SEARCH

SEARCH BY CITATION

Keywords:

  • carbon;
  • CO2;
  • Mojave Desert;
  • N cycling;
  • soil microbes

Abstract

Identifying soil microbial responses to anthropogenically driven environmental changes is critically important as concerns intensify over the potential degradation of ecosystem function. 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 with 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.