Though tropical forest ecosystems are among the largest natural sources of the potent greenhouse ... more Though tropical forest ecosystems are among the largest natural sources of the potent greenhouse gas nitrous oxide (N 2 O), the spatial distribution of emissions across landscapes is often poorly resolved. Leaf cutter ants (LCA; Atta and Acromyrmex, Myrmicinae) are dominant herbivores throughout Central and South America, and influence multiple aspects of forest structure and function. In particular, their foraging creates spatial heterogeneity by concentrating large quantities of organic matter (including nitrogen, N) from the surrounding canopy into their colonies, and ultimately into colony refuse dumps. Here, we demonstrate that refuse piles created by LCA species Atta colombica in tropical rainforests of Costa Rica provide ideal conditions for extremely high rates of N 2 O production (high microbial biomass, potential denitrification enzyme activity, N content and anoxia) and may represent an unappreciated source of heterogeneity in tropical forest N 2 O emissions. Average instantaneous refuse pile N 2 O fluxes surpassed background emissions by more than three orders of magnitude (in some cases exceeding 80 000 mg N 2 ON m 22 h 21) and generating fluxes comparable to or greater than those produced by engineered systems such as wastewater treatment tanks. Refuse-concentrating Atta species are ubiquitous in tropical forests, pastures and production ecosystems, and increase density strongly in response to disturbance. As such, LCA colonies may represent an unrecognized greenhouse gas point source throughout the Neotropics.
Journal of Geophysical Research Biogeosciences, 2018
Biological pumping of mineral elements (root uptake from the soil and concentration at the surfac... more Biological pumping of mineral elements (root uptake from the soil and concentration at the surface via litterfall) may be an important mechanism influencing their loss from terrestrial ecosystems by accelerating transport in runoff, though few estimates exist to assess this. In the Susquehanna Shale Hills Critical Zone Observatory (a temperate forested watershed in central Pennsylvania), we compared two independent methods (litterfall-based and transpiration flow-based) for estimating the total uptake of elements (Ca, K, Mg, Mn, Si, Sr, and Al) by canopy trees. Elemental concentrations were measured monthly in leaf tissue and xylem sap of dominant species Quercus rubra (chestnut oak), Q. prinus (red oak), and Acer saccharum (sugar maple) for two growing seasons. Species-specific litterfall and transpiration (estimated by eddy covariance) were used to scale concentrations to annual fluxes. For most elements, both methods generated comparable gross fluxes in the range of 53-85 (Ca), 9-19 (Mg), 14-28 (Mn), 0.2-0.6 (Al), and 0.1-0.3 (Sr) kg·ha À1 ·year À1. For K, litterfall-based methods generated substantially lower estimates than transpiration, though neither accounted comprehensively for leaching from live foliage, resorption, or potential recycling within the growing season. For most elements, uptake rates were similar in magnitude to stream losses, implying a low degree of recycling. For K and Mn however, biological uptake exceeded losses by 1-2 orders of magnitude, suggesting a biological role in ecosystem retention. We conclude that litter-and transpiration-based methods can be combined to broadly estimate the magnitude of biological pumping, with additional measures of wood-/root-based turnover necessary for more accurate budgets. Plain Language Summary Elements such as silicon and calcium that are weathered from rocks can be moved from deep in the soil to the surface through uptake by plant roots and subsequent litterfall. At the surface, these elements are more vulnerable to being lost from an ecosystem by being eroded or washed into waterways, where they can eventually influence downstream processes such as ocean carbon uptake. It is difficult to estimate how important transport by trees is for element loss (or for recycling within the ecosystem) without good measurement methods. Here we compared two methods for estimating tree uptake of seven elements by measuring their concentrations in either leaf litter or plant sap. We measured three dominant tree species (oaks and maples) in a temperate forest catchment in central Pennsylvania and used total litterfall and sap flow to scale these concentrations into annual uptake rates. For most elements, both methods generated similar estimates that showed that the amount taken up by trees was similar to the amount lost in streams every year. However, for potassium and manganese, tree uptake was much greater than stream loss, suggesting that incorporation into tree litter helps to retain these elements within the ecosystem.
Forest dynamics and tree species composition vary substantially between Pale-otropical and Neotro... more Forest dynamics and tree species composition vary substantially between Pale-otropical and Neotropical forests, but these broad biogeographic regions are treated uniformly in many land models. To assess whether these regional differences translate into variation in productivity and carbon (C) storage, we compiled a database of climate, tree stem growth, lit-terfall, aboveground net primary production (ANPP), and aboveground biomass across tropical rainforest sites spanning 33 countries throughout Central and South America, Asia, and Australasia, but excluding Africa due to a paucity of available data. Though the sum of litter-fall and stem growth (ANPP) did not differ between regions, both stem growth and the ratio of stem growth to litterfall were higher in Paleotropical forests compared to Neotropical forests across the full observed range of ANPP. Greater C allocation to woody growth likely explains the much larger aboveground biomass estimates in Paleotropical forests (~29%, or 80 Mg DW/ha, greater than in the Neotropics). Climate was similar in Paleo-and Neotropi-cal forests, thus the observed differences in C likely reflect differences in the evolutionary history of species and forest structure and function between regions. Our analysis suggests that Paleotropical forests, which can be dominated by tall-statured Dipterocarpaceae species, may be disproportionate hotspots for aboveground C storage. Land models typically treat these distinct tropical forests with differential structures as a single functional unit, but our findings suggest that this may overlook critical biogeographic variation in C storage potential among regions.
Primary tropical rainforests are generally considered to be relatively nitrogen (N) rich, with ch... more Primary tropical rainforests are generally considered to be relatively nitrogen (N) rich, with characteristically large hydrologic and gaseous losses of inorganic N. However, emerging evidence suggests that some tropical ecosystems can exhibit tight N cycling, with low biologically available losses. In this study, we combined isotopic data with a well-characterized watershed N mass balance to close the N budget and characterize gaseous N losses at the ecosystem scale in a lowland tropical rainforest on the Osa Peninsula in southwestern Costa Rica. We measured d 15 N and d 18 O of nitrate (NO 3-) in precipitation, surface, shallow and deep soil lysimeters and stream water biweekly for 1 year. Enrichment of both isotopes indicates that denitrification occurs predominantly as NO 3-moves from surface soil down to 15 cm depth or laterally to stream water, with little further processing in deeper soil. Two different isotopic modeling approaches suggested that the gaseous fraction comprises 14 or 32% of total N loss (2.7 or 7.5 kg N ha-1 y-1), though estimates are sensitive to selection of isotopic fractionation values. Gas loss estimates using the mass balance approach (3.2 kg N ha-1 y-1) fall within this range and include N 2 O losses of 0.9 kg N ha-1 y-1. Overall, gaseous and soluble hydrologic N losses comprise a modest proportion (25%) of the total N inputs to this ecosystem. By contrast, relatively large, episodic hydrologic losses of non-biologically available particulate N balance the majority of N inputs and may contribute to maintaining conservative N cycling in this lowland tropical forest. Similar patterns of N cycling may occur in other tropical forests with similar state factor combinations-high rainfall, steep topography, relatively fertile soils-such as the western arc of the Amazon Basin and much of In-doMalaysia, but this hypothesis remains untested.
