MEASUREMENT OF MICROBIAL BIOMASS N-C BY CHLOROFORM FUMIGATION-INCUBATION

We present a calculation for soil microbial biomass N:C ratio determin ed from a 10-d incubation following chloroform fumigation. The calcula tion is based on a mathematical model of the N content of the pre- and post-fumigation soil microbial biomass and the growth yield of the bi omass that develops after fumigation. Biomass N is calculated from the N:C ratio and biomass C. The mineralization of bacteria and fungi, wi th different N contents, added to fumigated soils was used to establis h the model parameters. The model was tested against an independent se t of measurements and considers two assumptions: 1) The ratio of N:C m ineralized from killed biomass is equal to the ratio of N:C mineralize d from soil non-biomass constituents. 2) More realistically, the N and C mineralization in the fumigated soil, from sources other than kille d biomass, is a residual fraction of the N and C mineralization in the unfumigated soil. Biomass C:N ratios calculated without a control cor rection (assumption 1) were, on average, 20% wider than corrected valu es (assumption 2). Biomass N calculated as the product of N:C and biom ass C was compared with published values for several data sets. The ne w calculation method was robust even when net immobilization of N foll owed fumigation.