Two soils (an Alfisol from Borgloon, Belgium, and a Spodosol from Meer, Belgium) were amended with C-14-labelled residues of maize plants and incubated for 7 days (Borgloon soil only) and 27 days (both soils). Samples from each soil were then subjected to either (1) drying at 40-degrees-C for 3 days, remoistening and incubation at 25-degrees-C for 10 days, or (2) storage at 4-degrees-C for 3 days and incubation. During drying or storage and subsequent incubation of soils, biomass C and C-14 were determined using a fumigation extraction method. Amounts of (CO2)-C-12 and (CO2)-C-14 evolved during the 10-day incubation period were monitored. Drying and rewetting of soils caused a decrease in microbial biomass and enhanced mineralization of carbon during subsequent moist incubation. During drying unlabelled biomass C declined to 70-82% of control soils. Labelled biomass C decreased relatively more by soil drying and rewetting than unlabelled biomass C. Largest relative decreases of biomass C-14 caused by soil drying and rewetting were observed when biomass C-14 showed the highest specific respiration activity. After 7 days of incubating Borgloon soil with labelled plant material, soil desiccation and remoistening decreased biomass C-14 to 37% of control soils. When this soil was subjected to drying after 27 days of incubation the relative decline in biomass C-14 was smaller, viz. to 60% of control soils. Corresponding respiration activities were 30 and 8 mug C mg-1 biomass C day-1 after incubating Borgloon soil for 7 and 27 days, respectively. After 27 days of incubation with labelled plant material, biomass C-14 of Meer soil decreased relatively more than biomass C-14 of Borgloon soil, previously incubated for 27 days. Specific respiration activity for biomass C-14 of Meer soil was 16 mug mg-1 biomass C day-1. It was concluded that the resistance of soil microorganisms to soil drying and rewetting is influenced by their metabolic activity (which is determined by their type and physiological state ), Other soil-related factors are not excluded, but appeared to be of minor importance only. The sources of enhanced mineralization after drying and rewetting are organic substrates derived from microorganisms killed by drying and, to a larger extent, from other non-living soil organic matter. The contribution of the non-biomass soil organic carbon to the mineralization flush after soil drying and remoistening is largely dependent on the localization of microrganisms in the soil structure relative to their substrates. Studying the effects of soil drying and remoistening is proposed as a possible way to unravel those important consumer-substrate relationships.
