Rotation cropping systems have important significance to soil fertility, productivity and ecological function in paddy field. We analyzed the long-term data from a field experiment started in 1982 to investigate the effects of rotation cropping of double-rice with various green manure on the dynamics of rice yield and the total and labile soil organic matter in Red Soil Experimental Station CAAS. Four rotation cropping systems were selected by three replicates, which were rotation of double rice with Chinese milk vetch (Astragalus sinicus L.), rape (Astragalus sinicus L.), ryegrass (lolium multiflorum) and winter fallow. Results showed that, during the duration from 1982 to 2008, the averaged annual rice yield were 10.8 t · hm^-2 · a^-1, 10.2 t · hm^-2 · a^-1 and 10.0 t · hm^-2 · a^-1 for the rotation cropping systems with Chinese milk vetch (Astragalus sinicus L.), rape (Astragalus sinicus L.) and ryegrass (Lolium multiflorum) respectively. Compared with double-rice and winter-fallow cropping, rice yield was increased significantly by 27.2%, 20.5%, and 18.1% under the rotation cropping with Chinese milk vetch (Astragalus sinicus L.), rape (Astragalus sinicus L.) and ryegrass (lolium multiflorum) respectively. There was no significant difference among these rotation cropping systems with three kinds of green manure during the early period from 1982 to 1993, while the rice yield under the rotation with cultivating Chinese milk vetch was significantly higher than that under the other two kinds green manure rotations at the latter period from 1994 to 2008. Sustainability index of grain yield (SYI) was a reliable parameter on evaluate the productivity of sustainable cropping systems. Cultivating Chinese milk vetch could fix atmospheric nitrogen and improve soil fertility. The Sustainability index of grain yield (SYI) and potential of yield increased with cultivating Chinese milk vetch was the highest. Soil organic matter showed an increasing trend under the rotation cropping of double rice and green manure. The rotation of double rice with Chinese milk vetch had the highest annually increasing rate (0.31g/kg) of soil organic matter, while the rate was 0.28 g/kg and 0.26 g/kg for the rotation cropping with ryegrass and rape, respectively. The labile soil organic matter under the rotation with Chinese milk vetch was the highest of all the treatments. Total organic carbon, total nitrogen, soil microbial biomass carbon and nitrogen under the rotation with three kinds of green manure were significantly higher than that under the winter-fallow cropping systems. Compared with rape, ryegrass has strong fibrous root system which content high organic matter and crude protein. More carbon and nitrogen were cumulated in the soil under the rotation of double rice with Chinese milk vetch and ryegrass than that under the other two cropping systems. Additionally, the two rotation cropping systems of double-rice with ryegrass and Chinese milk vetch increased the microbial biomass carbon content and the ratio of microbial biomass carbon to total organic carbon (C_mic-to-C_org ) significantly compared with the cropping of double-rice and rape or fallow.
In conclusion, rotation cropping of double rice with green manure had significant effects on enhancing rice yield and promoting soil organic matter quantity and quality. Rice cropping with Chinese milk rotation vetch was recommendable for rice productivity and soil fertility.