Distribution of microorganisms, total biomass, and enzyme activities in different particles of brown soil

A soil sample from the Ap horizon of an arable brown soil was fractionated by wet sieving, and seven size fractions of organic and mineral soil particles were separated. The organic fractions formed only 2.2% of the soil dry mass, but contained 41.5 and 29.12% of the total soil content of carbon and nitrogen, respectively, and thus represented an important reservoir of readily utilizable nutrients. Organic particles also accumulated most of the soil enzyme activities, determined asβ-glucosidase,β-acetylglucosaminidase, and proteinase activity. The highest counts of bacteria, actinomycetes, and fungi per gram of the soil fractions were obtained with the organic particles, but for the most part microorganisms accumulated in the silt-clay fraction. All soil fractions except the coarsest organic particles contained higher counts of oligotrophic bacteria than copiotrophic ones. Microbial counts, ATP contents, and enzyme activities decreased significantly with decrease in size of the organic soil particles, and increased with decrease in size of the mineral soil particles. Thus, the coarse organic particles >5 mm and the silt-clay fraction <0.05 mm represent the sites with the highest concentrations of microorganisms, ATP contents, and enzyme activities in the arable brown soil under test.