| In the winter wheat–summer maize rotation system on the Guanzhong Plain, long-term shallow straw returning often leads to surface carbon enrichment and subsurface compaction. Based on a long-term field experiment established in 2008 and sampled in 2024, six treatments were compared: no straw returning (CK), wheat high stubble retention (WH), wheat straw chopping returning (WS), maize straw deep incorporation every other year (MD), wheat and maize double-season chopping returning (WSMS), and wheat high stubble retention combined with maize straw deep incorporation every other year (WHMD). Crop yields over 16 years were analyzed, and soil organic carbon (SOC) and its fractions—particulate organic carbon (POC) and mineral-associated organic carbon (MAOC)—were measured in the 0~20 cm and 20~40 cm layers in 2024. The objective was to clarify the effects of different straw returning practices, especially deep incorporation, on crop yield improvement and soil carbon sequestration. The results showed that: (1) All straw returning treatments increased crop yields. The highest wheat yield was obtained in WSMS, which was 10.8% higher than CK; the highest maize yield was found in WHMD, which was 27.9% higher than CK. (2) Significant differences in SOC profile distribution and fractions were observed among treatments. WSMS mainly promoted SOC and POC accumulation in the 0~20 cm surface layer, increasing SOC stock and POC content by 42.8% and 44.2%, respectively, compared with CK. The deep incorporation treatments (MD and WHMD) markedly enhanced carbon sequestration in the 20~40 cm subsurface layer. WHMD increased subsurface net SOC sequestration by 135.2% relative to CK, while MD increased subsurface POC and MAOC contents by 213.6% and 34.9%, respectively. (3) Deep incorporation measures significantly altered both the physical properties and nutrient status of the subsoil. MD reduced subsurface bulk density by 14.0% and raised total porosity to 48.8%. WHMD increased dissolved organic nitrogen (DON), alkali-hydrolyzable nitrogen (AN), available phosphorus (AP), and microbial biomass carbon (MBC) concentrations by 12.0%, 34.8%, 53.2%, and 74.8%, respectively, compared with CK. In conclusion, long-term straw returning can synergistically enhance crop yield and soil carbon sequestration. Among the treatments, WHMD exhibited superior overall performance in strengthening subsurface carbon sequestration, ameliorating soil conditions, and maintaining relatively high yields. It can therefore be recommended as a promising practice that reconciles high crop productivity with subsurface carbon sequestration in the winter wheat–summer maize rotation system of the Guanzhong Plain. |