Depth-dependent straw incorporation governs soil organic carbon storage through microbial resource limitation in Mollisol agroecosystems of Northeast China

Sustainable management of cropland organic carbon is central to climate-resilient food systems, yet the depth at which crop residues are returned remains a critical, under-resolved lever in agroecosystem carbon management. Crop straw return is widely promoted to enhance soil organic carbon (SOC) in arable land, but the depth of incorporation governs how microbial communities mediate the partitioning of SOC into particulate (POC) and mineral-associated (MAOC) fractions across the plow layer. In this study, we used a 12-year field experiment in the Mollisol agroecosystem of Northeast China to test how four management strategies including conventional tillage without straw return (CT), shallow straw return (0–20 cm; SCT), deep straw incorporation across the full plow layer (0–35 cm; SIT), and subsoil-only straw return (20–35 cm; SST), reshape SOC fractions through microbial community structure and extracellular enzymatic resource limitation. Relative to CT, SCT and SIT increased topsoil MAOC and POC contents by 5.90–11.34% and 11.87–24.17%, whereas SIT and SST elevated subsoil POC by 30.54–50.87%. The fungi/bacteria (F/B) ratio and total PLFAs rose by 57.65–115.38% in topsoil under SCT and SIT, and by 37.99–190.00% in subsoil under SIT and SST. Enzymatic vector analysis revealed that straw return intensified microbial carbon limitation by 2.82–11.29% while alleviating phosphorus limitation by 1.07–6.41%. Partial least squares path modelling showed that reductions in soil pH and bulk density propagated through shifts in microbial community structure and resource limitation, ultimately driving POC/SOC enrichment and total SOC accumulation. Full-profile incorporation (SIT) homogenized SOC, microbial activity and resource demand across the plow layer, providing the most effective and durable strategy for combining carbon sequestration with crop productivity in temperate maize-dominated agroecosystems. These findings translate the biogeochemical mechanism of depth-dependent straw return into an actionable management principle for cropland carbon stewardship.