EFFECT OF SOIL TEXTURE ON THE SIZE OF THE MICROBIAL BIOMASS AND ON THE AMOUNT OF C AND N MINERALIZED PER UNIT OF MICROBIAL BIOMASS IN DUTCH GRASSLAND SOILS

The relationships between soil texture and the proportions of soil organic C and N present in the microbial biomass, the amounts of C and N mineralized per unit of microbial biomass and the C:N ratio of the microbial biomass in Dutch grassland soils were investigated. The proportions of both soil C and N in the microbial biomass were higher in fine-textured soils than in coarse-textured soils. The ratios between C mineralization and microbial biomass C (activity of the microbial biomass) and between N mineralization and microbial biomass C were both negatively correlated with the percentage of soil organic C in the microbial biomass. The activity of the biomass was twice as large in an average sandy or loam soil than in an average clay. While the activity of the microbial biomass was the same in an average sandy and loam soil, the amount of N mineralized per amount of microbial biomass was larger in the sandy soils. This was associated with a higher C:N ratio of the microbial biomass in the sandy soils (average 8) than in the loams (average 5). The amount of N mineralized per amount of microbial biomass was lowest in the clays. This was associated with the lower activity of the microbial biomass and its relatively low C:N ratio (average 6). The observed differences in N mineralization between soil types could be calculated well with a simple food web model using the observed C:N ratios of the microbial biomass.