MICROBIAL BIOMASS RESPONSES TO SOIL DRYING AND REWETTING - THE FATE OF FAST-GROWING AND SLOW-GROWING MICROORGANISMS IN SOILS FROM DIFFERENT CLIMATES

Two soils from a temperate humid climatic region and one soil from a tropical subhumid area were incubated with C-14-labelled plant material for 27 days. Samples from each soil were then subjected to (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. One of the soils (a silty loam from a temperate climate area) was also treated after 7 days of incubation with C-14-labelled plant material. During drying or storage and subsequent incubation of soils, biomass C-12 and C-14 were determined using a fumigation-extraction method. The amounts of (CO2)-C-12 and (CO2)-C-14 evolved and inorganic N released during 10 days' incubation were monitored. When the silty-loam soil was dried after 7 days of incubation with added plant material, biomass C-14 decreased by 63% when compared to stored control soils. A smaller relative decline in biomass C-14 (viz. 40%) was observed when this soil was subjected to drying 27 days after addition of plant material, suggesting that actively-growing microorganisms are more susceptible to desiccation than populations with slower growth rates. Relative decreases of biomass C-12 by drying were similar for the three soils of various texture classes and carbon contents, and ranged from 14 to 30%. It was concluded that desiccation resistance was mainly determined by intrinsic properties of soil biota, presumably cell wall characteristics. For both soils from temperate climatic regions, biomass C-14 declined relatively more after soil drying than biomass C-12. In contrast, similar relative decreases in biomass C-14 and biomass C-12 caused by drying were observed for the tropical soil. Adaptation of the microbial communities to frequent and severe desiccation of the latter soil in its natural environment, was suggested to explain those observations. Drying and rewetting of soils enhanced carbon and nitrogen mineralization during subsequent moist incubation. It was inferred that the sources of mineralization flushes were partly biomass killed by drying, and partly non-living (labelled and unlabelled) organic residues. The sizes of flushes appeared to be influenced by soil properties such as carbon content and texture.