Land Use Effects on the Distribution of Labile Organic Carbon Fractions through Soil Profiles

Labile fractions of organic matter can respond rapidly to changes in C supply and are considered to be important indicators of soil quality. However, much less is known on the impact of different land use systems and depth on labile organic matter fractions. The objective of this study was to estimate land use effects on a distribution of labile fraction organic C through soil profiles in the Sanjiang Plain of northeast China. Four land‐use types were selected: Deyeuxia angustifolia wetland, upland forest, two farmlands (cultivated 9 and 15 yr, respectively) of soils previously under Deyeuxia angustifolia wetland, and abandoned cultivated soil. Soil total organic C (TOC), dissolved organic C, microbial organic C, and hot water‐extractable C were measured. The results showed that the intact Deyeuxia angustifolia wetland soil had significantly higher labile fraction organic C contents in the topsoil when compared with upland forest, abandoned cultivated, and cultivated soils. However, there were no significant subsoil differences at all sites. The effects of land use on labile fraction organic C occurred mainly in the topsoil (0–20 cm). The labile fraction organic C contents decreased significantly with increasing soil depth in the intact Deyeuxia angustifolia wetland. However, the upland forest, abandoned cultivated, and cultivated soils showed a considerably smaller decrease in labile fraction organic C contents with increasing soil depth. The proportion of dissolved organic C, hot water‐extractable C, and microbial biomass C to TOC increased to a maximum at a depth of about 20 to 30 cm, and then decreased with increasing soil depth in the Deyeuxia angustifolia wetland but not the other land use types.