse habitats of biological promise and detecting the microbial hotspots within them.
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The hyper-arid core of the Atacama Desert (Chile) is the driest place on Earth and is considered a close analogue to the extremely arid conditions on the surface of Mars. Microbial life is very rare in soils of this hyper-arid region, and... more
The hyper-arid core of the Atacama Desert (Chile) is the driest place on Earth and is considered a close analogue to the extremely arid conditions on the surface of Mars. Microbial life is very rare in soils of this hyper-arid region, and autotrophic microorganisms are virtually absent. Instead, photosynthetic micro-organisms have successfully colonized the interior of halite crusts, which are widespread in the Atacama Desert. These endoevaporitic colonies are an example of life that has adapted to the extreme dryness by colonizing the interior of rocks that provide enhanced moisture conditions. As such, these colonies represent a novel example of potential life on Mars. Here, we present non-destructive Raman spectroscopical identification of these colonies and their organic remnants. Spectral signatures revealed the presence of UV-protective biomolecules as well as light-harvesting pigments pointing to photosynthetic activity. Compounds of biogenic origin identified within these rocks differed depending on the origins of specimens from particular areas in the desert, with differing environmental conditions. Our results also demonstrate the capability of Raman spectroscopy to identify biomarkers within rocks that have a strong astrobiological potential.
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China possesses large areas of plantation forests which take up great quantities of carbon. However, studies on soil respiration in these plantation forests are rather scarce and their soil carbon flux remains an uncertainty. In this... more
China possesses large areas of plantation forests which take up great quantities of carbon. However, studies on soil respiration in these plantation forests are rather scarce and their soil carbon flux remains an uncertainty. In this study, we used an automatic chamber system to measure soil surface flux of a 50-year-old mature plantation of Platycladus orientalis at Jiufeng Mountain, Beijing, China. Mean daily soil respiration rates (Rs) ranged from 0.09 to 4.87 mmol CO2 m22s21, with the highest values observed in August and the lowest in the winter months. A logistic model gave the best fit to the relationship between hourly Rs and soil temperature (Ts), explaining 82% of the variation in Rs over the annual cycle. The annual total of soil respiration estimated from the logistic model was 64565 g C m22 year21. The performance of the logistic model was poorest during periods of high soil temperature or low soil volumetric water content (VWC), which limits the model’s ability to predict the seasonal dynamics of Rs. The logistic model will potentially overestimate Rs at high Ts and low VWC. Seasonally, Rs increased significantly and linearly with increasing VWC in May and July, in which VWC was low. In the months from August to November, inclusive, in which VWC was not limiting, Rs showed a positively exponential relationship with Ts. The seasonal sensitivity of soil respiration to Ts (Q10) ranged from 0.76 in May to 4.38 in October. It was suggested that soil temperature was the main determinant of soil respiration when soil water was not limiting.
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Soil respiration is the primary path by which C02 fixed by land plants returns to the atmosphere. Estimated at approximately 75 x 1015 gC/yr, this large natural flux is likely to increase due to changes in the Earth's condition. The... more
Soil respiration is the primary path by which C02 fixed by land plants returns to the atmosphere. Estimated at approximately 75 x 1015 gC/yr, this large natural flux is likely to increase due to changes in the Earth's condition. The objective of this paper is to provide a brief scientific review for policymakers who are concerned that changes in soil respiration may contribute to the rise in C02 in Earth's atmosphere. Rising concentrations of CO2 in the atmosphere will increase the flux of CO2 from soils, while simultaneously leaving a greater store of carbon in the soil. Traditional tillage cultivation and rising temperature increase the flux of C02 from soils without increasing the stock of soil organic matter. Increasing deposition of nitrogen from the atmosphere may lead to the sequestration of carbon in vegetation and soils. The response of the land biosphere to simultaneous changes in all of these factors is unknown, but a large increase in the soil carbon pool seems unlikely to moderate the rise in atmospheric C02 during the next century.
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Managed grasslands are a predominant land use in northern temperate regions. Th ey are oft en on poorly drained soils and contain large stocks of soil organic carbon (SOC). It is important to understand C dynamics in grasslands to better... more
Managed grasslands are a predominant land use in northern temperate regions. Th ey are oft en on poorly drained soils and contain large stocks of soil organic carbon (SOC). It is important to understand C dynamics in grasslands to better assess their role in regional C budgets. Two adjacent grassland plots, one amended with 100 m3 ha−1 of liquid swine manure (LSM) annually since 1978, and another unamended were killed by glyphosate in the autumn or (i) left with vegetation intact. Those killed were either (ii) left as an undisturbed chemical fallow, (iii) plowed by full-inversion tillage (FIT) in the autumn, or (iv) in the spring. Following the autumn plowing, we monitored CO2 emissions from the fallow soil surface, CO2 concentrations in the soil profile, soil temperature, and soil water content for 1 yr. Changes in soil aggregation, the light fraction organic matter, and microbial biomass were also monitored. Plowing decreased the total microbial biomass on average by 27 g C m−2, the quantity of water-stable aggregates by 7 to 12% and with it the concentration of light fraction organic matter. Respiration was also reduced by 142 g CO2-C m−2 by autumn-FIT and 175 g CO2-C m−2 by spring-FIT on unamended soils, and 90 g CO2-C m−2 by autumn-FIT and 98 g CO2-C m−2 by spring-FIT on amended soils. Regression analyses suggested that reductions in CO2 emissions were due to the placement of surface SOC at depth where soil temperature and oxygen availability were attenuated. In these soils, the cool humid conditions at depth in the soil profile may act to counter-balance the physical effects of tillage thereby preserving C deep in the soil profile. Effective calculation of C budgets and changes in SOC stocks depends on the ability to reproduce the interaction between climate, soil type, land use, and management action. A more complete understanding of the effect of management actions that modify the vertical distribution of biogeochemical (particularly organic C) and environmental parameters on soil respiration in poorly drained soils is required.
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To better understand the biotic and abiotic factors that control soil CO2 efflux, we compared seasonal and diurnal variations in simultaneously measured forest-floor CO2 effluxes and soil CO2 concentration profiles in a 54-year-old... more
To better understand the biotic and abiotic factors that control soil CO2 efflux, we compared seasonal and diurnal variations in simultaneously measured forest-floor CO2 effluxes and soil CO2 concentration profiles in a 54-year-old Douglas fir forest on the east coast of Vancouver Island.We used small solid-state infrared CO2 sensors for long-term continuous real-time measurement of CO2 concentrations at different depths, and measured half-hourly soil CO2 effluxes with an automated non-steady-state chamber. We describe a simple steady-state method to measure CO2 diffusivity in undisturbed soil cores. The method accounts for the CO2 production in the soil and uses an analytical solution to the diffusion equation. The diffusivity was related to air-filled porosity by a power law function, which was independent of soil depth. CO2 concentration at all depths increased with increase in soil temperature, likely due to a rise in CO2 production, and with increase in soil water content due to decreased diffusivity or increased CO2 production or both. It also increased with soil depth reaching almost 10 mmol mol1 at the 50-cm depth. Annually, soil CO2 efflux was best described by an exponential function of soil temperature at the 5-cm depth, with the reference efflux at 10 8C (F10) of 2.6 mmol m2 s1 and the Q10 of 3.7. No evidence of displacement of CO2-rich soil air with rain was observed. Effluxes calculated from soil CO2 concentration gradients near the surface closely agreed with the measured effluxes. Calculations indicated that more than 75% of the soil CO2 efflux originated in the top 20 cm soil. Calculated CO2 production varied with soil temperature, soil water content and season, and when scaled to 10 8C also showed some diurnal variation. Soil CO2 efflux and concentrations as well as soil temperature at the 5-cm depth varied in phase. Changes in CO2 storage in the 0–50 cm soil layer were an order of magnitude smaller than measured effluxes. Soil CO2 efflux was proportional to CO2 concentration at the 50-cm depth with the slope determined by soil water content, which was consistent with a simple steadystate analytical model of diffusive transport of CO2 in the soil. The latter proved successful in calculating effluxes during 2004.
