Soil organic carbon and its labile fractions in paddy soil as influenced by water regimes and straw management

Understanding the effects of water regimes and straw management on soil organic carbon (SOC) pools is necessary to improve soil C sequestration and soil quality. Based on a long-term field experiment (15 years), we examined impacts of water regimes and straw management methods on total soil organic C (TOC), particulate organic C (POC) and potassium permanganate-oxidizable C (KMnO4-C) at two soil depths. Water regimes consisted of flooding cultivation and non-flooded cultivation. In non-flooded cultivation, limited irrigation was employed to keep the soil moist condition without standing water covering the field during the rice-growing season. It is substantially different from conventional flooded rice cultivation. The two straw management methods were straw mulching and straw removal. The results showed that water management had greater effects on TOC and KMnO4-C than did straw management at 0-10 cm soil layer. Compared to the non-flooded treatment, flooding cultivation resulted in 17.4% higher TOC and 31.4% higher KMnO4-C contents. Correspondingly, straw management showed dominant effects on POC. Straw mulching significantly increased POC content by 13.0% as compared to the straw removal treatment. However, at deeper soil depth (10-20 cm), labile SOC fractions were only affected by water regimes. The higher carbon management index (CMI) values at 0-20 cm were recorded in the flooding condition with straw mulching treatment. This was attributed to the increased carbon pool index (CPI), which contributed to the formation of more stable organic compounds that collectively act as a soil reservoir. The combined non-flooded cultivation and straw mulching method (NFC-SM) produced a similar 15-year average rice grain yield and TOC content in 2016 compared to the traditional flooding cultivation with straw removal treatment (FC-SR). Our data indicated that NFC-SM could be an ideal strategy not only to save water but also to maintain soil fertility.