Higher soil organic carbon accumulation in the subsoil layer by 37 years combined application of inorganic fertilizers with manure than with rice straw in a double-rice paddy soil

Organic incorporation is widely recommended to increase Soil Organic Carbon (SOC) content, but little was done to compare the effects of using manure and straw in different soil layers. The study was set up to assess the changes of SOC and oxidizable organic carbon fractions in three soil depths (0–15, 15–30 and 30–45 cm) in a double-rice paddy field subjected to 37 years fertilization schemes [control (CK), combined inorganic fertilization (NPK), inorganic fertilizer NK with pig manure (NKM), and inorganic fertilizer NP or NPK incorporated with rice straw (NPS and NPKS)]. The 37 years’ application of NPK, NKM, NPS and NPKS increased SOC content up to 16.5%, 38.7%, 24.4% and 24.2% across the three soil depths, respectively, compared with CK. Both NPS and NPKS treatment had 77% higher SOC accumulation by sequestering more labile SOC fraction (CHL) in topsoil but not the subsoil layers compared with NPK. Long-term NKM application performed best in SOC accumulation (threefold higher compared with NPK) in all studied soil depths, and in the content of CHL in topsoil. Overall, the integration of inorganic fertilizer with organic manure is more effective than straw incorporation on improving SOC accumulation and stability in not only the topsoil but also the subsoil layers.