Spatial distribution of microbial biomass in microaggregates of a silty-loam soil and the relation with the resistance of microorganisms to soil drying

To indicate factors determining the location of microorganisms in the soil structure, moist and dried-remoistened samples of a silty-loam soil were separated into particle-size fractions after gentle dispersion (shaking in water for 16 h). Microbial biomass C and total organic C and N contents of soil fractions were measured. The presence of microaggregates in each size fraction was determined by texture analysis of fractions. More than 50% of the microbial biomass C content of the total soil and 37% of the soil organic C content were associated with the aggregated clay (20-2 micrometer) and the clay-size (< 2 am) fraction. These represented respectively 19 and 5% of the total soil weight and contained respectively 40 and 23% of clay-size particles present in the whole soil. The coarse silt-size (50-20 micrometer) fraction, consisting mainly of non-aggregated mineral particles, contributed 38% of the total soil weight and contained only 7% of both total soil microbial biomass C and of total soil organic C. Multiple linear regression showed that the variation in biomass C content between size fractions was almost completely (R2 = 0.99) explained by its clay and organic C content. Such a close relationship would not have been observed if microporosity had determined the location of microorganisms in the soil structure, as fractions were very heterogeneous in respect to microaggregation. Thus, the results indicate that microaggregation per se had no effect in this soil on the microbial distribution in size fractions. The effects of other factors, such as substrate availability, are discussed. Soil drying and rewetting reduced the microbial biomass C content of unfractionated soil by 36% compared to moist soil. Microbial biomass concentrations of size fractions from dried soil (expressed per unit of soil- weight, fraction weight, organic C and clays) were compared with those of moist soil. The results indicated that microbial cells associated with the 20-2 micromolar size fraction were more susceptible to desiccation than those residing in other size fractions. This fraction contained relatively the most clay, suggesting that the presence of clay does not provide extra protection for soil organisms against severe soil drying.