Microbial activity in gradually dried or rewetted soils as governed by water and substrate availability

Abstract

Microbial respiration, mass and activity (respiration/mass) in three silt loam soils with different moisture regimes (1000-2700 mm annual rainfall) were measured as the field-moist soils were gradually air-dried at 25°C in the laboratory. Microbial CO2 and O2 respiration and mass (estimated from substrate-induced respiration), and oxidizable organic carbon, extractable in 0.5 M K2SO4, were monitored throughout the 60 h drying period and also in samples of these gradually dried soils which were rewetted with water (2 ml g-1 soil). Water availability was the major factor controlling microbial respiration and activity as the soils dried. Both respiration and activity declined continuously as gravimetric water content (W) decreased. Microbial mass also declined, but only after W fell below 0.1-0.3. The microbial mass of the lowest-rainfall soil resisted desiccation better than the higher-rainfall soils. Extractable carbon levels increased after considerable soil drying (when W < 0.1-0.2). This increase in extractable carbon could be accounted for by the carbon released from micro-organisms killed by drying. Microbial respiration and activity increased sharply within 30 min of rewetting gradually dried soils. These increases appeared to be largely due to the carbon released from micro-organisms, killed by drying, being metabolized by the surviving micro-organisms when water was present. The ability of the surviving micro-organisms to metabolize this (extractable) carbon was remarkably similar for all three soils, suggesting that the carbon released from killed cells is of a very similar qualitative nature, and/or that the surviving populations were also similar between the soils.