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Soil respiration on is known to be highly variable with time. Less is known, however, about the spatial variability of heterotrophic soil respiration at the plot scale. We simultaneously measured soil heterotrophic respiration, soil... more
Soil respiration on is known to be highly variable with time. Less is known, however, about the spatial variability of heterotrophic soil respiration at the plot scale. We simultaneously measured soil heterotrophic respiration, soil temperature, and soil water content at 48 locations with a nested sampling design and at 76 locations with a regular grid plus refinement within a 13- by 14-m bare soil plot for 15 measurement dates. Soil respiration was measured with a closed chamber covering a surface area of 0.032 m2. A geostatistical data analyses indicated a mean range of 2.7 m for heterotrophic soil respiration. We detected rather high coefficients of variation of CO2 respiration between 0.13 and 0.80, with an average of 0.33. The number of observations required to estimate average respiration fluxes at a 5% error level ranged between 5 and 123. The analysis of the temporal persistence revealed that a subset of 17 sampling locations is sufficient to estimate average respiration fluxes at a tolerable root mean square error of 0.15 g C m−2 d−1. Statistical analysis revealed that the spatiotemporal variability of heterotrophic soil respiration could be explained by the state variables soil temperature and water content. The spatial variability of respiration was mainly driven by variability in soil water content; the variability in the soil water content was almost an order of magnitude higher than the variability in soil temperature.
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We examined constraints on soil CO₂ respiration in natural oak woodlands, and adjacent vineyards that were converted approximately 30 yr ago from oak woodlands, in the Oakville Region of Napa Valley, California. All sites were located on... more
We examined constraints on soil CO₂ respiration in natural oak woodlands, and adjacent vineyards that were converted approximately 30 yr ago from oak woodlands, in the Oakville Region of Napa Valley, California. All sites were located on the same soil type, a Bale (variant) gravelly loam (fine-loamy, mixed, superactive, thermic Cumulic Ultic Haploxeroll) and dominated by C3 vegetation. Seasonal soil CO₂ efflux was greatest at the oak woodland sites, although during the summer drought the rates of soil CO₂ efflux measured from oak sites were generally similar to those measured from the vineyards. Soil profile CO₂ concentrations at the oak woodland sites were lower below 15 cm despite higher CO₂ efflux rates. Soil gas diffusion coefficients for oak sites were larger than for vineyard sites, and this indicated that the apparent discrepancy in soil profile carbon dioxide concentration ([CO₂]) may be caused by a diffusion limitation. Soil profile [CO₂] and δ¹³C values showed substantial temporal changes over the course of a year. Vineyard soil CO₂ was more depleted in ¹³CO₂ below 25 cm in the soil profile during the active growing season as indicated by more negative δ¹³C ratios. This result indicated that different C sources were being oxidized in vineyard soils. Annual C losses were less from vineyard soils (7.02 ± 0.58 Mg C ha¯¹ yr¯¹) as compared to oak soils (15.67 ± 1.44 Mg C ha¯¹ yr¯¹), and both were comparable to losses reported in previous investigations.
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The Atacama along the Pacific Coast of Chile and Peru is one of the driest and possibly oldest deserts in the world. It represents an extreme habitat for life on Earth and is an analog for life in dry conditions on Mars. We report on four... more
The Atacama along the Pacific Coast of Chile and Peru is one of the driest and possibly oldest deserts in the world. It represents an extreme habitat for life on Earth and is an analog for life in dry conditions on Mars. We report on four years (September 1994–October 1998) of climate and moisture data from the extreme arid region of the Atacama. Our data are focused on understanding moisture sources and their role in creating suitable environments for photosynthetic microorganisms in the desert surface. The average air temperature was 16.5°C and 16.6°C in 1995 and 1996, respectively. The maximum air temperature recorded was 37.9°C, and the minimum was 25.7°C. Annual average sunlight was 336 and 335 W m22 in 1995 and 1996, respectively. Winds averaged a few meters per second, with strong föhn winds coming from the west exceeding 12 m s21. During our 4 years of observation there was only one significant rain event of 2.3 mm, which occurred near midnight local time. We suggest that this event was a rainout of a heavy fog. It is of interest that the strong El Niño of 1997–1998 brought heavy rainfall to the deserts of Peru, but did not bring significant rain to the central Atacama in Chile. Dew occurred at our station frequently following high nighttime relative humidity, but is not a significant source of moisture in the soil or under stones. Groundwater also does not contribute to surface moisture. Only the one rain event of 2.3 mm resulted in liquid water in the soil and beneath stones for a total of only 65–85 h over 4 years. The paucity of liquid water under stones is consistent with the apparent absence of hypolithic (under-stone) cyanobacteria, the only known primary producers in such extreme deserts.
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Most terrestrial ecosystems support a similar suite of biogeochemical processes largely dependent on the availability of water and labile carbon (C). Here, we explored the biogeochemical potential of soils from Earth’s driest ecosystem,... more
Most terrestrial ecosystems support a similar suite of biogeochemical processes largely dependent on the availability of water and labile carbon (C). Here, we explored the biogeochemical potential of soils from Earth’s driest ecosystem, the Atacama Desert, characterized by extremely low moisture and organic C. We sampled surface soil horizons from sites ranging from the Atacama’s hyperarid core to less-arid locations at higher elevation that supported sparse vegetation. We performed laboratory incubations and measured fluxes of the greenhouse gases carbon dioxide (CO₂), nitrous oxide (N₂O), and methane (CH₄) as indices of potential biogeochemical activity across this gradient. We were able to stimulate trace gas production at all sites, and treatment responses often suggested the influence of microbial processes. Sites with extant vegetation had higher C concentrations (0.13–0.68%) and produced more CO₂ under oxic than sub-oxic conditions, suggesting the presence of aerobic microbial decomposers. In contrast, abiotic CO₂ production appeared to predominate in the most arid and C-poor (\0.08% C) sites without plants, with one notable exception. Soils were either a
weak source or sink of CH₄ under oxic conditions, whereas anoxia stimulated CH₄ production across.
weak source or sink of CH₄ under oxic conditions, whereas anoxia stimulated CH₄ production across.
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Surface and subsurface soil samples analyzed for this investigation were collected from the hyperarid Yungay region in the Atacama Desert, Chile. This report details the bacterial diversity derived from DNA and PLFA extracted directly... more
Surface and subsurface soil samples analyzed for this investigation were collected from the hyperarid Yungay region in the Atacama Desert, Chile. This report details the bacterial diversity derived from DNA and PLFA extracted directly from these extremely desiccated soils. Actinobacteria, Proteobacteria, Firmicutes and TM7 division bacteria were detected. Ninety-four percent of the 16S rRNA genes cloned from these soils belong to the Actinobacteria phylum, and the majority of these were most closely related to the genus Frankia. A 24-hour water activity (aw) time course showed a diurnal cycle that peaked at 0.52 in the early predawn hours, and ranged from 0.01–0.08 during the day. All measured water activity values were below the levels required for microbial growth or enzyme activity. Total organic carbon (TOC) concentrations were above the limit of detection and below the limit of quantification (i.e., 200 mg/g < TOC < 1000 mg/g), and phospholipid fatty acid (PLFA) concentrations ranged from 2 105 to 7 106 cell equivalents per gram of soil. Soil extracts analyzed for culturable biomass yielded mostly no growth on R2A media; the highest single extract yielded 47 colony forming units (CFU) per gram of soil.
