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United States Department of Agriculture

Agricultural Research Service

Title: Elevated Co2 Increases Microbial Carbon Substrate Use and N Cycling in Mojave Desert Soils

Authors
item Jin, Virginia
item Evans, R - WASHINGTON STATE UNIV

Submitted to: Global Change Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 4, 2006
Publication Date: February 2, 2007
Citation: Jin, V.L., Evans, R.D. 2007. Elevated CO2 increases microbial carbon substrate use and nitrogen cycling in Mojave Desert soils. Global Change Biology 13:452-465.

Interpretive Summary: Arid ecosystems comprise almost 35% of the total terrestrial surface area, and are one of the most rapidly expanding biomes globally. As anthropogenically-driven environmental changes continue, concerns have intensified over the potential degradation of ecosystem functioning. Because the soil microbial community is the primary mediator for the cycling of elements between plants, soil, and atmosphere, we measured various soil microbial aspects to evaluate how long-term exposure of an intact Mojave Desert ecosystem to elevated atmospheric carbon dioxide (CO2) concentrations affected microbial carbon (C) use and microbially-mediated nitrogen (N) cycling. Because desert landscapes are highly heterogeneous, we examined soil microbial activity from two contrasting microsites: unvegetated interspaces between plants (interspaces) and under the dominant shrub (Larrea tridentata) during the 2004-2005 growing season. Elevated CO2 increased microbial C use and increased the diversity of substrates used. It also altered the types of C substrates used by microbes, suggesting changes in the quantity and/or quality of soil C inputs. In contrast, elevated CO2 had a mixed effect on microbial N cycling, depending on when soils were sampled. Elevated CO2 decreased rates of N mineralization late in the growing season but increased nitrification early in the growing season. Overall, general increases in microbial activities under elevated CO2 are likely attributable to greater microbial biomass in interspace soils, and to increased microbial activity levels rather than pool size in soils under Larrea. Because soil water content and plant cover type dominates microbial C and N responses to CO2, the ability of desert landscapes to mitigate or intensify the impacts of global change will ultimately depend on how changes in precipitation and increasing atmospheric CO2 shift the spatial distribution of Mojave Desert plant communities.

Technical Abstract: We assessed the effects of elevated atmospheric CO2 on microbial carbon (C) and nitrogen (N) cycling in Mojave Desert soils using extracellular enzyme activities (EEAs), community-level physiological profiles (CLPPs), and gross N transformation rates. Soils were collected from unvegetated interspaces between plants and under the dominant shrub (Larrea tridentata) during the 2004-2005 growing season, an above-average rainfall year. Because most measured variables responded strongly to soil water availability, all significant effects of soil water content were used as covariates to remove potential confounding effects of water availability on microbial responses to experimental treatment effects of cover type, CO2, and sampling date. Microbial C and N activities were lower in interspace soils compared to soils under Larrea, and responses to date and CO2 treatments were cover-specific. Over the growing season, EEAs involved in cellulose (cellobiohydrolase) and orthophosphate (alkaline phosphatase) degradation decreased under ambient CO2, but increased under elevated CO2. Microbial C use and substrate use diversity in CLPPs decreased over time, and elevated CO2 positively affected both. Elevated CO2 also altered microbial C use patterns, suggesting changes in the quantity and/or quality of soil C inputs. In contrast, microbial biomass N was higher in interspace soils than soils under Larrea, and was lower in soils exposed to elevated CO2. Gross rates of NH4+ transformations increased over the growing season, and late-season NH4+ fluxes were negatively affected by elevated CO2. Gross NO3- fluxes decreased over time, with early-season interspace soils positively affected by elevated CO2. General increases in microbial activities under elevated CO2 are likely attributable to greater microbial biomass in interspace soils, and to increased microbial turnover rates and/or metabolic levels rather than pool size in soils under Larrea. Because soil water content and plant cover type dominates microbial C and N responses to CO2, the ability of desert landscapes to mitigate or intensify the impacts of global change will ultimately depend on how changes in precipitation and increasing atmospheric CO2 shift the spatial distribution of Mojave Desert plant communities.

Last Modified: 12/17/2014
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