14C specific activity and 15N atom% excess in the soil microbial biomass as measured by the fumigation method and by the nucleic acid base method

Abstract

14C specific activity and 15N atom% excess were measured in the soil microbial biomass by both the fumigation and the nucleic acid base methods in a Ca 2+-saturated soil that had been incubated with 14C and 15N labelled plant material. The specific activities of the 14C-labelled biomass and of the nucleic acid bases isolated from this biomass were similar by both methods. However 15N atom% excess was significantly higher in total biomass (measured by fumigation) than in the nucleic acid bases. This was assumed to be due to a different C and N turnover in nucleic acid bases and in the microorganisms as a whole. This assumption was tested with pure cultures of bacteria and fungi. These experiments suggested that the new microbial population could grow from different sources of soil carbon and nitrogen. Thus the new microbial biomass C could come from the microbial biomass C that was killed by fumigation and simultaneously the new microbial biomass N from soil mineral N.