Carbon dioxide fluxes from biologically-crusted Kalahari Sands after simulated wetting

We report surface CO2 efflux and subsoil CO2 concentrations in biologically-crusted soils from the Kalahari. Fluxes were determined in-situ using a closed chamber coupled to a portable gas chromatograph on dry soils and on soils subject to simulated light and heavy rainfall. Surface efflux was measured in an artificially darkened environment in order to determine by difference, whether photosynthesis was occurring. Dry soil efflux rates were 2.8-14.8 mg C m2 h−1 throughout a diurnal cycle. Light rainfall led to an immediate increase in efflux to a peak of 65.6 mg C m2 h−1. Heavy rainfall resulted in a large pulse of CO2 with efflux rates of 339.2 mg C m2 h−1 over the first hour after wetting. Peak rates remained high over the following 2 days (87.8 and 87.0 mg C m2 h−1). Given sufficient moisture, fluxes increased with temperature. We believe hydration of the subsoil stimulates microorganisms which repsire available C either from extracellular polysaccharide sheaths (EPS) or released into the soil through lysis of microbial cells. Higher fluxes from the soil kept in the dark suggests photosynthesis occurs in wetted crusts during the daytime but net C uptake is masked by respiration from other microorganisms.