Arid soils represent a substantial carbonate pool and may participate in surface-atmosphere CO 2 ... more Arid soils represent a substantial carbonate pool and may participate in surface-atmosphere CO 2 exchange via a diel cycle of carbonate dissolution and exsolution. We used a Keeling plot approach to determine the substrate δ 13 C of CO 2 emitted from carbonate-dominated soils in the Mojave desert and found evidence for a nonrespiratory source that increased with surface temperature. In dry soils at 25-30°C, the CO 2 substrate had δ 13 C values of À19.4 AE 4.2‰, indicative of respiration of organic material (soil organic matter = À23.1 AE 0.8‰). CO 2 flux increased with temperature; maximum fluxes occurred above 60°C, where δ 13 CO 2 substrate (À7.2‰ AE 2.8‰) approached soil carbonate values (0.2 AE 0.2‰). In wet soils, CO 2 emissions were not temperature dependent, and δ 13 CO 2 substrate was lower in vegetated soils with higher flux rates, higher organic C content, and potential root respiration. These data provide the first direct evidence of CO 2 emissions from alkaline desert soils derived from an abiotic source and that diurnal emission patterns are strongly driven by surface temperature.
Despite some well-documented drawbacks , the acetylene reduction assay (ARA) remains one of the m... more Despite some well-documented drawbacks , the acetylene reduction assay (ARA) remains one of the most widespread methods for measuring biological nitrogen (N 2) fixation (BNF) in symbiotic and free-living niches due to its low cost, simplicity, and high throughput potential. Because ARA measures a proxy reaction (the reduction of acetylene to ethylene by the nitrogenase enzyme), a conversion ratio ('R ratio') is required to estimate equivalent fixation of N 2. Based on the biochemistry of the reactions, the theoretical ratio is usually taken to be 3:1. However, 15 N 2 calibrations often generate ratios that deviate considerably from this value. We synthesized calibrated R ratios for terrestrial BNF studies, asking whether values converge on the theoretical ratio and vary across N-fixing niches. From 253 mean values (n = 2,072 samples), we find that some niches (legumes, soil, litter) do center on 3:1, while others fall significantly above (wood, lichen) or below (bio-crusts). Moss in particular shows a bimodal distribution that may indicate contributions from alternative nitrogenases. However, almost all niches have very wide distributions (up to 2 orders of magnitude); applying ratio values spanning even the 25th-75th percentile cause BNF rates to vary by a factor of 1.5-2.5, and up to[ 8. Despite this, only a minority of studies (* 30% of 345) perform calibrations, and this proportion has not increased over time. We conclude that high variability precludes the use of theoretical values to obtain accurate BNF estimates via ARA, and that historical data should be considered with appropriate caution. Values should be calibrated directly when the goal is to generate accurate rates or cross-condition comparisons.
The role of lowland tropical forest tree communities in shaping soil nutrient cycling has been ch... more The role of lowland tropical forest tree communities in shaping soil nutrient cycling has been challenging to elucidate in the face of high species diversity. Previously, we showed that differences in tree species composition and canopy foliar nitrogen (N) concentrations correlated with differences in soil N availability in a mature Costa Rican rainforest. Here, we investigate potential mechanisms explaining this correlation. We used imaging spectroscopy to identify study plots containing 10-20 canopy trees with either high or low mean canopy N relative to the landscape mean. Plots were restricted to an uplifted terrace with relatively uniform parent material and climate. In order to assess whether canopy and soil N could be linked by litterfall inputs, we tracked litter production in the plots and measured rates of litter decay and the carbon and N content of leaf litter and leaf litter leachate. We also compared the abundance of putative N fixing trees and rates of free-living N fixation as well as soil pH, texture, cation exchange capacity, and topographic curvature to assess whether biological N fixation and/or soil properties could account for differences in soil N that were, in turn, imprinted on the canopy. We found no evidence of differences in legume communities, free-living N fixation, or abiotic properties. However, soils beneath Responsible Editor: John Harrison. Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10533-020-00643-0) contains supplementary material, which is available to authorized users.
Mangrove forests play an important role in climate change adaptation and mitigation by maintainin... more Mangrove forests play an important role in climate change adaptation and mitigation by maintaining coastline elevations relative to sea level rise, protecting coastal infrastructure from storm damage, and storing substantial quantities of carbon (C) in live and detrital pools. Determining the efficacy of mangroves in achieving climate goals can be complicated by difficulty in quantifying C inputs (i.e., differentiating newer inputs from younger trees from older residual C pools), and mitigation assessments rarely consider potential offsets to CO 2 storage by methane (CH 4) production in mangrove sediments. The establishment of non-native Rhizophora mangle along Hawaiian coastlines over the last century offers an opportunity to examine the role mangroves play in climate mitigation and adaptation both globally and locally as novel ecosystems. We quantified total ecosystem C storage, sedimentation, accretion, sediment organic C burial and CH 4 emissions from ~70 year old R. mangle stands and adjacent uninvaded mudflats. Ecosystem C stocks of mangrove stands exceeded mudflats by 434 ± 33 Mg C/ha, and mangrove establishment increased average coastal accretion by 460%. Sediment organic C burial increased 10-fold (to 4.5 Mg C ha −1 year −1), double the global mean for old growth mangrove forests, suggesting that C accumulation from younger trees may occur faster than previously thought, with implications for mangrove restoration. Simulations indicate that increased CH 4 emissions from sediments offset ecosystem CO 2 storage by only 2%-4%, equivalent to 30-60 Mg CO 2-eq/ha over mangrove lifetime (100 year sustained global warming potential). Results highlight the importance of mangroves as novel systems that can rapidly accumulate C, have a net positive atmospheric greenhouse gas removal effect, and support shoreline accretion rates that outpace current sea level rise. Sequestration potential of novel mangrove forests should be taken into account when considering their removal or management, especially in the context of climate mitigation goals. K E Y W O R D S 210 Pb, methane, Moloka'i, non-native species, restoration, Rhizophora mangle, sediment 4316 | SOPER Et al.
