Different Rotation and Double Straw Returning Significantly Increase Liable Organic Carbon Content and Yield of Double Cropping Paddy Field in Southern China

The liable organic carbon (LOC) plays an imperative role in soil fertility and rice production. The diverse crop rotations and straw returning appreciably improve the crop yield and soil health. Therefore, the present experiment was conducted from 2016 to 2017 comprising five treatments including winter fallow, winter Chinese milk vetch (CMV), winter rape, winter garlic, winter rotation-early rice-late rice to determine the impacts on liable carbon fractions and rice yield. The results showed that compared with the winter fallow treatment, except winter garlic treatment, all other winter cropping and double straw returning improved the soil organic carbon (SOC), total organic carbon (TOC) and microbial biomass carbon (MBC). However, water-soluble organic carbon (WSOC) content of winter CMV, rape, and garlic treatments decreased, but easily oxidizable carbon (EOC) and soil particulate organic carbon (POC) content of winter rotation treatment was increased. At the same time, organic carbon content in 0–15 cm soil was significantly higher than that in 15–30 cm soil layer (P < 0.05). The soil carbon pool management index (CPMI) of both earlier and late rice in the soil layer of winter treatment except 15–30 cm of winter garlic treatment was significantly improved and the CPMI was higher in 0–15 cm soil as compared to 15–30 cm. The two-year average yield of late rice was better than that of early rice and rice yield was positively correlated with TOC, WSOC and MBC, and negatively correlated with EOC and POC. In conclusion, different rotations and double straw returns improved LOC, which was conducive for improving yields of both early and late rice.