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The Life in the Atacama project investigated the regional distribution of life and habitats in the Atacama Desert of Chile. We sought to create biogeologic maps through survey traverses across the desert using a rover carrying biologic... more
The Life in the Atacama project investigated the regional distribution of life and habitats in the Atacama Desert of Chile. We sought to create biogeologic maps through survey traverses across the desert using a rover carrying biologic and geologic instruments. Elements of our science approach were to: Perform ecological transects from the relatively wet coastal range to the arid core of the desert; use converging evidence from science instruments to reach conclusions about microbial abundance; and develop and test exploration strategies adapted to the search of scattered surface and shallow subsurface microbial oases. Understanding the ability of science teams to detect and characterize microbial life signatures remotely using a rover became central to the project. Traverses were accomplished using an autonomous rover in a method that is technologically relevant to Mars exploration. We present an overview of the results of the 2003, 2004, and 2005 field investigations. They include: The confirmed identification of microbial habitats in daylight by detecting fluorescence signals from chlorophyll and dye probes; the characterization of geology by imaging and spectral measurement; the mapping of life along transects; the characterization of environmental conditions; the development of mapping techniques including homogeneous biological scoring and predictive models of habitat location; the development of exploration strategies adapted to the search for life with an autonomous rover capable of up to 10 km of daily traverse; and the autonomous detection of life by the rover as it interprets observations on-the-fly and decides which targets to pursue with further analysis.
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Background: Mowing is a widely adopted management practice for the semiarid steppe in China and affects CH₄ exchange. However, the magnitude and the underlying mechanisms for CH₄ uptake in response to mowing remain uncertain.... more
Background: Mowing is a widely adopted management practice for the semiarid steppe in China and affects CH₄ exchange. However, the magnitude and the underlying mechanisms for CH₄ uptake in response to mowing remain uncertain. Methodology/Principal Findings: In two consecutive growing seasons, we measured the effect of mowing on CH₄ uptake in a steppe community. Vegetation was mowed to 2 cm (M2), 5 cm (M5), 10 cm (M10), 15 cm (M15) above soil surface, respectively, and control was set as non-mowing (NM). Compared with control, CH₄ uptake was substantially enhanced at almost all the mowing treatments except for M15 plots of 2009. CH₄ uptake was significantly correlated with soil microbial biomass carbon, microbial biomass nitrogen, and soil moisture. Mowing affects CH₄ uptake primarily through its effect on some biotic factors, such as net primary productivity, soil microbial C\N supply and soil microbial activities, while soil temperature and moisture were less important. Conclusions/Significance: This study found that mowing affects the fluxes of CH₄ in the semiarid temperate steppe of north China.
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Methane (CH₄) and nitrous oxide (N₂O) emission estimates were made for Vigna mungo and Vigna radiata legumes. The affecting soil parameters like redox potential, soil temperature were studied to evaluate CH₄ and N₂O emissions. The CH₄ was... more
Methane (CH₄) and nitrous oxide (N₂O) emission estimates were made for Vigna mungo and Vigna radiata legumes. The affecting soil parameters like redox potential, soil temperature were studied to evaluate CH₄ and N₂O emissions. The CH₄ was negative and N₂O was positive for Vigna mungo, almost throughout the cropping period. The redox potential was more than +100 mV during the entire cropping period with a maximum N₂O flux of 11.67 μg m–2 h–1. The raise in soil temperature and the redox potential during harvest further increased the N₂O flux to 18.38 μg m–2 h–1. The seasonally integrated flux E(SIF) for CH₄ and N₂O for Vigna mungo was calculated to be –4.06 g.m–2 and 3.38 mg m–2 respectively. Similarly E(SIF) values estimated for Vigna radiata cropping season were 0.009 g m–2 and –7.6 mg m–2, whereas for the post harvesting period the fluxes were 0.02 g m–2 and 4.06 mg m–2 for CH₄ and N₂O respectively. The soil parameters like organic carbon and nutrients such as ammonia, nitrate and nitrite during the cropping season were evaluated. The emission of greenhouse gases (GHG) was also correlated to various physico-chemical parameters of soil.
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Greenhouse gas (GHG) fluxes from a seminatural, extensively sheep grazed drained moorland and intensively sheep grazed fertilised grassland in SE Scotland were compared over 4 yr (2007–2010). Nitrous oxide and CH₄ fluxes were measured by... more
Greenhouse gas (GHG) fluxes from a seminatural, extensively sheep grazed drained moorland and intensively sheep grazed fertilised grassland in SE Scotland were compared over 4 yr (2007–2010). Nitrous oxide and CH₄ fluxes were measured by static chambers, respiration from soil including ground vegetation by a flow through chamber and the net ecosystem exchange of CO₂ by eddy covariance. All GHG fluxes displayed high temporal and interannual variability. Temperature, radiation, water table height and precipitation could explain a significant percentage of seasonal and interannual variations. Greenhouse gas fluxes were dominated by the net ecosystem exchange of CO₂, emissions of N₂O from the grazed grassland (384 gCO₂eqm¯² yr¯¹) and emissions of CH₄ from ruminant fermentation (147 gCO₂eqm¯² yr¯¹). Methane emissions from the moorland were small (6.7 gCO₂eqm¯² yr¯¹). Net ecosystem exchange of CO₂ and respiration were much larger on the productive fertilised grassland (−1624 and + 7157 gCO₂eqm¯² yr¯¹, respectively) than the seminatural moorland (−338 and + 2554 gCO₂eqm¯² yr¯¹, respectively). Large CH₄ and N₂O losses from the grazed grassland counteracted the CO₂ uptake by 35 %, whereas the small N₂O and CH₄ emissions from the moorland did only impact the NEE by 2 %. The 4 yr average GHG budget for the grazed grassland was 1006 gCO₂eqm¯² yr¯¹ and 331 gCO₂eqm¯² yr¯¹ for the moorland.
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Soil respiration is an important component of the net carbon dioxide exchange between agricultural ecosystems and the atmosphere, and reliable estimates of soil respiration are required in carbon balance studies. Most of the field... more
Soil respiration is an important component of the net carbon dioxide exchange between agricultural ecosystems and the atmosphere, and reliable estimates of soil respiration are required in carbon balance studies. Most of the field measurements of soil respiration reported in the literature have been made using alkali traps. The use of portable CO2 analysers in dynamic closed chamber systems is recent. The introduction of this new technique requires its evaluation against existing methods in order to compare new information with older data. Nine intercomparisons between dynamic systems and alkali traps were made. Measurements of Fc,s obtained by both chambers showed a good agreement in all but two comparisons in which alkali trap measurements were lower than the dynamic chamber by about 22%. This first report of agreement between both techniques suggests that many measurements made in the past using alkali traps may be comparable to the measurements made more recently using the dynamic chambers. Analysis of the soil temperature and CO2 concentration inside the alkali traps failed to explain why the alkali traps occasionally underestimated the fluxes. Soil respiration measured with a dynamic closed chamber were also compared to eddycorrelation measurements. The results did not reveal any consistent bias between techniques but the scattering was large. This dispersion is likely the result of the difference between the areas measured by the two techniques.
