Nitrogen fractions in the microbial biomass in soils of southern Brazil

The reaction of nitrogen compounds with ninhydrin can be used as an indicator of cytoplasmic materials released from microbial cells killed by fumigation. Total-N, ninhydrin-reactive-N (NR-N), ammonium-N (A-N), and α-amino-N in the microbial biomass of soils from the State of Rio Grande do Sul, Brazil, were determined, in 1996, in 0.5 mol L-1 K2SO4 extracts of fumigated and non-fumigated soils. Total-N varied from 20.3 to 104.4 mg kg-1 and the ninhydrin-reactive-N corresponded, in average, to 27% of this. The ninhydrin-reactive-N was made up of 67% ammonium-N and 33% aminoacids with the amino group at the α-carbon position. It was concluded that colorimetric analysis of NR-N and A-N may be used as a direct measure of microbial N in soil. This simple and rapid procedure is adequate for routine analyses.