The different pathways of microbial regulation of organic carbon turnover in the topsoil and subsoil of coastal saline soil after long-term stubble return and subsoiling

Abstract

Long-term stubble return and subsoiling increased the soil organic carbon (SOC) in coastal saline soil; however, the changes in SOC components and their regulatory role of microorganisms in this process remain unclear. Therefore, a 9-year experiment assessed stubble management (removal or return) and tillage (non-subsoiling or subsoiling) effects on SOC components and microbial communities to clarify microbial regulation of SOC turnover. Results showed that stubble return and subsoiling improved the physicochemical properties of the topsoil (0–20 cm) and subsoil (20–40 cm), and increased SOC and its components, including dissolved organic carbon (DOC) by 160 %, easily oxidizable carbon (EOC) by 59 %, microbial biomass carbon (MBC) by 171 %, particulate organic carbon (POC) by 103 %, and mineral-associated organic carbon (MOC) by 22 %. In the subsoil, stubble management significantly increased DOC, EOC, MBC, POC, and MOC by 21 %, 62 %, 27 %, 72 %, and 37 %, respectively. Through high-throughput sequencing and Biolog-ECO microplate, we found significant changes in the structure and metabolic function of microbial communities after stubble return and subsoiling. Notably, stubble return and subsoiling altered the microbial regulatory pathways in SOC turnover in different soil layers. In the topsoil, microorganisms primarily utilize EOC to convert it into POC, while the direct adsorption of DOC by minerals serves as the main pathway for MOC synthesis. In the subsoil, DOC serves as the primary carbon source for microorganisms, leading to the conversion to MBC and the promotion of MOC formation. Our results highlight that stubble return and subsoiling can optimize the microbial community structure and metabolic function and increase SOC sequestration with different regulation pathways in SOC component turnover, and the measures can be effective measures to improve the status of coastal saline soil.