CARBON TURNOVER IN A RANGE OF ALLOPHANIC SOILS AMENDED WITH C-14-LABELED GLUCOSE

The influence of soil allophane (a short-range-order mineral) content on organic-C turnover was determined with C-14-labelled glucose. Samples from four soils, providing a range of allophane, organic C, clay contents, and some other characteristics, were incubated with C-14-labelled glucose for 28 days. During incubation, microbial biomass C-12 and C-14 were determined using the fumigation-extraction technique. The amounts of (CO2)-C-12 and (CO2)-C-14 evolved during incubation were also monitored, and residual C-14 concentrations determined. Biomass C-14 was highest in the soil with the highest allophane content (13%) and least in the soil with least allophane content (<2%). In contrast, (CO2)-C-14 production from the [C-14]glucose was highest (63%) in the soil with least allophane content and lowest (54%) in the soil with the most allophane. It was concluded, from first-order decay rate constants for residual C-14 and exponential decay rate constants for biomass C-14, that allophane retards the turnover rates of C-14 derived from added glucose by stabilization of microbial biomass, and also by protection of microbial products. During a 28 day incubation, ca 0.8% more C was diverted from respired CO2 to new biomass with each 1% increase in allophane content. For allophanic soils, inclusion of mineral surface area rather than clay content should provide a better quantification of the organic matter turnover rate.