MICROBIAL BIOMASS AND C AND N MINERALIZATION IN FOREST SOILS

Microbial biomass and activity were determined in three Cambisols and two Rankers developed over different kinds of rocks. The soils contained 0.3-1.7 g C kg-1 soil as biomass, which represented 0.8-1.5% of the total C Whether the fumigation-incubation or fumigation-extraction method was employed, nonsignificant differences were found in microbial biomass values. The microbial density, which ranged from 10(7) to 10(8) microorganisms g-1 soil, was drastically reduced by CHCl3-fumigation; however, the liberation of cell metabolites significantly increased the microbial growth rate. During 10 days of soil incubation at 25-degrees-C 0.1-0.2 g C kg-1 soil (0.1-0.5% of total soil C) was released and 1-5 mg N 100 g-1 soil (0.1-1.5% of total soil N) was mineralized A significant increase (approximately threefold) in the average C and N mineralization was found as a consequence of microbial lysis by fumigation. The results indicated that these soils have a considerable quantity of microbial biomass but a relatively low metabolic activity and that C and N mineralization may be significantly improved when microbial constituents are liberated into the soil.