CARBON TURNOVER AND NITROGEN TRANSFORMATIONS IN AN ALFISOL AND VERTISOL AMENDED WITH [U-C-14] GLUCOSE AND [N-15] AMMONIUM-SULFATE

Two topsoils, a Vertisol and an Alfisol, and the Alfisol subsoil were each amended with [U-C-14] glucose and (N-15) (NH4)2SO4, and incubated for 112 days. Glucose was decomposed without lag in the two topsoils. Throughout the incubation the extent of decomposition of C-14-labelled residues was greater, and the accumulation of residual C-14 as microbial biomass C-14 was less, in the lighter-textured, weakly-aggregated Alfisol topsoil than in the well-structured, high-clay Vertisol. In the Alfisol topsoil, net immobilization of inorganic N during glucose metabolism included both the added N-15-labelled NH4+ N and also the NO3- N present in the soil initially, to yield microbial residues of relatively low C:N ratio (ca 7-8: 1). By contrast, during the rapid phase of net N immobilization in the Vertisol added NH4+ N was utilized to the exclusion of NO3- N, thus resulting in residues of high C:N ratio (> 20:1). Net mineralization of immobilized N and N-15 occurred sooner and was more extensive in the Alfisol topsoil than in the Vertisol. Decomposition of glucose in the Alfisol subsoil, of similar clay content as the Vertisol but of weak aggregate stability, occurred after a 3-4 day lag. Decomposition of C-14 residues continued to an extent intermediate between those obtained with the topsoils, but the proportions of residual organic C-14 present as biomass C-14 were relatively very low. The subsoil contained low NO3- concentrations initially, and products of immobilization during rapid glucose metabolism were of C: N ratios similar to that of the Vertisol. From measurements of (CO2)-C-14 evolution rates and net changes in glucose-derived organic C-14 and microbial biomass C-14, calculations were made of gross rates of turnover of C-14 through the biomass in each of the three soils. First-order decay rates of biomass C-14 and non-biomass C-14 declined with decreases in the assigned efficiencies of substrate utilization, and with incubation time. For any assigned level of substrate utilization efficiency, the observed differences in total C-14 behaviour in the two topsoils were due mainly to major differences in the calculated gross rates of decline of biomass C-14, those in the low-clay Alfisol being 2.0-2.8 times greater than those in the high-clay Vertisol. Relative differences in the calculated gross rates of decline of non-biomass C-14 in the two topsoils were far less (1.0-1.2 times). Thus, analysis of C-14 turnover rates indicated that differences in the stabilization of biomass C-14, rather than in the stabilization of the C-14 of the products of cell death and metabolism, were responsible for the short-term (112 days) differences in C-14 behaviour in the topsoils. Of the three soils, rates of decay of C-14 in biomass were most rapid, and rates of decay C-14 in non-biomass were least rapid in the subsoil, resulting in the very low proportions of residual organic C-14 of this soil present as biomass C-14.