Elucidation of the source and turnover of water soluble and microbial biomass carbon in agricultural soils

Understanding the dynamics of soil C is key to managing soil organic matter to enhance soil quality and ecosystem functioning, and reduce trace gas em issions from soils. Our objective was to determine the source and turnover of C pools in some agricultural soils in eastern Canada. Soils from five fi eld experiments under continuous maize cropping for 4-37 yr were sampled, a nd the organic C content and stable C isotope (C-13) composition of whole s oil and water soluble and microbial biomass fractions determined. The C-13 results showed a clear distinction between the water soluble organic C and microbial biomass C, with the water soluble organic C more like the whole s oil and the microbial biomass more like the maize residues. A simple linear model was used to explore the relationship among the soil organic constitu ents and evaluate the turnover of these carbon pools. Even though the water soluble organic C had a higher turnover rate than the microbial biomass C, the proportion of C-4-derived C in the biomass was about 2.5 times greater than that in water soluble organic C, Apparently the large amount of nativ e soil C, the small amount of water soluble organic C, and its equilibrium with the native soil C, cause humus to dominate the isotopic composition of water soluble organic C even though the water soluble C is very active. Ou r results suggest that the quantity, as well as the turnover rate, of soil organic matter constituents that are in equilibrium influence the isotopic composition of such constituents. Crown Copyright (C) 2000 Published by Els evier Science Ltd. All rights reserved.