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Soil respiration forms a substantial part of the ecosystem respiration. However, the respiration measurements conducted with chambers on the soil surface do not give information on the vertical distribution of CO2 sources and its seasonal... more
Soil respiration forms a substantial part of the ecosystem respiration. However, the respiration measurements conducted with chambers on the soil surface do not give information on the vertical distribution of CO2 sources and its seasonal dynamics. We used permanently installed CO2 probes to determine the CO2 profile in a boreal coniferous forest soil and calculated the CO2 efflux from the concentration profile with a dynamic model. There was an increase in both soil CO2 efflux and soil air CO2 concentration over the 2.5-mo study period between April 15 and June 30. The CO2 efflux determined from the concentration profile was in relatively good agreement with the CO2 efflux measured by open dynamic chamber method. We also determined the respiration of different soil horizons and estimated the contribution of recent photosynthate to total respiration. Humus layer and A-horizon contributed 69.9%, B-horizon 19.8%, and C-horizon 10.4% of the total CO2 efflux. The Q10 values determined over 7-d periods were on average 2.54, 3.66, and 13.14 in A-, B-, and C-horizons, respectively. However, when fitted over the whole 2.5-mo time period, the Q10 values were 3.56, 5.57, and 17.45 in A-, B-, and C-horizons, respectively. Based on the different temperature responses obtained over 7-d and 2.5-mo time periods, we estimated the contribution of recent photosynthate to soil respiration. In April, the respiration originating from recent photosynthate contributed about 2% of the total respiration in the A-, B-, and C-horizons and increased to 32, 35, and 24% by June 30 in the respective soil horizons.
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The McMurdo Dry Valleys in Antarctica are a cold hyperarid polar desert that present extreme challenges to life. Here, we report a culture-independent survey of multidomain microbial biodiversity in McKelvey Valley, a pristine example of... more
The McMurdo Dry Valleys in Antarctica are a cold hyperarid polar desert that present extreme challenges to life. Here, we report a culture-independent survey of multidomain microbial biodiversity in McKelvey Valley, a pristine example of the coldest desert on Earth. We demonstrate that life has adapted to form highly specialized communities in distinct lithic niches occurring concomitantly within this terrain. Endoliths and chasmoliths in sandstone displayed greatest diversity, whereas soil was relatively depauperate and lacked a significant photoautotrophic component, apart from isolated islands of hypolithic cyanobacterial colonization on quartz rocks in soil contact. Communities supported previously unreported polar bacteria and fungi, but archaea were absent from all niches. Lithic community structure did not vary significantly on a landscape scale and stochastic moisture input due to snowmelt resulted in increases in colonization frequency without significantly affecting diversity. The findings show that biodiversity near the cold-arid limit for life is more complex than previously appreciated, but communities lack variability probably due to the high selective pressures of this extreme environment.
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Rates of atmospheric CH₄ consumption of soils in temperate forest were compared in plots continuously enriched with CO₂ at 200 mL L±1 above ambient and in control plots exposed to the ambient atmosphere of 360 mL CO₂ L±1. The purpose was... more
Rates of atmospheric CH₄ consumption of soils in temperate forest were compared in plots continuously enriched with CO₂ at 200 mL L±1 above ambient and in control plots exposed to the ambient atmosphere of 360 mL CO₂ L±1. The purpose was to determine if ecosystem atmospheric CO₂ enrichment would alter soil microbial CH₄ consumption at the forest floor and if the effect of CO₂ would change with time or with environmental conditions. Reduced CH₄ consumption was observed in CO₂-enriched plots relative to control plots on 46 out of 48 sampling dates, such that CO₂-enriched plots showed annual reductions in CH₄ consumption of 16% in 1998 and 30% in 1999. No significant differences were observed in soil moisture, temperature, pH, inorganic-N or rates of N-mineralization between CO₂-enriched and control plots, indicating that differences in CH₄ consumption between treatments were likely the result of changes in the composition or size of the CH₄-oxidizing microbial community. A repeated measures analysis of variance that included soil moisture, soil temperature (from 0 to 30 cm), and time as covariates indicated that the reduction of CH₄ consumption under elevated CO₂ was enhanced at higher soil temperatures. Additionally, the effect of elevated CO₂ on CH₄ consumption increased with time during the two year study. Overall, these data suggest that rising atmospheric CO₂ will reduce atmospheric CH₄ consumption in temperate forests and that the effect will be greater in warmer climates. A 30% reduction in atmospheric CH₄ consumption by temperate forest soils in response to rising atmospheric CO₂ will result in a 10% reduction in the sink strength of temperate forest soils in the atmospheric CH₄ budget and a positive feedback to the greenhouse effect.
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El metano, producido mayoritariamente en la degradación microbiana de compuestos orgánicos bajo condiciones anaeróbicas, es el segundo gas en importancia que interviene en el efecto invernadero del planeta. Su potencial de absorción de... more
El metano, producido mayoritariamente en la degradación microbiana de compuestos orgánicos bajo condiciones anaeróbicas, es el segundo gas en importancia que interviene en el efecto invernadero del planeta. Su potencial de absorción de rayos infrarrojos procedentes de la tierra es 11 veces superior a la del CO₂. Por este motivo, a pesar de su baja concentración en la atmósfera (1.7 ppm frente a 334 ppm de CO₂), el CH₄ contribuye en aproximadamente 17 % al calentamiento actual del planeta (Intergo-vernamental Panel on Climate Change, 1990).
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This protocol addresses N₂O, CO₂ and CH₄ flux measurement by soil chamber methodology. The reactivities of other gasses of interest such as NOx O₃, CO, and NH₃ will require different chambers and associated instrumentation. Carbon dioxide... more
This protocol addresses N₂O, CO₂ and CH₄ flux measurement by soil chamber methodology. The reactivities of other gasses of interest such as NOx O₃, CO, and NH₃ will require different chambers and associated instrumentation. Carbon dioxide is included as an analyte with this protocol; however, when plants are present, interpretation of soil CO₂ flux data in the context of net GHG flux is not straightforward because soil CO₂ emissions do not represent net ecosystem CO₂-C exchange. This protocol adopts chamber-based flux methodology (the least expensive option available) in order to allow inclusion of as many sites as possible. Since micrometeorological techniques require expensive instrumentation, they will be used only at locations with current micrometeorological capability. In deciding on a chamber design, our goal was to adopt methodology which is sensitive, unbiased, has low associated variance, and allows accurate interpolation/extrapolation over time and space. Because of our inability, at this time, to precisely assess the extent of bias associated with a given chamber design and sampling protocol under the range of conditions which might exist, we have adopted our 'best guess' protocol. Assessment, refinement and/or modifications of this protocol may continue in the future. At some sites this may include evaluation of chambers against fluxes determined by micrometeorology or performing comparisons of alternate chamber designs. Recognizing that any measurement technique will have disadvantages, the best we can do at this time is to select a technique which minimizes potential problems. In addition, adoption of common methodology will aid in site inter-comparisons. To facilitate the adoption of a common technique, it is important to attain a common understanding of the potential shortcomings associated with chamber-based flux measurement techniques (Rochette and Eriksen-Haamel, 2008). The following section summarizes some of these issues.
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Renewed interest in quantifying greenhouse gas emissions from soil has led to an increase in the application of chamber-based flux measurement techniques. Despite the apparent conceptual simplicity of chamber-based methods, nuances in... more
Renewed interest in quantifying greenhouse gas emissions from soil has led to an increase in the application of chamber-based flux measurement techniques. Despite the apparent conceptual simplicity of chamber-based methods, nuances in chamber design, deployment, and data analyses can have marked effects on the quality of the flux data derived. In many cases, fluxes are calculated from chamber headspace vs. time series consisting of three or four data points. Several mathematical techniques have been used to calculate a soil gas flux from time course data. This paper explores the influences of sampling and analytical variability associated with trace gas concentration quantification on the flux estimated by linear and nonlinear models. We used Monte Carlo simulation to calculate the minimum detectable fluxes (α = 0.05) of linear regression (LR), the Hutchinson/Mosier (H/M) method, the quadratic method (Quad), the revised H/M (HMR) model, and restricted versions of the Quad and H/M methods over a range of analytical precisions and chamber deployment times (DT) for data sets consisting of three or four time points. We found that LR had the smallest detection limit thresholds and was the least sensitive to analytical precision and chamber deployment time. The HMR model had the highest detection limits and was most sensitive to analytical precision and chamber deployment time. Equations were developed that enable the calculation of flux detection limits of any gas species if analytical precision, chamber deployment time, and ambient concentration of the gas species are known.
