Labile soil carbon pools in subtropical forest and agricultural ecosystems as influenced by management practices and vegetation types

Carbon storage in agricultural and forest soils has attracted attention recently due to its potential as a substantial carbon sink. Labile soil C pools are especially important because they are more vulnerable to climatic change and disturbance and play vital roles in nutrient cycling. Southern Appalachian forest soils and those from conventional tillage (CT), no-tillage (NT) and fescue sods at three sites in the Georgia piedmont were analyzed for total C, total N, carbohydrates, and microbial biomass C. The sizes of soil labile C pools (carbohydrates and microbial biomass) and their contributions to the total soil C pool differed significantly among ecosystems. The highest carbohydrate contents and microbial biomass C were found in forest soils, but agricultural soils had a significantly higher proportion of the soil organic matter present as carbohydrates and as microbial biomass. This difference probably reflects the quality of soil organic matter. Soil microbial biomass C was more sensitive to changes in management regimes than soil carbohydrates. Management practices signfiicantly affected organic C, carbohydrate contents, microbial biomass C and organic C turnover rates in agricultural soils, whereas differences in the quality of organic input due to different vegetation types substantially influenced soil labile C pools in forest soils. High mannose-to-xylose ratios in highly sandy agricultural soils indicate that plant-derived materials are rapidly metabolized by microorganisms and that organic C protection in sandy soils is largely dependent on reducing microbial access through effective residue management such as surface placement.