Biogeochemistry Lab

ABSTRACT Drying and rewetting of soil can have large effects on carbon (C) and nitrogen (N) dynamics. Drying-rewetting effects have mostly been studied in the absence of plants, although it is well known that plant–microbe interactions can substantially alter soil C and N dynamics. We investigated for the first time how drying and rewetting affected rhizodeposition, its utilization by microbes, and its stabilization into soil (C associated with soil mineral phase). We also investigated how drying and rewetting influenced N mineralization and loss. We grew wheat (Triticum aestivum) in a controlled environment under constant moisture and under dry-rewetting cycles, and used a continuous 13C-labeling method to partition plant and soil organic matter (SOM) contribution to different soil pools. We applied a 15N label to the soil to determine N loss. We found that dry-rewetting decreased total input of plant C in microbial biomass (MB) and in the soil mineral phase, mainly due to a reduction of plant biomass. Plant derived C in MB and in the soil mineral phase were positively correlated (R2 = 0.54; P = 0.0012). N loss was reduced with dry rewetting cycles, and mineralization increased after each rewetting event. Overall drying and rewetting reduced rhizodeposition and stabilization of new C, primary through biomass reduction. However, frequency of rewetting and intensity of drought may determine the fate of C in MB and consequently into the soil mineral phase. Frequency and intensity may also be crucial in stimulating N mineralization and reducing N loss in agricultural soils.

... PRS™-probes were used to measure K supply rates in three soils differing in soil texture, clay content, and thereby K buffering power. ... 1). Bradwell soil was sandy and had a low K buffering power, resulting in a rapid decline in measured K supply rates. ...

The Prairie Heating and CO2 Enrichment (PHACE) experiment was initiated in Spring, 2007 to evaluate the combined effects of warming and elevated CO2 on a northern mixed-grass prairie. Thirty 3-m diameter circular experimental plots were installed in Spring, 2006 at the USDA-ARS High Plains Grasslands Research Station, just west of Cheyenne, WY, USA. Twenty plots were assigned to a two-level

ABSTRACT Background/Question/Methods Elevated CO2 and warming are both known to stimulate soil respiration rates, leading to concerns regarding soil-related feedback effects on climate change. We investigated soil C cycling at the Prairie Heating and CO2 Enrichment (PHACE) experiment near Cheyenne, WY, a factorial experiment combining FACE (ambient and elevated [600 ppm] CO2 concentration), experimental warming (1.5ºC daytime, 3ºC nighttime) and irrigation to evaluate direct, indirect and interactive effects of global changes on native grassland structure and function. We measured soil respiration rates and applied two methods (vegetation removal by herbicide and stable isotopes) to partition the total flux into root respiration and decomposition components, during the growing season of 2008. We hypothesized that soil respiration and decomposition would be stimulated by elevated CO2 and experimental warming, and that vegetation removal would provide more comprehensive results in comparison with stable isotope partitioning. Results/Conclusions Experimental warming did not alter soil respiration or decomposition rates during the study period. Elevated CO2 did not alter soil respiration rates on undisturbed mixed-grass prairie, but it stimulated decomposition on non-vegetated plots by ~40% at ambient temperature and 60% at elevated temperature. These results are consistent with a priming effect due to increased labile C allocation belowground under elevated CO2 that enhances decomposition. Vegetation removal suggested that decomposition from ambient CO2 plots was 35% of total soil respiration, and that from elevated CO2 plots was 52% of soil respiration. The δ13C value of soil respiration in non-vegetated, elevated CO2 plots was similar to that in undisturbed plots, showing that labile substrates remained available for decomposition for several months following herbicide application. Stable isotopes suggested that warming in combination with elevated CO2 enhanced decomposition of older soil organic matter, in comparison to unwarmed, elevated CO2 plots. This research suggests that although vegetation removal causes disturbance, it can be applied in situations that do not allow stable isotope partitioning, and is reasonably straightforward to interpret. Uncertainties in stable isotope partitioning owing to determination of “end member” isotope values and non-steady state respiration conditions make it more challenging to apply. Additional research to quantify effects of elevated CO2 and warming on decomposition of soil organic matter will reduce uncertainties in model predictions of future climate.

Background/Question/Methods Carbon allocation and N acquisition by plants following defoliation may be linked through plant-microbe interactions in the rhizosphere. Feedbacks between herbivory and plant-microbe interactions may also be affected by increasing ...

In desert ecosystems, plant growth and nutrient uptake are restricted by availability of soil nitrogen (N) and phosphorus (P). The effects of both climate and soil nutrient conditions on N and P concentrations among desert plant life forms (annual, perennial and shrub) remain unclear. We assessed leaf N and P levels of 54 desert plants and measured the corresponding soil N and P in shallow (0-10 cm), middle (10-40 cm) and deep soil layers (40-100 cm), at 52 sites in a temperate desert of northwest China. Leaf P and N:P ratios varied markedly among life forms. Leaf P was higher in annuals and perennials than in shrubs. Leaf N and P showed a negative relationship with mean annual temperature (MAT) and no relationship with mean annual precipitation (MAP), but a positive relationship with soil P. Leaf P of shrubs was positively related to soil P in the deep soil. Our study indicated that leaf N and P across the three life forms were influenced by soil P. Deep-rooted plants may enhance the availability of P in the surface soil facilitating growth of shallow-rooted life forms in this N and P limited system, but further research is warranted on this aspect.

Semiarid rangelands are a significant global sink for methane (CH4), but this sink strength may be altered by climate change. Methane uptake is sensitive to soil moisture showing a hump-shaped relationship with a distinct optimum soil moisture level. Both CO2 and temperature affect soil moisture, but the direction of CH4 uptake response may depend on if the system is below