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Laboratory and field CO₂ efflux measurements were used to investigate the influence of soil organic C (SOC) decomposability and soil microclimate on summer SOC dynamics in seasonally dry montane forest and rangeland soils at the T.W.... more
Laboratory and field CO₂ efflux measurements were used to investigate the influence of soil organic C (SOC) decomposability and soil microclimate on summer SOC dynamics in seasonally dry montane forest and rangeland soils at the T.W. Daniel Experimental Forest in northern Utah. Soil respiration, soil temperature, and soil moisture content (SMC) were measured between July and October 2004 and 2005 in 12 control and 12 irrigated plots laid out in a randomized block design in adjacent forest (aspen or conifer) and rangeland (sagebrush [Artemisia tridentata Nutt.] or grass–forb) sites. Irrigated plots received a single water addition of 2.5 cm in July 2004 and two additions in July 2005. Th e SOC decomposability in mineral soil samples (0–10, 10–20, and 20–30 cm) was derived from 10-mo lab incubations. The amount of SOC accumulated in the A horizon (16 Mg ha¯¹) and the top 1 m (74 Mg ha¯¹) of the mineral soil did not differ significantly among vegetation type, but upper forest soils tended to contain more decomposable SOC than rangeland soils. The CO₂ efflux measured in the field varied significantly with vegetation cover (aspen > conifer = sagebrush > grass–forb), ranging from 12 kg CO₂–C ha¯¹ d¯¹ in aspen to 5 kg CO₂–C ha¯¹ d¯¹ in the grass–forb sites. It increased (~35%) immediately following water additions, with treatment effects dissipating within 1 wk. Soil temperature and SMC, which were negatively correlated (r =−0.53), together explained ~60% of the variability in summer soil respiration. Our study suggests that vegetation cover influences summer CO₂ efflux rates through its effect on SOC quality and the soil microclimate.
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Continuous changes in methane (CH₄) and carbon dioxide (CO₂) concentrations inside a closed chamber were measured on the forest floor at three sites: a deciduous forest and a coniferous forest in Hokkaido, Japan, and a birch forest in... more
Continuous changes in methane (CH₄) and carbon dioxide (CO₂) concentrations inside a closed chamber were measured on the forest floor at three sites: a deciduous forest and a coniferous forest in Hokkaido, Japan, and a birch forest in West Siberia, Russian Federation. Flux estimations by three types of regression methods, exponential, nonlinear, and linear, were examined using field-collected concentration data. The pattern of change with time of the gas concentration in the headspace differed, mainly according to site but also, to a lesser extent, according to the gas. This was a function of both the chamber height and surface soil property relating to soil gas diffusion and the gas concentration profile. Flux estimations did not differ statistically between the exponential and nonlinear methods for either gas at any site, because both of those regression methods were based on diffusion theory. However, the flux values estimated by linear regression were significantly different from those estimated by the other two methods for both CH₄ and CO₂ at the deciduous forest site and for CO₂ at the coniferous forest site. Shortening the chamber deployment period improved the linearity of the curve, but did not completely eliminate the error. Our results suggest that linear regression is not a good model of the change in headspace concentration with time.
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The δ¹³C of the soil surface efflux of carbon dioxide (δ¹³ CRs) has emerged as a powerful tool enabling investigation of a wide range of soil processes from characterising entire ecosystem respiration to detailed compound-specific isotope... more
The δ¹³C of the soil surface efflux of carbon dioxide (δ¹³ CRs) has emerged as a powerful tool enabling investigation of a wide range of soil processes from characterising entire ecosystem respiration to detailed compound-specific isotope studies. δ¹³CRs can be used to trace assimilated carbon transfer below ground and to partition the overall surface efflux into heterotrophic and autotrophic components. Despite this wide range of applications no consensus currently exists on the most appropriate method of sampling this surface efflux of CO2 in order to measure δ¹³CRs. Here we consider and compare the methods which have been used, and examine the pitfalls. We also consider a number of analysis options, isotope ratio mass spectrometry (IRMS), tuneable diode laser spectroscopy (TDLS) and cavity ring-down laser spectroscopy (CRDS). δ¹³CRs is typically measured using chamber systems, which fall into three types: closed, open and dynamic. All are imperfect. Closed chambers often rely on Keeling plots to estimate δ¹³CRs, which may not be appropriate without free turbulent air mixing. Open chambers have the advantage of being able to maintain steady-state conditions but analytical errors may become limiting with low efflux rates. Dynamic chambers like open chambers are complex, and controlling pressure fluctuations caused by air movement is a key concern. Both open and dynamic chambers in conjunction with field portable TDLs and CRDs analysis systems have opened up the possibility of measuring δ¹³CRs in real time permitting new research opportunities and are on balance the most suited to this type of measurement.
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Variability in seasonal soil moisture (SM) and temperature (T) can alter ecosystem/atmosphere exchange of the trace gases carbon dioxide (CO₂), nitrous oxide (N₂O), and methane (CH₄). This study reports the impact of year-round SM status... more
Variability in seasonal soil moisture (SM) and temperature (T) can alter ecosystem/atmosphere exchange of the trace gases carbon dioxide (CO₂), nitrous oxide (N₂O), and methane (CH₄). This study reports the impact of year-round SM status on trace gas fluxes in three semiarid vegetation zones, mesquite (30 g organic C kg¯¹ soil), open/ forb (6 g organic C kg¯¹ soil), and sacaton (18 g organic C kg¯¹ soil) from July 2002 – September 2003 in southeastern Arizona. Carbon dioxide and N₂O emissions were highly dependent on available SM and T. During the heavy rains of the 2002 monsoon (238 mm total rainfall), large differences in soil C content did not correlate with variations in CO₂ production, as efflux averaged 235.6 ± 39.5 mg CO₂ m¯² h¯¹ over all sites. In 2003, limited monsoon rain (95 mm total rainfall) reduced CO₂ emissions by 19% (mesquite), 40% (open), and 30% (sacaton), compared with 2002. Nitrous oxide emissions averaged 21.1 ± 13.4 (mesquite), 2.1 ± 4.4 (open), and 3.9 ± 5.2 mg N₂O m¯² h¯¹ (sacaton) during the 2002 monsoon. Limited monsoon 2003 rainfall reduced N₂O emissions by 47% in the mesquite, but N₂O production increased in the open (55%) and sacaton (5%) sites. Following a dry winter and spring 2002 (15 mm total rainfall), premonsoon CH₄ consumption at all sites was close to zero, but following monsoon moisture input, the CH₄ sink averaged 26.1 ± 6.3 mg CH₄ m¯² h¯¹ through April 2003. Laboratory incubations showed potentials for CH₄ oxidation from 0 to 45 cm, suggesting that as the soil surface dried, CH₄ oxidation activity shifted downward in the sandy soils. Predicted climate change shifts in annual precipitation from one dominated by summer monsoon rainfall to one with higher winter precipitation may reduce soil CO₂ and N₂O emissions while promoting CH₄ oxidation rates in semiarid riparian soils of the Southwest, potentially acting as a negative feedback for future global warming.
