Turnover of carbon, nitrogen and phosphorus through the microbial biomass in soils incubated with 14-C-, 15N- and 32P-labelled bacterial cells

Abstract

A mixture of triply-labelled ( 14C, 15N, 32P) bacterial cells was added as a fresh suspension to two soils, a sandy-loam and a clay soil. The soils were incubated for up to 101 days under continuously moist conditions, or with periodic drying. Rates of mineralization of 14C, and especially of 15N and 32P, indicated rapid death and extensive decomposition of the added cells within a few days of incubation. Throughout, decomposition was more rapid in the sandy-loam soil than in the clay soil; and lower proportions of applied 14C, 15N and 32P were accounted for in the soil biota of the sandy-loam, as judged from assays of biomass 14C, 15N and 32P respectively. In general, the relative behaviour of the isotopes in the two soils with respect to their incorporation into biomass and subsequent net decay were similar to that found for soils in which 14C, 15N and 32P were added as soluble substrates and in which biomass and metabolites were formed in situ. The proportions of bacterial 14C released as 14CO 2 over 130 days were unaffected by the amounts added to soils, whereas the proportions of added 14C present as biomass 14C were slightly greater, the less added initially. Periodic drying of soils marginally decreased 14CO 2 evolution and increased 15N mineralization, accompanied by decreased recoveries of 15N in biomass. Differences in the behaviour of C and N in the two soils, and under different incubation conditions, have been described by a simulation model, which accommodates differences in the rates of turnover of microbial biomass C and N.