1. The mechanistic links between nitrogen (N) availability and investment in plant phosphorus (P)... more 1. The mechanistic links between nitrogen (N) availability and investment in plant phosphorus (P) acquisition have important implications for plant growth, species distributions, and responses to CO 2 fertilization under global change, especially in P-poor tropical ecosystems. Currently, it is unclear whether investment in strategies that enhance plant P acquisition (arbuscular mycorrhizal, AM; colonization or root phosphatase activity, RPA) are determined primarily by phylogeny, or whether these strategies differ among N 2-fixing legumes and nonfixing plants as a result of differing N availability. 2. We hypothesized that plant N status, which can vary widely independent of N fixation, correlates with investment in P acquisition, because: (a) N and P concentrations scale in plant tissue indicative of coupled demand and (b) plants with more N may have more resources available to allocate to acquisition strategies. 3. We grew seedlings of eight tropical tree species from three families (including three N 2-fixing and one nonfixing legume) under greenhouse conditions in native forest soil for four months. Species represented almost the full range of foliar N observed in tropical trees. 4. Neither foliar N nor P concentrations correlated with investment in P acquisition. Across all species, we found an inverse relationship between investment in AM colonization and RPA, but this trade-off was unrelated to foliar N or P and did not differ between functional types (i.e., N 2 fixers vs. nonfixers). 5. Within legumes (family Fabaceae), two strategies were evident that were unrelated to fixation status. High-fixing Inga and nonfixing Dialium displayed high foliar N and P concentrations and greater proportional investment in RPA versus AM, while lower fixing Ormosia species were characterized by lower foliar nutrient concentrations and proportionally more investment in AM. 6. Synthesis. Investment in P acquisition strategies in tropical trees is not dependent on foliar N or functional group, but instead may be controlled in part by resource trade-offs. High diversity in nutrient strategies between related species cautions again the use of simple functional groupings to draw conclusions about nutrient acquisition in tropical trees. K E Y W O R D S arbuscular mycorrhizal fungi, Fabaceae, legume, nitrogen fixation, phosphatase enzymes, phylogeny, plant-soil (below-ground) interactions, stoichiometry
Tropical forests exhibit significant heterogeneity in plant functional and chemical traits that m... more Tropical forests exhibit significant heterogeneity in plant functional and chemical traits that may contribute to spatial patterns of key soil biogeochemical processes, such as carbon storage and greenhouse gas emissions. Although tropical forests are the largest ecosystem source of nitrous oxide (N 2 O), drivers of spatial patterns within forests are poorly resolved. Here, we show that local variation in canopy foliar N, mapped by remote-sensing image spectroscopy, correlates with patterns of soil N 2 O emission from a lowland tropical rainforest. We identified ten 0.25 ha plots (assemblages of 40-70 individual trees) in which average remotely-sensed canopy N fell above or below the regional mean. The plots were located on a single minimally-dissected terrace (<1 km 2) where soil type, vegetation structure and climatic conditions were relatively constant. We measured N 2 O fluxes monthly for 1 yr and found that high canopy N species assemblages had on average threefold higher total mean N 2 O fluxes than nearby lower canopy N areas. These differences are consistent with strong differences in litter stoichiometry, nitrification rates and soil nitrate concentrations. Canopy N status was also associated with microbial community characteristics: lower canopy N plots had twofold greater soil fungal to bacterial ratios and a significantly lower abundance of ammonia-oxidizing archaea, although genes associated with denitrification (nirS, nirK, nosZ) showed no relationship with N 2 O flux. Overall, landscape emissions from this ecosystem are at the lowest end of the spectrum reported for tropical forests, consist with multiple metrics indicating that these highly productive forests retain N tightly and have low plant-available losses. These data point to connections between canopy and soil processes that have largely been overlooked as a driver of denitrification. Defining relationships between remotely-sensed plant traits and soil processes offers the chance to map these processes at large scales, potentially increasing our ability to predict N 2 O emissions in heterogeneous landscapes.
Arid soils represent a substantial carbonate pool and may participate in surface-atmosphere CO 2 ... more Arid soils represent a substantial carbonate pool and may participate in surface-atmosphere CO 2 exchange via a diel cycle of carbonate dissolution and exsolution. We used a Keeling plot approach to determine the substrate δ 13 C of CO 2 emitted from carbonate-dominated soils in the Mojave desert and found evidence for a nonrespiratory source that increased with surface temperature. In dry soils at 25–30°C, the CO 2 substrate had δ 13 C values of À19.4 AE 4.2‰, indicative of respiration of organic material (soil organic matter = À23.1 AE 0.8‰). CO 2 flux increased with temperature; maximum fluxes occurred above 60°C, where δ 13 CO 2 substrate (À7.2‰ AE 2.8‰) approached soil carbonate values (0.2 AE 0.2‰). In wet soils, CO 2 emissions were not temperature dependent, and δ 13 CO 2 substrate was lower in vegetated soils with higher flux rates, higher organic C content, and potential root respiration. These data provide the first direct evidence of CO 2 emissions from alkaline desert soils derived from an abiotic source and that diurnal emission patterns are strongly driven by surface temperature.
Difficulty in quantifying rates of biological N fixa-tion (BNF), especially over long time scales... more Difficulty in quantifying rates of biological N fixa-tion (BNF), especially over long time scales, remains a major impediment to defining N budgets in many ecosystems. To estimate N additions from BNF, we applied a tree-scale N mass balance approach to a well-characterized chronosequence of woody legume (Prosopis glandulosa) encroachment into subtropical grasslands. We defined spatially discrete single Prosopis clusters (aged 28–99 years), and for each calculated BNF as the residual of: soil N (0–30 cm), above-and below-ground biomass N, wet and dry atmospheric N deposition, N trace gas and N 2 loss, leaching loss, and baseline grassland soil N at time of establishment. Contemporary BNF for upland savanna woodland was estimated at 10.9 ± 1.8 kg N ha-1 y-1 , equal to a total of 249 ± 60 kg N ha-1 over about 130 years of encroachment at the site. Though these BNF values are lower than previous estimates for P. glandulosa, this likely reflects lower plant density as well as low water availability at this site. Uncertainty in soil and biomass parameters affected BNF estimates by 6–11%, with additional sensitivity of up to 18% to uncertainty in other scaling parameters. Differential N deposition (higher rates of dry N deposition to Prosopis canopies versus open grasslands) did not explain N accrual beneath trees; iterations that represented this scenario reduced estimated BNF estimates by a maximum of 1.5 kg N ha-1 y-1. We conclude that in this relatively well-constrained system, small-scale mass balance provides a reasonable method of estimating BNF and could provide an opportunity to cross-calibrate alternative estimation approaches.
