Title: The Biogeochemistry of Drought
Abstract: One might assume that drought is stressful to soil microorganisms. Why then does total microbial biomass increase over time once soil drops beneath a moisture threshold? Why also does the pool of extractable organic matter increase as soils remain dry but decline as soon as soils dry out? We have explored several hypotheses to explain these patterns, including microbes synthesizing osmolytes and exoenzymes breaking down dead roots. Our research in California annual grasslands and chaparral, however, suggest that neither of these can explain these observed patterns. Rather, it appears that the supply of bioavailable C comes from physically protected, mineral-sorbed organic matter. In a field experiment, we thinned plants during the winter growing season to regulate the input of fresh C during the summer. Yet, removing plants had no effects on the biomass of microbes, the accumulation of extractable organic matter, or even on the in situ fluxes of CO2 from the soil during the early part of the growing season. Only later, as the peak of the growing season developed did CO2 fluxes match in the control and bare plots. The mechanisms involved in regulating these C-fluxes remain unclear. Capturing dry season processes in simulation models requires formulations that incorporate the release of organic matter by abiotic processes. In this talk, I will discuss some of the approaches we have developed to analyze and parameterize dry season processes.