14C-labelled glucose turnover in New Zealand soils

The influence of soil mineralogy, as well as texture, on organic-C turnover was determined with 14C-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 14C-labelled glucose for 35 d. The amounts of 12CO2 and 14CO2 evolved during incubation were monitored and the residual 14C concentrations determined. Periodically, the samples were removed and microbial biomass 12C and 14C determined using the fumigation-extraction technique. System mean residence times (MRTs) were obtained by three independent methods: (i) a compartmental model using 14C microbial biomass data, (ii) a non-compartmental model using 14C microbial biomass data and (iii) a biexponential equation as an empirical equation from residual 14C data. The effect of soil characteristics on MRTs was compared. The 14CO2 respired, after 35 d incubation, accounted for 51 to 66% of the glucose 14C input to these soils. The soils differed significantly in their amounts of 14CO2 evolution and in the proportions of labelled 14C in the biomass. The extent of mineralization of 14C-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 14C); 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 14C and microbial biomass 14C during the 35 d glucose decomposition for most of the soils. MRTs for microbial biomass 14C 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 14C are similar despite differences in the chemical nature of the applied 14C-labelled substrate. However, the MRAT for humus 14C 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.