Savanna ecosystems are a major source of nitrogen (N) trace gases that influence air quality and ... more Savanna ecosystems are a major source of nitrogen (N) trace gases that influence air quality and climate. These systems are experiencing widespread encroachment by woody plants, frequently associated with large increases in soil N, with no consensus on implications for trace gas emissions. We investigated the impact of encroachment by N-fixing tree Prosopis glandulosa on total reactive N gas flux (N t = NO + N 2 O + NO y + NH 3) from south Texas savanna soils over 2 years. Contrary to expectations, upland Prosopis groves did not have greater N t fluxes than adjacent unencroached grasslands. However, abiotic conditions (temperature, rainfall, and topography) were strong drivers. Emissions from moist, low-lying Prosopis playas were up to 3 times higher than from Prosopis uplands. Though NO dominated emissions, NH 3 and NO y (non-NO oxidized N) comprised 12–16% of the total summer N flux (up to 7.9 μg N m À2 h À1). Flux responses to soil wetting were temperature dependent for NO, NH 3 , and NO y : a 15 mm rainfall event increased flux 3-fold to 22-fold after 24 h in summer but had no effect in winter. Repeated soil wetting reduced N flux responses, indicating substrate depletion as a likely control. Rapid (<1 min) increases in NO emissions following wetting of dry soils suggested that abiotic chemodenitrification contributes to pulse emissions. We conclude that temperature and wetting dynamics, rather than encroachment, are primary drivers of N flux from these upland savannas, with implications for future emission patterns under altered precipitation regimes.
Information on denitrification (particularly N 2) losses from dry ecosystems is limited despite t... more Information on denitrification (particularly N 2) losses from dry ecosystems is limited despite their large area. Here, we present the first direct denitrifica-tion measurements for a northern hemisphere savanna, a Prosopis-dominated grassland/grove matrix in south Texas. We used the gas-flow intact soil core method to quantify N 2 , N 2 O and CO 2 losses and compared these with field measurements of N 2 O, NO y , NH 3 and CO 2 .
Water and nitrogen (N) interact to influence soil N cycling and plant N acquisition. We studied i... more Water and nitrogen (N) interact to influence soil N cycling and plant N acquisition. We studied indices of soil N availability and acquisition by woody plant taxa with distinct nutritional specialisations along a north Australian rainfall gradient from monsoonal savanna (1,600–1,300 mm annual rainfall) to semi-arid woodland (600–250 mm). Aridity resulted in increased ‘openness’ of N cycling, indicated by increasing δ15Nsoil and nitrate:ammonium ratios, as plant communities transitioned from N to water limitation. In this context, we tested the hypothesis that δ15Nroot xylem sap provides a more direct measure of plant N acquisition than δ15Nfoliage. We found highly variable offsets between δ15Nfoliage and δ15Nroot xylem sap, both between taxa at a single site (1.3–3.4 ‰) and within taxa across sites (0.8–3.4 ‰). As a result, δ15Nfoliage overlapped between N-fixing Acacia and non-fixing Eucalyptus/Corymbia and could not be used to reliably identify biological N fixation (BNF). However, Acacia δ15Nroot xylem sap indicated a decline in BNF with aridity corroborated by absence of root nodules and increasing xylem sap nitrate concentrations and consistent with shifting resource limitation. Acacia dominance at arid sites may be attributed to flexibility in N acquisition rather than BNF capacity. δ15Nroot xylem sap showed no evidence of shifting N acquisition in non-mycorrhizal Hakea/Grevillea and indicated only minor shifts in Eucalyptus/Corymbia consistent with enrichment of δ15Nsoil and/or decreasing mycorrhizal colonisation with aridity. We propose that δ15Nroot xylem sap is a more direct indicator of N source than δ15Nfoliage, with calibration required before it could be applied to quantify BNF.
Though tropical forest ecosystems are among the largest natural sources of the potent greenhouse ... more Though tropical forest ecosystems are among the largest natural sources of the potent greenhouse gas nitrous oxide (N 2 O), the spatial distribution of emissions across landscapes is often poorly resolved. Leaf cutter ants (LCA; Atta and Acromyrmex, Myrmicinae) are dominant herbivores throughout Central and South America, and influence multiple aspects of forest structure and function. In particular, their foraging creates spatial heterogeneity by concentrating large quantities of organic matter (including nitrogen, N) from the surrounding canopy into their colonies, and ultimately into colony refuse dumps. Here, we demonstrate that refuse piles created by LCA species Atta colombica in tropical rainforests of Costa Rica provide ideal conditions for extremely high rates of N 2 O production (high microbial biomass, potential denitrification enzyme activity, N content and anoxia) and may represent an unappreciated source of heterogeneity in tropical forest N 2 O emissions. Average instantaneous refuse pile N 2 O fluxes surpassed background emissions by more than three orders of magnitude (in some cases exceeding 80 000 mg N 2 ON m 22 h 21) and generating fluxes comparable to or greater than those produced by engineered systems such as wastewater treatment tanks. Refuse-concentrating Atta species are ubiquitous in tropical forests, pastures and production ecosystems, and increase density strongly in response to disturbance. As such, LCA colonies may represent an unrecognized greenhouse gas point source throughout the Neotropics.
Journal of Geophysical Research Biogeosciences, 2018
Biological pumping of mineral elements (root uptake from the soil and concentration at the surfac... more Biological pumping of mineral elements (root uptake from the soil and concentration at the surface via litterfall) may be an important mechanism influencing their loss from terrestrial ecosystems by accelerating transport in runoff, though few estimates exist to assess this. In the Susquehanna Shale Hills Critical Zone Observatory (a temperate forested watershed in central Pennsylvania), we compared two independent methods (litterfall-based and transpiration flow-based) for estimating the total uptake of elements (Ca, K, Mg, Mn, Si, Sr, and Al) by canopy trees. Elemental concentrations were measured monthly in leaf tissue and xylem sap of dominant species Quercus rubra (chestnut oak), Q. prinus (red oak), and Acer saccharum (sugar maple) for two growing seasons. Species-specific litterfall and transpiration (estimated by eddy covariance) were used to scale concentrations to annual fluxes. For most elements, both methods generated comparable gross fluxes in the range of 53-85 (Ca), 9-19 (Mg), 14-28 (Mn), 0.2-0.6 (Al), and 0.1-0.3 (Sr) kg·ha À1 ·year À1. For K, litterfall-based methods generated substantially lower estimates than transpiration, though neither accounted comprehensively for leaching from live foliage, resorption, or potential recycling within the growing season. For most elements, uptake rates were similar in magnitude to stream losses, implying a low degree of recycling. For K and Mn however, biological uptake exceeded losses by 1-2 orders of magnitude, suggesting a biological role in ecosystem retention. We conclude that litter-and transpiration-based methods can be combined to broadly estimate the magnitude of biological pumping, with additional measures of wood-/root-based turnover necessary for more accurate budgets. Plain Language Summary Elements such as silicon and calcium that are weathered from rocks can be moved from deep in the soil to the surface through uptake by plant roots and subsequent litterfall. At the surface, these elements are more vulnerable to being lost from an ecosystem by being eroded or washed into waterways, where they can eventually influence downstream processes such as ocean carbon uptake. It is difficult to estimate how important transport by trees is for element loss (or for recycling within the ecosystem) without good measurement methods. Here we compared two methods for estimating tree uptake of seven elements by measuring their concentrations in either leaf litter or plant sap. We measured three dominant tree species (oaks and maples) in a temperate forest catchment in central Pennsylvania and used total litterfall and sap flow to scale these concentrations into annual uptake rates. For most elements, both methods generated similar estimates that showed that the amount taken up by trees was similar to the amount lost in streams every year. However, for potassium and manganese, tree uptake was much greater than stream loss, suggesting that incorporation into tree litter helps to retain these elements within the ecosystem.
