Enhancing soil organic carbon sequestration through slope-to-terrace conversion and mixed planting: Insights from carbon pool composition and biomarker dynamics

Soil organic carbon (SOC) plays a critical role in climate regulation and soil fertility. Extensively distributed agricultural lands hold significant potential for ecological carbon sequestration, the process by which natural and managed ecosystems capture and store atmospheric carbon over time, making the improvement of SOC storage and stability essential for sustainable farmland management. In this study, we examined the effects of slope-to-terrace conversion and mixed planting on SOC pool composition and stability in hilly farmlands of southwestern China. Soil samples were collected from sloped monoculture, terraced monoculture, and mixed-planting terraced farmlands to assess SOC content and its critical components. Microbial necromass and plant lignin inputs were quantified using amino sugar and lignin phenol biomarkers. Results showed that slope-to-terrace conversion significantly increased SOC and mineral-associated organic carbon (MAOC) contents by 105–204 % and 116–338 %, respectively, indicating improved soil carbon retention via terracing. Mixed planting further enhanced labile carbon fractions (e.g. easily oxidizable organic carbon, particulate organic carbon), microbial biomass carbon (MBC), and microbial necromass contents by over 10 %, particularly bacterial-derived carbon in maize systems and fungal-derived carbon in citrus systems, emphasizing the role of diversified plant litter inputs in supporting microbial carbon turnover and cycling. However, the relative contributions of microbial residues and plant lignin to total SOC remained unchanged, suggesting no substantial improvement in long-term SOC stability. These findings demonstrate that while slope-to-terrace conversion and mixed planting effectively expand the SOC pool size and microbial inputs, they do not significantly enhance long-term SOC stabilization.