Divergent Stabilization Characteristics of Soil Organic Carbon between Topsoil and Subsoil Under Different Land Use Types

Abstract

Carbon sequestration in soils is widely regarded as a potential strategy to mitigate global warming and improve soil quality. However, the underlying stabilization mechanisms of soil organic carbon (SOC) remain ambiguous, especially in the soil profile under different land use types. We characterized the response of SOC concentration, stock and functional fractions (particular organic carbon, POC; mineral-associated organic carbon, MAOC) to land-use changes (i.e., forestland, cropland and greenhouse) down to 60 cm soil profile and analysed SOC stabilization characteristics. We found SOC stock was considerably lost by 18% (cropland) and 10% (greenhouse) in the entire profile (0–60 cm), as a result of converting from natural forestland to arable lands. Majority of SOC was stored in the form of MAOC (63–87%), and the percentage of MAOC to SOC increased with soil depths. The SOC concentration showed positive relationships with soil total nitrogen, POC, MAOC and aggregate stability in the topsoil (0–10 cm). In the subsoil (40–60 cm), however, SOC concentration was only linked to soil total nitrogen, MAOC and soil pH. The structural equation model (SEM) results showed that differences in SOC concentration was mainly attributed to POC, followed by soil physical properties (i.e. soil specific surface area, bulk density and aggregate stability) in the topsoil (0–10 cm). However, MAOC was the dominant driver controlling SOC concentrations in the subsoil (40–60 cm). Our results highlighted that POC and MAOC mainly regulated SOC sequestration in the topsoil and subsoil, respectively. Therefore, the control factors of different soil layers need to be considered to better predict and manage soil carbon in terrestrial ecosystems.

Keywords: soil organic carbon, particular organic carbon, mineral associated organic carbon, aggregate stability, land use types