Forest dynamics and tree species composition vary substantially between Pale-otropical and Neotro... more Forest dynamics and tree species composition vary substantially between Pale-otropical and Neotropical forests, but these broad biogeographic regions are treated uniformly in many land models. To assess whether these regional differences translate into variation in productivity and carbon (C) storage, we compiled a database of climate, tree stem growth, lit-terfall, aboveground net primary production (ANPP), and aboveground biomass across tropical rainforest sites spanning 33 countries throughout Central and South America, Asia, and Australasia, but excluding Africa due to a paucity of available data. Though the sum of litter-fall and stem growth (ANPP) did not differ between regions, both stem growth and the ratio of stem growth to litterfall were higher in Paleotropical forests compared to Neotropical forests across the full observed range of ANPP. Greater C allocation to woody growth likely explains the much larger aboveground biomass estimates in Paleotropical forests (~29%, or 80 Mg DW/ha, greater than in the Neotropics). Climate was similar in Paleo-and Neotropi-cal forests, thus the observed differences in C likely reflect differences in the evolutionary history of species and forest structure and function between regions. Our analysis suggests that Paleotropical forests, which can be dominated by tall-statured Dipterocarpaceae species, may be disproportionate hotspots for aboveground C storage. Land models typically treat these distinct tropical forests with differential structures as a single functional unit, but our findings suggest that this may overlook critical biogeographic variation in C storage potential among regions.
Primary tropical rainforests are generally considered to be relatively nitrogen (N) rich, with ch... more Primary tropical rainforests are generally considered to be relatively nitrogen (N) rich, with characteristically large hydrologic and gaseous losses of inorganic N. However, emerging evidence suggests that some tropical ecosystems can exhibit tight N cycling, with low biologically available losses. In this study, we combined isotopic data with a well-characterized watershed N mass balance to close the N budget and characterize gaseous N losses at the ecosystem scale in a lowland tropical rainforest on the Osa Peninsula in southwestern Costa Rica. We measured d 15 N and d 18 O of nitrate (NO 3-) in precipitation, surface, shallow and deep soil lysimeters and stream water biweekly for 1 year. Enrichment of both isotopes indicates that denitrification occurs predominantly as NO 3-moves from surface soil down to 15 cm depth or laterally to stream water, with little further processing in deeper soil. Two different isotopic modeling approaches suggested that the gaseous fraction comprises 14 or 32% of total N loss (2.7 or 7.5 kg N ha-1 y-1), though estimates are sensitive to selection of isotopic fractionation values. Gas loss estimates using the mass balance approach (3.2 kg N ha-1 y-1) fall within this range and include N 2 O losses of 0.9 kg N ha-1 y-1. Overall, gaseous and soluble hydrologic N losses comprise a modest proportion (25%) of the total N inputs to this ecosystem. By contrast, relatively large, episodic hydrologic losses of non-biologically available particulate N balance the majority of N inputs and may contribute to maintaining conservative N cycling in this lowland tropical forest. Similar patterns of N cycling may occur in other tropical forests with similar state factor combinations-high rainfall, steep topography, relatively fertile soils-such as the western arc of the Amazon Basin and much of In-doMalaysia, but this hypothesis remains untested.
Arid soils represent a substantial carbonate pool and may participate in surface-atmosphere CO 2 ... more Arid soils represent a substantial carbonate pool and may participate in surface-atmosphere CO 2 exchange via a diel cycle of carbonate dissolution and exsolution. We used a Keeling plot approach to determine the substrate δ 13 C of CO 2 emitted from carbonate-dominated soils in the Mojave desert and found evidence for a nonrespiratory source that increased with surface temperature. In dry soils at 25-30°C, the CO 2 substrate had δ 13 C values of À19.4 AE 4.2‰, indicative of respiration of organic material (soil organic matter = À23.1 AE 0.8‰). CO 2 flux increased with temperature; maximum fluxes occurred above 60°C, where δ 13 CO 2 substrate (À7.2‰ AE 2.8‰) approached soil carbonate values (0.2 AE 0.2‰). In wet soils, CO 2 emissions were not temperature dependent, and δ 13 CO 2 substrate was lower in vegetated soils with higher flux rates, higher organic C content, and potential root respiration. These data provide the first direct evidence of CO 2 emissions from alkaline desert soils derived from an abiotic source and that diurnal emission patterns are strongly driven by surface temperature.
Despite some well-documented drawbacks , the acetylene reduction assay (ARA) remains one of the m... more Despite some well-documented drawbacks , the acetylene reduction assay (ARA) remains one of the most widespread methods for measuring biological nitrogen (N 2) fixation (BNF) in symbiotic and free-living niches due to its low cost, simplicity, and high throughput potential. Because ARA measures a proxy reaction (the reduction of acetylene to ethylene by the nitrogenase enzyme), a conversion ratio ('R ratio') is required to estimate equivalent fixation of N 2. Based on the biochemistry of the reactions, the theoretical ratio is usually taken to be 3:1. However, 15 N 2 calibrations often generate ratios that deviate considerably from this value. We synthesized calibrated R ratios for terrestrial BNF studies, asking whether values converge on the theoretical ratio and vary across N-fixing niches. From 253 mean values (n = 2,072 samples), we find that some niches (legumes, soil, litter) do center on 3:1, while others fall significantly above (wood, lichen) or below (bio-crusts). Moss in particular shows a bimodal distribution that may indicate contributions from alternative nitrogenases. However, almost all niches have very wide distributions (up to 2 orders of magnitude); applying ratio values spanning even the 25th-75th percentile cause BNF rates to vary by a factor of 1.5-2.5, and up to[ 8. Despite this, only a minority of studies (* 30% of 345) perform calibrations, and this proportion has not increased over time. We conclude that high variability precludes the use of theoretical values to obtain accurate BNF estimates via ARA, and that historical data should be considered with appropriate caution. Values should be calibrated directly when the goal is to generate accurate rates or cross-condition comparisons.
