Direct and indirect effects of shifting rainfall on soil microbial respiration and enzyme activity in a semi-arid system

Higher interannual precipitation variability is predicted for Southern California's shrub-dominated systems, promoting soil moisture variation and changing community composition. We asked if soil microbial responses to rainfall regime will depend on litter inputs; showing direct effects of altered precipitation through soil moisture and indirect effects resulting from shifting litter inputs. Soils were collected from a 2-year field rainfall manipulation experiment. Under lab conditions soils were subjected to high or low moisture pulses with litter amendments from native and exotic species in all combinations. Soil respiration was higher with larger water pulses, but rose over time in low pulse treatments (direct response). Litter additions from exotic species promoted greater respiration, and results were stronger under higher soil moisture (indirect response). Extracellular enzyme activities generally were higher with exotic litter and under high moisture pulses. Those involved in N-cycling had much larger increases activity for the exotic litter addition - high moisture pulse scenarios compared to other treatments. Our results indicate the potential for microbial acclimation to drought conditions over short timescales and that below-ground processes are sensitive to direct and indirect effects of shifting rainfall regimes, especially where invasion is promoted by future climate change.