Profile distribution and accumulation characteristics of organic carbon in a karst hillslope based on particle-size fractionation and stable isotope analysis.

Recent studies have highlighted tight coupling between soil aggregate fractions and soil organic carbon (SOC) turnover. However, large uncertainties remain and a mechanistic understanding of geomorphic and land use change effects on carbon storage in soil is still lacking. Taking typical slope of vegetation recovery in karst area as object, the present study analyzed organic carbon content and stable carbon isotope composition (δ13C value) of soil organic matter in bulk and particle size separates of soil on profiles at different topographic positions. The results showed that SOC content decreased gradually in downhill direction. Organic carbon content of sandy soil (50-2000 μm) accounted above 50% in the upper slope positions but in the middle and lower slope soil profiles, organic carbon was mainly stored in silts (2-50 μm) and clays (< 2 μm) which belonged to stable and highly humified SOC. The composition difference of δ13C values in soil profiles reflected the input of plant residues and accumulation characteristics. Organic matter was deposited in different soil particle sizes owing to different degrees of decomposition. Hence, δ13C value can help in identifying the storage and decomposition rates of soil organic matter.