The role of lowland tropical forest tree communities in shaping soil nutrient cycling has been ch... more The role of lowland tropical forest tree communities in shaping soil nutrient cycling has been challenging to elucidate in the face of high species diversity. Previously, we showed that differences in tree species composition and canopy foliar nitrogen (N) concentrations correlated with differences in soil N availability in a mature Costa Rican rainforest. Here, we investigate potential mechanisms explaining this correlation. We used imaging spectroscopy to identify study plots containing 10-20 canopy trees with either high or low mean canopy N relative to the landscape mean. Plots were restricted to an uplifted terrace with relatively uniform parent material and climate. In order to assess whether canopy and soil N could be linked by litterfall inputs, we tracked litter production in the plots and measured rates of litter decay and the carbon and N content of leaf litter and leaf litter leachate. We also compared the abundance of putative N fixing trees and rates of free-living N fixation as well as soil pH, texture, cation exchange capacity, and topographic curvature to assess whether biological N fixation and/or soil properties could account for differences in soil N that were, in turn, imprinted on the canopy. We found no evidence of differences in legume communities, free-living N fixation, or abiotic properties. However, soils beneath Responsible Editor: John Harrison. Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10533-020-00643-0) contains supplementary material, which is available to authorized users.
Mangrove forests play an important role in climate change adaptation and mitigation by maintainin... more Mangrove forests play an important role in climate change adaptation and mitigation by maintaining coastline elevations relative to sea level rise, protecting coastal infrastructure from storm damage, and storing substantial quantities of carbon (C) in live and detrital pools. Determining the efficacy of mangroves in achieving climate goals can be complicated by difficulty in quantifying C inputs (i.e., differentiating newer inputs from younger trees from older residual C pools), and mitigation assessments rarely consider potential offsets to CO 2 storage by methane (CH 4) production in mangrove sediments. The establishment of non-native Rhizophora mangle along Hawaiian coastlines over the last century offers an opportunity to examine the role mangroves play in climate mitigation and adaptation both globally and locally as novel ecosystems. We quantified total ecosystem C storage, sedimentation, accretion, sediment organic C burial and CH 4 emissions from ~70 year old R. mangle stands and adjacent uninvaded mudflats. Ecosystem C stocks of mangrove stands exceeded mudflats by 434 ± 33 Mg C/ha, and mangrove establishment increased average coastal accretion by 460%. Sediment organic C burial increased 10-fold (to 4.5 Mg C ha −1 year −1), double the global mean for old growth mangrove forests, suggesting that C accumulation from younger trees may occur faster than previously thought, with implications for mangrove restoration. Simulations indicate that increased CH 4 emissions from sediments offset ecosystem CO 2 storage by only 2%-4%, equivalent to 30-60 Mg CO 2-eq/ha over mangrove lifetime (100 year sustained global warming potential). Results highlight the importance of mangroves as novel systems that can rapidly accumulate C, have a net positive atmospheric greenhouse gas removal effect, and support shoreline accretion rates that outpace current sea level rise. Sequestration potential of novel mangrove forests should be taken into account when considering their removal or management, especially in the context of climate mitigation goals. K E Y W O R D S 210 Pb, methane, Moloka'i, non-native species, restoration, Rhizophora mangle, sediment 4316 | SOPER Et al.
1. The mechanistic links between nitrogen (N) availability and investment in plant phosphorus (P)... more 1. The mechanistic links between nitrogen (N) availability and investment in plant phosphorus (P) acquisition have important implications for plant growth, species distributions, and responses to CO 2 fertilization under global change, especially in P-poor tropical ecosystems. Currently, it is unclear whether investment in strategies that enhance plant P acquisition (arbuscular mycorrhizal, AM; colonization or root phosphatase activity, RPA) are determined primarily by phylogeny, or whether these strategies differ among N 2-fixing legumes and nonfixing plants as a result of differing N availability. 2. We hypothesized that plant N status, which can vary widely independent of N fixation, correlates with investment in P acquisition, because: (a) N and P concentrations scale in plant tissue indicative of coupled demand and (b) plants with more N may have more resources available to allocate to acquisition strategies. 3. We grew seedlings of eight tropical tree species from three families (including three N 2-fixing and one nonfixing legume) under greenhouse conditions in native forest soil for four months. Species represented almost the full range of foliar N observed in tropical trees. 4. Neither foliar N nor P concentrations correlated with investment in P acquisition. Across all species, we found an inverse relationship between investment in AM colonization and RPA, but this trade-off was unrelated to foliar N or P and did not differ between functional types (i.e., N 2 fixers vs. nonfixers). 5. Within legumes (family Fabaceae), two strategies were evident that were unrelated to fixation status. High-fixing Inga and nonfixing Dialium displayed high foliar N and P concentrations and greater proportional investment in RPA versus AM, while lower fixing Ormosia species were characterized by lower foliar nutrient concentrations and proportionally more investment in AM. 6. Synthesis. Investment in P acquisition strategies in tropical trees is not dependent on foliar N or functional group, but instead may be controlled in part by resource trade-offs. High diversity in nutrient strategies between related species cautions again the use of simple functional groupings to draw conclusions about nutrient acquisition in tropical trees. K E Y W O R D S arbuscular mycorrhizal fungi, Fabaceae, legume, nitrogen fixation, phosphatase enzymes, phylogeny, plant-soil (below-ground) interactions, stoichiometry
Tropical forests exhibit significant heterogeneity in plant functional and chemical traits that m... more Tropical forests exhibit significant heterogeneity in plant functional and chemical traits that may contribute to spatial patterns of key soil biogeochemical processes, such as carbon storage and greenhouse gas emissions. Although tropical forests are the largest ecosystem source of nitrous oxide (N 2 O), drivers of spatial patterns within forests are poorly resolved. Here, we show that local variation in canopy foliar N, mapped by remote-sensing image spectroscopy, correlates with patterns of soil N 2 O emission from a lowland tropical rainforest. We identified ten 0.25 ha plots (assemblages of 40-70 individual trees) in which average remotely-sensed canopy N fell above or below the regional mean. The plots were located on a single minimally-dissected terrace (<1 km 2) where soil type, vegetation structure and climatic conditions were relatively constant. We measured N 2 O fluxes monthly for 1 yr and found that high canopy N species assemblages had on average threefold higher total mean N 2 O fluxes than nearby lower canopy N areas. These differences are consistent with strong differences in litter stoichiometry, nitrification rates and soil nitrate concentrations. Canopy N status was also associated with microbial community characteristics: lower canopy N plots had twofold greater soil fungal to bacterial ratios and a significantly lower abundance of ammonia-oxidizing archaea, although genes associated with denitrification (nirS, nirK, nosZ) showed no relationship with N 2 O flux. Overall, landscape emissions from this ecosystem are at the lowest end of the spectrum reported for tropical forests, consist with multiple metrics indicating that these highly productive forests retain N tightly and have low plant-available losses. These data point to connections between canopy and soil processes that have largely been overlooked as a driver of denitrification. Defining relationships between remotely-sensed plant traits and soil processes offers the chance to map these processes at large scales, potentially increasing our ability to predict N 2 O emissions in heterogeneous landscapes.