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Variability in seasonal soil moisture (SM) and temperature (T) can alter ecosystem/atmosphere exchange of the trace gases carbon dioxide (CO₂), nitrous oxide (N₂O), and methane (CH₄). This study reports the impact of year-round SM status... more
Variability in seasonal soil moisture (SM) and temperature (T) can alter ecosystem/atmosphere exchange of the trace gases carbon dioxide (CO₂), nitrous oxide (N₂O), and methane (CH₄). This study reports the impact of year-round SM status on trace gas fluxes in three semiarid vegetation zones, mesquite (30 g organic C kg¯¹ soil), open/ forb (6 g organic C kg¯¹ soil), and sacaton (18 g organic C kg¯¹ soil) from July 2002–September 2003 in southeastern Arizona. Carbon dioxide and N₂O emissions were highly dependent on available SM and T. During the heavy rains of the 2002 monsoon (238 mm total rainfall), large differences in soil C content did not correlate with variations in CO₂ production, as efflux averaged 235.6 ± 39.5 mg CO₂ m¯² h¯¹ over all sites. In 2003, limited monsoon rain (95 mm total rainfall) reduced CO₂ emissions by 19% (mesquite), 40% (open), and 30% (sacaton), compared with 2002. Nitrous oxide emissions averaged 21.1 ± 13.4 mesquite), 2.1 ± 4.4 (open), and 3.9 ± 5.2 mg N₂O m¯² h¯¹ (sacaton) during the 2002 monsoon. Limited monsoon 2003 rainfall reduced N₂O emissions by 47% in the mesquite, but N₂O production increased in the open (55%) and sacaton (5%) sites. Following a dry winter and spring 2002 (15 mm total rainfall), premonsoon CH₄ consumption at all sites was close to zero, but following monsoon moisture input, the CH₄ sink averaged 26.1 ± 6.3 mg CH₄ m¯² h¯¹ through April 2003. Laboratory incubations showed potentials for CH₄ oxidation from 0 to 45 cm, suggesting that as the soil surface dried, CH₄ oxidation activity shifted downward in the sandy soils. Predicted climate change shifts in annual precipitation from one dominated by summer monsoon rainfall to one with higher winter precipitation may reduce soil CO₂ and N₂O emissions while promoting CH₄ oxidation rates in semiarid riparian soils of the Southwest, potentially acting as a negative feedback for future global warming.
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Besides agricultural soils, temperate forest soils have been identified as significant sources of or sinks for important atmospheric trace gases (N₂O, NO, CH₄, and CO₂). Although the number of studies for this ecosystem type increased... more
Besides agricultural soils, temperate forest soils have been identified as significant sources of or sinks for important atmospheric trace gases (N₂O, NO, CH₄, and CO₂). Although the number of studies for this ecosystem type increased more than tenfold during the last decade, studies covering an entire year and spanning more than 1–2 years remained scarce. This study reports the results of continuous measurements of soil-atmosphere C- and N-gas exchange with high temporal resolution carried out since 1994 at the Höglwald Forest spruce site, an experimental field station in Southern Germany. Annual soil N₂O, NO and CO₂ emissions and CH₄ uptake (1994–2010) varied in a range of 0.2 – 3.0 kg N₂O-N ha¯¹ yr¯¹, 6.4–11.4 kg NO-N ha¯¹ yr¯¹, 7.0– 9.2 t CO₂-C ha¯¹ yr¯¹, and 0.9–3.5 kg CH₄-C ha¯¹ yr¯¹ respectively. The observed high fluxes of N-trace gases are most likely a consequence of high rates of atmospheric nitrogen deposition (>20 kg N ha¯¹ yr¯¹) of NH₃ and NOx to our site. For N₂O, cumulative annual emissions were 0.8 kg N₂O-N ha¯¹ yr¯¹ in years with freeze-thaw events (5 out 14 of years). This shows that long-term, multi-year measurements are needed to obtain reliable estimates of N₂O fluxes for a given ecosystem. Cumulative values of soil respiratory CO₂ fluxes tended to be highest in years with prolonged freezing periods, i.e. years with below average annual
mean soil temperatures and high N₂O emissions (e.g. the years 1996 and 2006). Furthermore, based on our unique database on trace gas fluxes we analyzed if soil temperature, soil moisture measurements can be used to approximate trace gas fluxes at daily, weekly, monthly, or annual scale. Our analysis shows that simple-to-measure environmental drivers such as soil temperature or soil moisture are suitable to approximate fluxes of NO and CO₂ at weekly and monthly resolution reasonably well (accounting for up to 59% of the variance). However, for CH₄ we so far failed to find meaningful correlations, and also for N₂O the predictive power is rather low. This is most likely due to the complexity of involved processes and counteracting effects of soil moisture and temperature, specifically with regard to N₂O production and consumption by denitrification and microbial community dynamics. At monthly scale, including information on gross primary production (CO₂, NO), and N deposition (N₂O), increased significantly the explanatory power of the obtained empirical regressions (CO₂: r² = 0.8; NO: r² = 0.67; N₂O, all data: r² = 0.5; N₂O, with exclusion of freeze-thaw periods: r² = 0.65).
mean soil temperatures and high N₂O emissions (e.g. the years 1996 and 2006). Furthermore, based on our unique database on trace gas fluxes we analyzed if soil temperature, soil moisture measurements can be used to approximate trace gas fluxes at daily, weekly, monthly, or annual scale. Our analysis shows that simple-to-measure environmental drivers such as soil temperature or soil moisture are suitable to approximate fluxes of NO and CO₂ at weekly and monthly resolution reasonably well (accounting for up to 59% of the variance). However, for CH₄ we so far failed to find meaningful correlations, and also for N₂O the predictive power is rather low. This is most likely due to the complexity of involved processes and counteracting effects of soil moisture and temperature, specifically with regard to N₂O production and consumption by denitrification and microbial community dynamics. At monthly scale, including information on gross primary production (CO₂, NO), and N deposition (N₂O), increased significantly the explanatory power of the obtained empirical regressions (CO₂: r² = 0.8; NO: r² = 0.67; N₂O, all data: r² = 0.5; N₂O, with exclusion of freeze-thaw periods: r² = 0.65).
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The rewetting of dry soils and the thawing of frozen soils are short-term, transitional phenomena in terms of hydrology and the thermodynamics of soil systems. The impact of these short-term phenomena on larger scale ecosystem fluxes is... more
The rewetting of dry soils and the thawing of frozen soils are short-term, transitional phenomena in terms of hydrology and the thermodynamics of soil systems. The impact of these short-term phenomena on larger scale ecosystem fluxes is increasingly recognized, and a growing number of studies show that these events affect fluxes of soil gases such as carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), ammonia (NH₃) and nitric oxide (NO). Global climate models predict that future climatic change is likely to alter the frequency and intensity of drying-rewetting events and thawing of frozen soils. These future scenarios highlight the importance of understanding how rewetting and thawing will influence dynamics of these soil gases. This study summarizes findings using a new database containing 338 studies conducted from 1956 to 2011, and highlights open research questions. The database revealed conflicting results following rewetting and thawing in various terrestrial ecosystems and among soil gases, ranging from large increases in fluxes to non-significant changes. Studies reporting lower gas fluxes before rewetting tended to find higher post-rewetting fluxes for CO₂, N₂O and NO; in addition, increases in N₂O flux following thawing were greater in warmer climate regions. We discuss possible mechanisms and controls that regulate flux responses, and recommend that a high temporal resolution of flux measurements is critical to capture rapid changes in gas fluxes after these soil perturbations. Finally, we propose that future studies should investigate the interactions between biological (i.e., microbial community and gas production) and physical (i.e., porosity, diffusivity, dissolution) changes in soil gas fluxes, apply techniques to capture rapid changes (i.e., automated measurements), and explore synergistic experimental and modelling approaches.
