Comparison of microbial C, N-flush and ATP, and certain enzyme activities of different textured soils subject to gradual drying

Abstract

A clay loam, a silt loam and a sand soil were gradually dried from field moisture content to air-dryness at 25°C in the laboratory. Microbial C measured by substrate-induced respiration (SIR), fumigation-incubation (FI) and fumigation-extraction (FE), microbial N-flush measured by FI and FE, microbial ATP content, and soil phosphatase and sulfatase activities were monitored throughout a drying period of approx. 60 h achieved over 16 days. All the microbial and enzyme variables declined as the gravimetric soil water content ( W) decreased to air-dryness. In general, the relationship between microbial C or N-flush and W was linear, but was exponential between ATP or phosphatase and W. Soil texture appeared to affect the rates of decline and also the amounts of the microbial and enzyme variables remaining in air-dry soil; e.g., the lowest rate of microbial C decline and the largest amount remaining at air-dryness occurred in the clay loam soil. Sulfatase activity was not significantly affected by soil drying. Agreement between the SIR and FE estimates of microbial C was good (r = 0.92***). These two methods were applicable over a wide range of water contents. Microbial N-flush, estimated by the FE method, also showed a consistent trend and correlated highly with microbial C estimated by SIR or FE. In contrast, microbial C and N-flush estimated by the FI method were not significantly correlated with W or any of the other variables. ATP and phosphatase activity appeared to relate more closely to microbial activity (CO2 respiration/microbial C) than microbial mass. The reliability of the methods to measure the biomass and the influence of soil texture, water and carbon contents on microbial survival are discussed.