Effects of straw returning depth on soil organic carbon sequestration and crop yield in China: A meta-analysis

Carbon sequestration is a crucial strategy for mitigating carbon dioxide emissions and addressing global climate change, with straw returning playing a key role in enhancing soil organic carbon (SOC) storage. However, most studies have focused on surface-level straw returning and its impact on topsoil SOC, with limited attention to how different straw returning depths (RD) on SOC stocks (SOCS) in topsoil (0−30 cm) and subsoil (30−60 cm). This study conducted a meta-analysis of 2290 observations from China to evaluate the effects of varying straw returning depths [0 cm (RD₀), 0−20 cm (RD₀₋₂₀), 20−30 cm (RD₂₀₋₃₀), and 30−60 cm (RD₃₀₋₆₀)] on SOCS in two soil layers and their relationship with crop yield. All straw returning depths significantly improved topsoil and subsoil SOCS and enhanced soil physicochemical properties. RD₂₀₋₃₀ showed the strongest effect on topsoil SOCS (14.3 %), whereas RD₃₀₋₆₀ had the weakest (5.6 %). Conversely, RD₃₀₋₆₀ had the strongest effect on subsoil SOCS (30.3 %), followed by RD₂₀₋₃₀ (15.8 %). Crop yield increased under all straw returning depths, with the greatest gains under RD₀₋₂₀ (11.2 %) and RD₂₀₋₃₀ (10.0 %). However, the effectiveness of each depth varied with environmental and management conditions. For example, RD₀ increased SOCS the most under straw inputs below 8000 kg·ha^–1 and loam soils, while RD_0–20 was most effective in single cropping systems, upland regions, clay loam soils, low temperature and precipitation regions, and maize straw returning. RD_20–30 proved beneficial in regions with double cropping systems, paddy and paddy-upland soils, high temperatures, rainy regions, and wheat or rice straw returning. These findings offer evidence-based insights to support sustainable straw returning strategies in China's agricultural systems.