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Soil gas flux is commonly measured by monitoring the change in headspace gas concentration over time within a sealed compartment at the soil surface. Often, more than one trace gas is monitored at a time (e.g., CO₂ and CH₄), but the data... more
Soil gas flux is commonly measured by monitoring the change in headspace gas concentration over time within a sealed compartment at the soil surface. Often, more than one trace gas is monitored at a time (e.g., CO₂ and CH₄), but the data fit separately. Flux estimates for CO₂ and CH₄ were obtained simultaneously by minimizing a weighted sum-of-squares error. The approximation of one model parameter for CH₄, through theoretical relationship to the respective CO₂ parameter, reduced the total parameter count by one and allowed for the joint estimation of one parameter using the combined CO₂ and CH₄ datasets. The method of joint optimization was compared with separate optimization for two nonlinear models, using both real and simulated data. The datasets were best fit with the jointly optimized models. Furthermore, the jointly optimized models more accurately estimated initial soil–air fluxes (simulated data only). the method of joint optimization is recommended as a means to apply better-fitting nonlinear models to typically small gas sample sets. This method is applicable to any number of trace gases monitored simultaneously.
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Relative contributions of diverse, managed ecosystems to greenhouse gases are not completely documented. Th is study was conducted to estimate soil surface fluxes of carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) as affected... more
Relative contributions of diverse, managed ecosystems to greenhouse gases are not completely documented. Th is study was conducted to estimate soil surface fluxes of carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) as affected by management practices and weather. Gas fluxes were measured by vented, static chambers in Drummer and Raub soil series during two growing seasons. Treatments evaluated were corn cropped continuously (CC) or in rotation with soybean (CS) and fertilized with in-season urea-ammonium nitrate (UAN) or liquid swine manure applied in the spring (SM) or fall (FM). Soybean (SC) rotated with CS and restored prairie grass (PG) were also included. The CO₂ fluxes correlated (P ≤ 0.001) with soil temperature (ρ: 0.74) and accumulated rainfall 120 h before sampling (ρ: 0.53); N₂O fluxes correlated with soil temperature (ρ: 0.34). Seasonal CO₂–C emissions were not different across treatments (4.4 Mg ha¯¹ yr¯¹) but differed between years. Manured soils were net seasonal CH₄–C emitters (0.159–0.329 kg ha¯¹ yr¯¹), whereas CSUAN and CCUAN exhibited CH₄–C uptake (−0.128 and −0.177 kg ha¯¹ yr¯¹, respectively). Treatments significantly influenced seasonal N₂O–N emissions (P < 0.001) and ranged from <1.0 kg ha¯¹ yr¯¹ in PG and SC to between 3 and 5 kg ha¯¹ yr¯¹ in CCFM and CSUAN and >8 kg ha¯¹ yr¯¹ in CCSM; differences were driven by pulse emissions after N fertilization in concurrence with major rainfall events. These results suggest fall manure application, corn–soybean rotation, and restoration of prairies may diminish N₂O emissions and hence contribute to global warming mitigation.
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Intensive agriculture and increased N fertilizer use have contributed to elevated emissions of the greenhouse gases carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O). In this study, the exchange of CO₂, N₂O, and CH₄ between a... more
Intensive agriculture and increased N fertilizer use have contributed to elevated emissions of the greenhouse gases carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O). In this study, the exchange of CO₂, N₂O, and CH₄ between a Quincy fine sand (mixed, mesic Xeric Torripsamments) soil and atmosphere was measured in a sweet corn (Zea mays L.)–sweet corn–potato (Solanum tuberosum L.) rotation during the 2005 and 2006 growing seasons under irrigation in eastern Washington. Gas samples were collected using static chambers installed in the second-year sweet corn and potato plots under conventional tillage or reduced tillage. Total emissions of CO₂–C from sweet corn integrated over the season were 2071 and 1684 kg CO₂–C ha¯¹ for the 2005 and 2006 growing seasons, respectively. For the same period, CO₂ emissions from potato plots were 1571 and 1256 kg of CO₂–C ha¯¹. Cumulative CO₂ fluxes from sweet corn and potato fields were 17 and 13 times higher, respectively, than adjacent non-irrigated, native shrub steppe vegetation (NV). Nitrous oxide losses accounted for 0.5% (0.55 kg N ha¯¹) of the applied fertilizer (112 kg N ha¯¹) in corn and 0.3% (0.59 kg N ha¯¹) of the 224 kg N ha¯¹ applied fertilizer. Sweet corn and potato plots, on average, absorbed 1.7 g CH₄–C ha¯¹ d¯¹ and 2.3 g CH₄–C ha¯¹ d¯¹, respectively. The global warming potential contributions from NV, corn, and potato fields were 459, 7843, and 6028 kg CO₂–equivalents ha¯¹, respectively, for the 2005 growing season and were 14% lower in 2006.
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Understanding how carbon, nitrogen, and key soil attributes affect gas emissions from soil is crucial for alleviating their undesirable residual effects that can linger for years after termination of manure and compost applications. This... more
Understanding how carbon, nitrogen, and key soil attributes affect gas emissions from soil is crucial for alleviating their undesirable residual effects that can linger for years after termination of manure and compost applications. This study was conducted to evaluate the emission of soil CO₂, N₂O, and CH₄ and soil C and N indicators four years after manure and compost application had stopped. Experimental plots were treated with annual synthetic N fertilizer (FRT), annual and biennial manure (MN1 and MN2, respectively), and compost (CP1 and CP2, respectively) from 1992 to 1995 based on removal of 151 kg N ha1 yr1 by continuous corn (Zea mays L.). The control (CTL) plots received no input. After 1995, only the FRT plots received N fertilizer in the spring of 1999. In 1999, the emissions of CO₂ were similar between control and other treatments. The average annual carbon input in the CTL and FRT plots were similar to soil CO₂–C emission (4.4 and 5.1 Mg C ha1 yr1, respectively). Manure and compost resulted in positive C and N balances in the soil four years after application. Fluxes of CH₄–C and N₂O-N were nearly zero, which indicated that the residual effects of manure and compost four years after application had no negative influence on soil C and N storage and global warming. Residual effects of compost and manure resulted in 20 to 40% higher soil microbial biomass C, 42 to 74% higher potentially mineralizable N, and 0.5 unit higher pH compared with the FRT treatment. Residual effects of manure and compost on CO₂, N₂O, and CH₄ emissions were minimal and their benefits on soil C and N indicators were more favorable than that of N fertilizer.
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At most sites the magnitude of soil-atmosphere exchange of nitrous dioxide (N₂O), carbon dioxide (CO₂) and methane (CH₄) was estimated based on a few chambers located in a limited area. Topography has been demonstrated to influence the... more
At most sites the magnitude of soil-atmosphere exchange of nitrous dioxide (N₂O), carbon dioxide (CO₂) and methane (CH₄) was estimated based on a few chambers located in a limited area.
Topography has been demonstrated to influence the production and consumption of these gases in temperate ecosystems, but this aspect has often been ignored in tropical areas. In this study, we investigated spatial variability of the net fluxes of these gases
along a 100 m long slope of a evergreen broadleaved forest in southern China over a whole year. We expected that the lower part of slope would release more N₂O and CO₂, but take up less atmospheric CH₄ than the upper part due to different availability of water and nutrients. Our results showed that the soil moisture (Water Filled Pore Space, WFPS) decreased along the slope from bottom to top as we expected, but among the three gases only N₂O emissions followed this pattern. Annual means of WFPS ranged from 27.7% to 52.7% within the slope, and annual emissions of N₂O ranged from 2.0 to 4.4 kg N ha¯¹ year¯¹, respectively. These two variables were highly and positively correlated across the slope. Neither potential rates of net N mineralization and nitrification, nor N₂O emissions in the laboratory incubated soils varied with slope positions. Soil CO₂ release and CH₄ uptake appeared to be independent on slope position in this study. Our results suggested that soil water content and associated N₂O emissions are likely to be influenced by topography even in a short slope, which may need to be taken into account in field measurements and modelling.
