14C-labelled glucose turnover in New Zealand soils

The influence of soil mineralogy, as well as texture, on organic-C turnover was determined with C-14-labelled glucose. Samples of 16 soils from major mineralogical classes of New Zealand pastures and providing a range of organic C, clay contents and surface area, were incubated with C-14-labelled glucose for 35 d. The amounts of (CO2)-C-12 and (CO2)-C-14 evolved during incubation were monitored and the residual C-14 concentrations determined. Periodically, the samples were removed and microbial biomass C-12 and C-14 determined using the fumigation-extraction technique. System mean residence times (MRTs) were obtained by three independent methods: (i) a compartmental model using C-14 microbial biomass data, (ii) a non-compartmental model using C-14 microbial biomass data and (iii) a biexponential equation as an empirical equation from residual C-14 data. The effect of soil characteristics on MRTs was compared. The (CO2)-C-14 respired, after 35 d incubation, accounted for 51 to 66% of the glucose C-14 input to these soils. The soils differed significantly in their amounts of (CO2)-C-14 evolution and in the proportions of labelled C-14 in the biomass, The extent of mineralization of C-14-labelled glucose was influenced by soil clay content and clay surface area. Soils of low clay content (3-12%) had high biophysical quotients (respired: residual C-14); the highest (1.93) was in the soil with least clay (3%) and lowest mineral surface area, suggesting that clay is effective in C stabilization immediately after substrate assimilation. A biexponential model was found to be suitable for describing changes in the residual C-14 and microbial biomass C-14 during the 35 d glucose decomposition for most of the soils, MRTs for microbial biomass C-14 were correlated with clay content (P < 0.001), surface area estimated by para-nitrophenol (pNP) (P < 0.003) and pH (P < 0.01). Our results also showed that the MRTs of microbially assimilated C-14 are similar despite differences in the chemical nature of the applied C-14-labelled substrate. However, the MRT for humus C-14 differed with the chemical nature of the applied substrate. Clay and surface area played a major role in controlling the decomposition of added substrate through the stabilization and protection of the microbial biomass. (C) 1999 Elsevier Science Ltd. All rights reserved.