Quantification of the microbial biomass by determining ninhydrin-reactive N

The fumigation-extraction method was used to investigate the relationship between microbial biomass C and biomass ninhydrin-reactive N (Enin) in a group of 110 soils. This large soil group was used to assess the effects of soil conditions (pH, cation exchange capacity, K2SO4 extractable C, soil organic C and biomass NH4+) or the form of land use (arable, grassland and forest soils) on the relationship between biomass C and Enin and the biomass C-to-Enin ratio. The Enin contents ranged from 1.8 to 139.2 μg N g−1 soil and were significantly correlated with biomass C (r = 0.94). This relationship was affected mainly by the pH but also by the formation of biomass NH4+. In acid soils only small amounts of biomass N were converted to NH4+. The form of land use had no effect on the relationship between biomass C and Enin if the differences in pH were considered. No uniform conversion factor can be proposed for the estimation of microbial biomass C by measuring Enin. Different factors are proposed for two groups differentiated according to the pH: soils pH > 5.0: biomass C=22.0 ×Enin;soils pH ≤ 5.0: biomass C=35.3 × Enin. more precise quantification of the microbial biomass by measuring the Enin content, it would be necessary to determine the specific biomass C-to-Enin ratio for each soil before commencement of an experiment.