Soil wetting-respiration integrates microbial and extracellular carbon sources across a lithologic gradient

Background/Question/Methods . Wetting of dry soil causes a brief, several-fold increase in microbial respiration rates. Wetting respiration may be fueled by carbon that has accumulated in microbial biomass during the dry period. Alternatively, wetting may disrupt physical barriers that separate microbes from extracellular organic carbon under dry conditions, briefly increasing the supply of bioavailable carbon to microbes. We evaluated these two possibilities by comparing wetting-respiration rates at two soil depths (0-10cm, 40-50cm) in 37 soil profiles sampled across a lithologic gradient. This sampling approach allowed us to separate the contributions of microbial biomass (controlled by depth) and extracellular organic carbon availability (controlled by lithology) to wetting respiration.

Results/Conclusions . We found that the amount of carbon released after wetting soil from 0-10cm depth is equivalent to the amount of carbon typically lost from the microbial biomass after wetting (20% of the biomass). In contrast, the amount of carbon released after wetting at 40-50cm depth is unrelated to microbial biomass, and is equivalent to 50% of the water extractable organic carbon obtained from dry soil. In combination, these observations indicate that wetting respiration integrates both microbial and extracellular sources of carbon, and that the relative contributions of these two sources are controlled by depth.