THE TURNOVER OF ORGANIC CARBON IN SOIL (BIOMASS, ORGANIC CARBON)

Available from UMI in association with The British Library.; This thesis investigates the turnover of soil organic matter, the process which governs the accumulation and degredation of organic matter in soil. The soil microbial biomass is studied as the agent of transformation of both fresh organic inputs to soil and of native soil organic matter itself.; Microbial biomass C and organic C were measured in 21 soils selected from 5 classical and long-term field experiments at Rothamsted and Woburn. The results were used to discuss the effects of agricultural practices on the formation of organic C in these soils and the relationships between biomass C and total organic C. This suggests that changes in soil biomass C provide an early indication of changes in total soil organic C following changes in soil management.; The dynamics of decomposition and transformation of different substrates (glucose, ryegrass and straw) and the effects of the substrate incorporation on the turnover of soil biomass C and the decomposition of soil organic C were studied in 7 soils with different characteristics. From this the mechanisms of priming effects were established.; A procedure to determine the turnover time of soil biomass C was proposed. Measurements suggested that the turnover time of biomass C was closely related to the decomposition of organic C in soils and was greatly affected by soil clay content.; The relationships between soil respiration, soil biomass C and soil organic C were studied in the laboratory. Soils containing more organic C and receiving larger fresh organic C inputs also have faster rates of soil organic C mineralization, suggesting that the turnover of organic C in such soils is probably faster than in soils containing less organic C and receiving less fresh organic C inputs.; This study suggests that soil clay content is the main factor increasing the stabilization of organic C inputs as soil organic matter. Soils with high clay content also show decreased biological activity of the soil microbial biomass, and decreased turnover of soil organic C. Physical disturbance of soil and air-drying-rewetting treatments increased the mineralization of soil organic C.; The validity of the Rothamsted carbon model to predict amounts of both soil organic C and biomass C under different agricultural systems at Rothamsted was examined. The agreements between the predicted and measured quantities of both soil organic C and biomass C were good in most soils studied. (Abstract shortened by UMI.)...