Topography has been demonstrated to influence the production and consumption of these gases in temperate ecosystems, but this aspect has often been ignored in tropical areas. In this study, we investigated spatial variability of the net fluxes of these gases
along a 100 m long slope of a evergreen broadleaved forest in southern China over a whole year. We expected that the lower part of slope would release more N₂O and CO₂, but take up less atmospheric CH₄ than the upper part due to different availability of water and nutrients. Our results showed that the soil moisture (Water Filled Pore Space, WFPS) decreased along the slope from bottom to top as we expected, but among the three gases only N₂O emissions followed this pattern. Annual means of WFPS ranged from 27.7% to 52.7% within the slope, and annual emissions of N₂O ranged from 2.0 to 4.4 kg N ha¯¹ year¯¹, respectively. These two variables were highly and positively correlated across the slope. Neither potential rates of net N mineralization and nitrification, nor N₂O emissions in the laboratory incubated soils varied with slope positions. Soil CO₂ release and CH₄ uptake appeared to be independent on slope position in this study. Our results suggested that soil water content and associated N₂O emissions are likely to be influenced by topography even in a short slope, which may need to be taken into account in field measurements and modelling.
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CO₂ exchange was measured on the forest floor of a coastal temperate Douglas-fir forest located near Campbell River, British Columbia, Canada. Continuous measurements were obtained at six locations using an automated chamber system... more
CO₂ exchange was measured on the forest floor of a coastal temperate Douglas-fir forest located near Campbell River, British Columbia, Canada. Continuous measurements were obtained at six locations using an automated chamber system between April and December, 2000. Fluxes were measured every half hour by circulating chamber headspace air through a sampling manifold assembly and a closed-path infrared gas analyzer. Maximum CO₂ fluxes measured varied by a factor of almost 3 between the chamber locations, while the highest daily average fluxes observed at two chamber locations occasionally reached values near 15 mol C m¯² s¯¹. Generally, fluxes ranged between 2 and 10 mol C m¯² s¯¹ during the measurement period. CO₂ flux from the forest floor was strongly related to soil temperature with the highest correlation found with 5 cm depth temperature. A simple temperature dependent exponential model fit to the nighttime fluxes revealed Q10 values in the normal range of 2–3 during the warmer parts of the year, but values of 4–5 during cooler periods. Moss photosynthesis was negligible in four of the six chambers, while at the other locations, it reduced daytime half-hourly net CO₂ flux by about 25%. Soil moisture had very little effect on forest floor CO₂ flux. Hysteresis in the annual relationship between chamber fluxes and soil temperatures was observed. Net exchange from the six chambers was estimated to be 1920 ± 530 g C m¯² per year, the higher estimates exceeding measurement of ecosystem respiration using year-round eddy correlation above the canopy at this site. This discrepancy is attributed to the inadequate number of chambers to obtain a reliable estimate of the spatial average soil CO₂ flux at the site and uncertainty in the eddy covariance respiration measurements.
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Concentrations of CO₂ and other greenhouse gases (GHG's) have been increasing dramatically in earth’s atmosphere since the industrial revolution, and are expected to continue increasing from ~385 ppmv today to more than 600 ppmv by the... more
Concentrations of CO₂ and other greenhouse gases (GHG's) have been increasing dramatically in earth’s atmosphere since the industrial revolution, and are expected to continue increasing from ~385 ppmv today to more than 600 ppmv by the end of this century (IPCC, 2007). Global surface temperatures are expected to rise between 1.1 to 5.4 °C by 2100, depending on how fast greenhouse gas concentrations increase. Precipitation dynamics are also predicted to change, although there is still considerable uncertainty in these projections. While some of the details of these events are unclear, most agree climate change has already affected agroecosystems worldwide, and will have even more profound effects as climate change accelerates (Solomon et al., 2009). Important feedback exists between the atmosphere and the soil (Heimann and Reichstein, 2008), and a clear understanding of how climate change and rising atmospheric CO₂ might affect soil C sequestration and greenhouse gas exchange in agroecosystems is urgently needed.
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Quantifying carbon (C) sequestration in paddy soils is necessary to help better understand the effect of agricultural practices on the C cycle. The objective of the present study was to assess the effects of tillage practices... more
Quantifying carbon (C) sequestration in paddy soils is necessary to help better understand the effect of agricultural practices on the C cycle. The objective of the present study was to assess the effects of tillage practices [conventional tillage (CT) and no-tillage (NT)] and the application of nitrogen (N) fertilizer (0 and 210 kg N ha¯¹) on fluxes of CH₄ and CO₂, and soil organic C (SOC) sequestration during the 2009 and 2010 rice growing seasons in central China. Application of N fertilizer significantly increased CH₄ emissions by 13%–66% and SOC by 21%–94% irrespective of soil sampling depths, but had no effect on CO₂ emissions in either year. Tillage significantly affected CH₄ and CO₂ emissions, where NT significantly decreased CH₄ emissions by 10%–36% but increased CO₂ emissions by 22%–40% in both years. The effects of tillage on the SOC varied with the depth of soil sampling. NT significantly increased the SOC by 7%–48% in the 0–5 cm layer compared with CT. However, there was no significant difference in the SOC between NT and CT across the entire 0–20 cm layer. Hence, our results suggest that the potential of SOC sequestration in NT paddy fields may be overestimated in central China if only surface soil samples are considered.
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The impact of fire on soil fluxes of CO₂, CH₄ and N₂O was investigated in a tropical grassland in Congo Brazzaville during two field campaigns in 2007–2008. The first campaign was conducted in the middle of the dry season and the second... more
The impact of fire on soil fluxes of CO₂, CH₄ and N₂O was investigated in a tropical grassland in Congo Brazzaville during two field campaigns in 2007–2008. The first campaign was conducted in the middle of the dry season and the second at the end of the growing season, respectively one and eight months after burning. Gas fluxes and several soil parameters were measured in each campaign from burned plots and from a close by control area preserved from fire. Rain events were simulated at each campaign to evaluate the magnitude and duration of the generated gas flux pulses. In laboratory experiments, soil samples from field plots were analysed for microbial biomass, net N mineralization, net nitrification, N₂O, NO and CO₂ emissions under different water and temperature soil regimes. One month after burning, field CO₂ emissions were significantly lower in burned plots than in the control plots, the average daily CH₄ flux shifted from net emission in the unburned area to net consumption in burned plots, no significant effect of fire was observed on soil N₂O fluxes. Eight months after burning, the average daily fluxes of CO₂, CH₄ and N₂O measured in control and burned plots were not significantly different. In laboratory, N₂O fluxes from soil of burned plots were significantly higher than fluxes from soil of unburned plots only above 70% of maximum soil water holding capacity; this was never attained in the field even after rain simulation. Higher NO emissions were measured in the lab in soil from burned plots at both 10% and 50% of maximum soil water holding capacity. Increasing the incubation temperature from 25 °C to 37 °C negatively affected microbial growth, mineralization and nitrification activities but enhanced N₂O and CO₂ production. Results indicate that fire did not increase postburning soil GHG emissions in this tropical grasslands characterized by acidic, well drained and nutrient-poor soil.