Formation of microbial biomass during the decomposition of C-14 labelled ryegrass in soil

Ryegrass uniformly labelled with 14C was incubated aerobically at 25°C for 62 days in two contrasting soils, a near‐neutral (pH 6.8) palcudalf from England and a strongly acid (pH 3.6) haplorthox from Brazil. Decomposition of the labelled plant material was faster in the near‐neutral soil throughout the whole of the incubation period. In neither soil did the addition of fresh plant material significantly accelerate the evolution of CO2 from organic matter already in the soil, i.e. there was no priming action. In the near‐neutral soil there was a rapid build up of labelled microbial biomass in the first 6 days, followed by a much slower increase that continued throughout the whole incubation period. After 62 days 22.5% of the labelled C remaining in the near‐neutral soil was in the biomass. The yield coefficient (the fraction of the incoming plant C converted to microbial C) of this stabilized or ‘resting’ biomass was 0.15. Much less labelled microbial biomass was formed in the acid soil than in the near‐neutral soil. By the end of 62 days only 6.2% of the labelled carbon remaining in the acid soil was in the biomass. Biomass C measurements in strongly acid soils must however be treated with caution as the technique used has not yet been adequately validated for such soils