UC Santa Barbara

Wetting of dry soil triggers a pulse of elevated microbial respiration. This pulse may occur because microbes metabolize cellular carbon as they acclimate to moist conditions. Alternatively, wetting may mobilize extracellular soil carbon that is otherwise unavailable to microbes, fueling microbial growth and respiration. The latter mechanism can be expected to increase C availability in upper soil horizons that experience frequent cycles of drying and wetting, helping to explain the widely-observed relationship between C residence times and depth. I evaluated the relative importance of cellular and extracellular C sources across a range of grassland soils in California, and found that cellular carbon is likely a major contributor to respiration after wetting in surface horizons, while extracellular carbon availability is a more important C-source in deeper horizons. High-resolution measurements coupled with a model revealed that respiration after wetting in surface horizons incorporates a blend of cellular and extracellular sources. In addition, by translocating deep soil to the soil surface for 18 months, I found that surface microclimate increased microbial biomass without changing soluble C concentrations. These results indicate that wetting and drying cycles control microbial access to extracellular carbon, and thus have the potential to influence the residence time of soil C and the global C cycle.