MICROBIAL ACTIVITY AND SURVIVAL IN SOILS DRIED AT DIFFERENT RATES

The changes in microbial biomass C, soil respiration, microbial activity (respiration/microbial C) and the content of oxidizable organic C extracted by 0.5 M K2SO4, were measured in four soils of contrasting characteristics (a sand, two silt loam soils and a peat) which were air-dried at 22-degrees-C at three different rates in the laboratory. Respiration was also measured on samples of the drying soils rewetted with water. The rates of drying were: < 10 h (fast), < 33 h (medium) and < 62 h (slow); drying was carried out for 6 h on consecutive days, with overnight storage. Measurements were also made on soils stored at field-moisture content over the 15 day duration of the experiment. Respiration and activity declined continuously and in a generally linear manner as the volumetric water content (W(v)) decreased. The decline in respiration in relation to water content W(v) was similar for all four soils and for the three rates of drying. Microbial biomass C also declined but generally only after a considerable initial period of drying (after the soils had reached W(v) of 0.1-0.3). Extractable C values increased, but only after an initial drying period (W(v) below 0.06-0.12). The increases in extractable C were approximately coincident with the decreases in microbial C, but only part of the increase in extractable C could be accounted for by the decrease in microbial C. Rewetting of dried soils caused a marked increase in respiration, particularly when the rewetted soils had reached W(v) values where extractable C had begun to increase. The relationship between microbial activity and extractable C was similar for all four soils and was not affected by the rate of drying. The similarity of the microbial responses in these contrasting soils, and the absence of any detectable differences between rates of drying suggest that the microbial communities had similar survival strategies to resist desiccation, and occupied comparable physical niches in the soils, despite these soils having widely differing textures, organic matter content, and soil moisture characteristics.