Enhancing subsoil organic carbon (SOC) is a critical strategy for improving soil sustainability and mitigating climate change. However, how straw incorporation depth regulates subsoil SOC accrual and stabilization remains poorly understood. A two-year field experiment was conducted in a temperate cropland to compare four straw return practices: no straw return, straw mulching, shallow incorporation into the 0–20 cm layer, and deep incorporation into the 20–40 cm layer. Straw incorporation exhibited clear depth-dependent effects on SOC distribution and associated soil properties. Shallow incorporation primarily improved SOC, nutrient availability, and aggregate stability in the 0–20 cm topsoil, whereas deep straw incorporation significantly increased SOC in the 20–40 cm subsoil by 32.4% compared with the control. The SOC increases in the respective straw-amended soil layers under shallow and deep incorporation were mainly associated with substantial increases in particulate organic carbon, whereas mineral-associated organic carbon showed only limited changes. Deep incorporation also enhanced dissolved organic carbon, microbial biomass carbon, and humic organic carbon fractions in the subsoil, accompanied by improved aggregate stability and increases in Fe-related fractions, suggesting a potential enhancement of Fe-associated organic carbon protection. These results indicate that subsoil SOC accrual under deep straw incorporation was initially dominated by labile and particulate organic carbon pools, while physicochemical stabilization processes were activated concurrently. Overall, deep straw incorporation increased subsoil SOC and altered the balance between organic carbon accumulation and stabilization pathways during the early stage of straw decomposition. This study highlights the potential of depth-specific residue management as a practical strategy for enhancing subsoil organic carbon accumulation in sustainable agricultural systems, although long-term validation is required to assess the persistence of these SOC gains. • Deep straw incorporation significantly increased SOC in the 20–40 cm subsoil. • Subsoil SOC accrual was mainly associated with particulate organic carbon accumulation. • Labile organic carbon fractions and microbial biomass carbon increased at depth. • Aggregate stability and Fe-related fractions indicated potential stabilization pathways. • Organic carbon accumulation and stabilization-related processes occurred concurrently.
Zhang et al. (Fri,) studied this question.