Arid soils represent a substantial carbonate pool and may participate in surface-atmosphere CO 2 ... more Arid soils represent a substantial carbonate pool and may participate in surface-atmosphere CO 2 exchange via a diel cycle of carbonate dissolution and exsolution. We used a Keeling plot approach to determine the substrate δ 13 C of CO 2 emitted from carbonate-dominated soils in the Mojave desert and found evidence for a nonrespiratory source that increased with surface temperature. In dry soils at 25–30°C, the CO 2 substrate had δ 13 C values of À19.4 AE 4.2‰, indicative of respiration of organic material (soil organic matter = À23.1 AE 0.8‰). CO 2 flux increased with temperature; maximum fluxes occurred above 60°C, where δ 13 CO 2 substrate (À7.2‰ AE 2.8‰) approached soil carbonate values (0.2 AE 0.2‰). In wet soils, CO 2 emissions were not temperature dependent, and δ 13 CO 2 substrate was lower in vegetated soils with higher flux rates, higher organic C content, and potential root respiration. These data provide the first direct evidence of CO 2 emissions from alkaline desert soils derived from an abiotic source and that diurnal emission patterns are strongly driven by surface temperature.
Difficulty in quantifying rates of biological N fixa-tion (BNF), especially over long time scales... more Difficulty in quantifying rates of biological N fixa-tion (BNF), especially over long time scales, remains a major impediment to defining N budgets in many ecosystems. To estimate N additions from BNF, we applied a tree-scale N mass balance approach to a well-characterized chronosequence of woody legume (Prosopis glandulosa) encroachment into subtropical grasslands. We defined spatially discrete single Prosopis clusters (aged 28–99 years), and for each calculated BNF as the residual of: soil N (0–30 cm), above-and below-ground biomass N, wet and dry atmospheric N deposition, N trace gas and N 2 loss, leaching loss, and baseline grassland soil N at time of establishment. Contemporary BNF for upland savanna woodland was estimated at 10.9 ± 1.8 kg N ha-1 y-1 , equal to a total of 249 ± 60 kg N ha-1 over about 130 years of encroachment at the site. Though these BNF values are lower than previous estimates for P. glandulosa, this likely reflects lower plant density as well as low water availability at this site. Uncertainty in soil and biomass parameters affected BNF estimates by 6–11%, with additional sensitivity of up to 18% to uncertainty in other scaling parameters. Differential N deposition (higher rates of dry N deposition to Prosopis canopies versus open grasslands) did not explain N accrual beneath trees; iterations that represented this scenario reduced estimated BNF estimates by a maximum of 1.5 kg N ha-1 y-1. We conclude that in this relatively well-constrained system, small-scale mass balance provides a reasonable method of estimating BNF and could provide an opportunity to cross-calibrate alternative estimation approaches.
Savanna ecosystems are a major source of nitrogen (N) trace gases that influence air quality and ... more Savanna ecosystems are a major source of nitrogen (N) trace gases that influence air quality and climate. These systems are experiencing widespread encroachment by woody plants, frequently associated with large increases in soil N, with no consensus on implications for trace gas emissions. We investigated the impact of encroachment by N-fixing tree Prosopis glandulosa on total reactive N gas flux (N t = NO + N 2 O + NO y + NH 3) from south Texas savanna soils over 2 years. Contrary to expectations, upland Prosopis groves did not have greater N t fluxes than adjacent unencroached grasslands. However, abiotic conditions (temperature, rainfall, and topography) were strong drivers. Emissions from moist, low-lying Prosopis playas were up to 3 times higher than from Prosopis uplands. Though NO dominated emissions, NH 3 and NO y (non-NO oxidized N) comprised 12–16% of the total summer N flux (up to 7.9 μg N m À2 h À1). Flux responses to soil wetting were temperature dependent for NO, NH 3 , and NO y : a 15 mm rainfall event increased flux 3-fold to 22-fold after 24 h in summer but had no effect in winter. Repeated soil wetting reduced N flux responses, indicating substrate depletion as a likely control. Rapid (<1 min) increases in NO emissions following wetting of dry soils suggested that abiotic chemodenitrification contributes to pulse emissions. We conclude that temperature and wetting dynamics, rather than encroachment, are primary drivers of N flux from these upland savannas, with implications for future emission patterns under altered precipitation regimes.
Information on denitrification (particularly N 2) losses from dry ecosystems is limited despite t... more Information on denitrification (particularly N 2) losses from dry ecosystems is limited despite their large area. Here, we present the first direct denitrifica-tion measurements for a northern hemisphere savanna, a Prosopis-dominated grassland/grove matrix in south Texas. We used the gas-flow intact soil core method to quantify N 2 , N 2 O and CO 2 losses and compared these with field measurements of N 2 O, NO y , NH 3 and CO 2 .
