Soil water availability and microsite mediate fungal and bacterial phospholipid fatty acid biomarker abundances in Mojave Desert soils exposed to elevated atmospheric CO 2

Changes in the rates of nitrogen (N) cycling, microbial carbon (C) substrate use, and extracellular enzyme activities in a Mojave Desert ecosystem exposed to elevated atmospheric CO 2 suggest shifts in the size and/or functional characteristics of microbial assemblages in two dominant soil microsites: plant interspaces and under the dominant shrub Larrea tridentata . We used ester‐linked phospholipid fatty acid (PLFA) biomarkers as a proxy for microbial biomass to quantify spatial and temporal differences in soil microbial communities from February 2003 to May 2005. Further, we used the 13 C signature of the fossil CO 2 source for elevated CO 2 plots to trace recent plant C inputs into soil organic matter (SOM) and broad microbial groups using δ 13 C (‰). Differences between individual δ 13 C PLFA and δ 13 C SOM for fungal biomarkers indicated active metabolism of newer C in elevated CO 2 soils. Total PLFA‐C was greater in shrub microsites compared to plant interspaces, and CO 2 treatment differences within microsites increased under higher soil water availability. Total, fungal, and bacterial PLFA‐C increased with decreasing soil volumetric water content (VWC) in both microsites, suggesting general adaptations to xeric desert conditions. Increases in fungal‐to‐bacterial PLFA‐C ratio with decreasing VWC reflected functional group‐specific responses to changing soil water availability. While temporal and spatial extremes in resource availability in desert ecosystems contribute to the difficulty in identifying common trends or mechanisms driving microbial responses in less extreme environments, we found that soil water availability and soil microsite interacted with elevated CO 2 to shift fungal and bacterial biomarker abundances in Mojave Desert soils.