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 DUTCHGRASSLAND SOILS

The relationships between soil texture and the proportions of soil org anic 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 mic robial biomass in Dutch grassland soils were investigated. The proport ions 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 mi neralization and microbial biomass C (activity of the microbial biomas s) and between N mineralization and microbial biomass C were both nega tively correlated with the percentage of soil organic C in the microbi al biomass. The activity of the biomass was twice as large in an avera ge sandy or loam soil than in an average clay. While the activity of t he microbial biomass was the same in an average sandy and loam soil, t he 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 (a verage 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.