Water and nitrogen (N) interact to influence soil N cycling and plant N acquisition. We studied i... more Water and nitrogen (N) interact to influence soil N cycling and plant N acquisition. We studied indices of soil N availability and acquisition by woody plant taxa with distinct nutritional specialisations along a north Australian rainfall gradient from monsoonal savanna (1,600–1,300 mm annual rainfall) to semi-arid woodland (600–250 mm). Aridity resulted in increased ‘openness’ of N cycling, indicated by increasing δ15Nsoil and nitrate:ammonium ratios, as plant communities transitioned from N to water limitation. In this context, we tested the hypothesis that δ15Nroot xylem sap provides a more direct measure of plant N acquisition than δ15Nfoliage. We found highly variable offsets between δ15Nfoliage and δ15Nroot xylem sap, both between taxa at a single site (1.3–3.4 ‰) and within taxa across sites (0.8–3.4 ‰). As a result, δ15Nfoliage overlapped between N-fixing Acacia and non-fixing Eucalyptus/Corymbia and could not be used to reliably identify biological N fixation (BNF). However, Acacia δ15Nroot xylem sap indicated a decline in BNF with aridity corroborated by absence of root nodules and increasing xylem sap nitrate concentrations and consistent with shifting resource limitation. Acacia dominance at arid sites may be attributed to flexibility in N acquisition rather than BNF capacity. δ15Nroot xylem sap showed no evidence of shifting N acquisition in non-mycorrhizal Hakea/Grevillea and indicated only minor shifts in Eucalyptus/Corymbia consistent with enrichment of δ15Nsoil and/or decreasing mycorrhizal colonisation with aridity. We propose that δ15Nroot xylem sap is a more direct indicator of N source than δ15Nfoliage, with calibration required before it could be applied to quantify BNF.
ABSTRACT Background/Question/Methods Predicting the magnitude of symbiotic nitrogen (N) fixation ... more ABSTRACT Background/Question/Methods Predicting the magnitude of symbiotic nitrogen (N) fixation by woody plants has traditionally been impeded by the difficulty of measuring fixation rates in the field. Foliar δ15N measurement is the most commonly applied method for estimating fixation in woody perennials, especially when root nodules cannot be reliably recovered. However foliar δ15N values record an integrated signal of fixation over leaf lifetime and must rely on interspecies comparisons. The influence of factors such as climatic variation or phenology-driven N demand on N fixation is therefore difficult to quantify, because these processes operate on sub-annual scales. Furthermore, intrinsic differences in uptake and tissue fractionation among species are a source of inherent error associated with the foliar method. We attempted to improve the temporal resolution of δ15N data by sampling tree xylem sap (a N pool with a much shorter turnover time than foliage) at multiple time points within 12 months. We compared these data to foliar and rooting-zone soil soluble δ15N samples and tracked individual trees over time. The method was applied to N-fixing tree Prosopis glandulosa (honey mesquite), a widespread facilitator of woody encroachment in south Texas. N-fixing trees are common facilitators of encroachment worldwide and drive significant ecosystem N inputs. Results/Conclusions We observed significant seasonal variation in P. glandulosa xylem sap δ15N that could not be explained by variation in the soil inorganic N source, suggesting that sub-annual environmental or phenological processes may influence plant N fixation rates. Furthermore, we observed that the difference between xylem sap and foliar δ15N within P. glandulosa was substantially different to that within the best available woody non-fixing reference species in this system. This was sufficient to cause a two-fold difference in the calculated %NDFA (percent nitrogen derived from fixation) derived from foliar values compared with those derived from xylem sap δ15N. Between individuals, the range of variation in xylem sap δ15N at a single time point was 2.5 ‰ and within individuals, this value varied by up to 2 ‰ across seasons. Significant linear relationships between the sap δ15N values of individual trees over time suggest that some trees consistently fix more N or access soil N pools that are consistently isotopically distinct. The variation in potential fixation between trees could not be explained by tree age, soil total N or inorganic N concentrations; instead we propose that genotype interactions or extent of rhizobial colonization may account for this observation.
Organic nitrogen (ON) is the most abundant nitrogen (N) form in most soils and includes compounds... more Organic nitrogen (ON) is the most abundant nitrogen (N) form in most soils and includes compounds ranging from single amino acids to high molecular weight proteins. Despite increasing recognition that many species access amino acids as a N source, more complex forms of ON have not previously been considered accessible to plants. Furthering the recent suggestion that some plants can use di-peptides and proteins as an N source under sterile conditions, we assessed the capacity of two functionally different species to access a ...
Woody encroachment of nitrogen (N)-fixing trees has occurred extensively throughout semi-arid, su... more Woody encroachment of nitrogen (N)-fixing trees has occurred extensively throughout semi-arid, subtropical grasslands and savannas over the last 150 years. In the Rio Grande Plains of Texas, encroachment of N-fixing Prosopis glandulosa (honey mesquite) is widespread and leads to soil N accretion, presumably from large inputs of fixed N. These N inputs have consequences for carbon storage and trace gas emissions over large areas. Predicting future N inputs on a regional scale requires identification of environmental and plant ...
ABSTRACT Background/Question/Methods Predicting the magnitude of symbiotic nitrogen (N) fixation ... more ABSTRACT Background/Question/Methods Predicting the magnitude of symbiotic nitrogen (N) fixation by woody plants has traditionally been impeded by the difficulty of measuring fixation rates in the field. Foliar δ15N measurement is the most commonly applied method for estimating fixation in woody perennials, especially when root nodules cannot be reliably recovered. However foliar δ15N values record an integrated signal of fixation over leaf lifetime and must rely on interspecies comparisons. The influence of factors such as climatic variation or phenology-driven N demand on N fixation is therefore difficult to quantify, because these processes operate on sub-annual scales. Furthermore, intrinsic differences in uptake and tissue fractionation among species are a source of inherent error associated with the foliar method. We attempted to improve the temporal resolution of δ15N data by sampling tree xylem sap (a N pool with a much shorter turnover time than foliage) at multiple time points within 12 months. We compared these data to foliar and rooting-zone soil soluble δ15N samples and tracked individual trees over time. The method was applied to N-fixing tree Prosopis glandulosa (honey mesquite), a widespread facilitator of woody encroachment in south Texas. N-fixing trees are common facilitators of encroachment worldwide and drive significant ecosystem N inputs. Results/Conclusions We observed significant seasonal variation in P. glandulosa xylem sap δ15N that could not be explained by variation in the soil inorganic N source, suggesting that sub-annual environmental or phenological processes may influence plant N fixation rates. Furthermore, we observed that the difference between xylem sap and foliar δ15N within P. glandulosa was substantially different to that within the best available woody non-fixing reference species in this system. This was sufficient to cause a two-fold difference in the calculated %NDFA (percent nitrogen derived from fixation) derived from foliar values compared with those derived from xylem sap δ15N. Between individuals, the range of variation in xylem sap δ15N at a single time point was 2.5 ‰ and within individuals, this value varied by up to 2 ‰ across seasons. Significant linear relationships between the sap δ15N values of individual trees over time suggest that some trees consistently fix more N or access soil N pools that are consistently isotopically distinct. The variation in potential fixation between trees could not be explained by tree age, soil total N or inorganic N concentrations; instead we propose that genotype interactions or extent of rhizobial colonization may account for this observation.
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