Effects of co-incorporation of green manure planting and rice straw on soil organic carbon pool and soil enzyme activity in a mono-rice cropping system

Abstract: Green manure planting and rice straw returning are two effective ways of improving soil fertility of paddy fields in South China. The study on the effects of straw management practices on soil fertility during the winter fallow season is of great significance for providing a scientific basis for the rational utilization of straw and land resources in the rice cropping system of South China. Recently, the technique for co-incorporation of green manure planting and rice straw has been developed though little is known about the soil carbon pool and soil enzyme activity under co-application of the two organic materials in a mono-rice cropping system. This study is based on a three-year field experiment conducted during the winter fallow seasons from September 2010 to September 2013 in Jianghan Plain, Central China. The aim was to examine the effects of different treatments on soil organic carbon components, Carbon Pool Management Index (CPMI), soil enzyme activities and crop yield in a mono-rice cropping system were studied. The experiments included six treatments: 1) Rice Straw Mulching (RSM), 2) Rice Straw Burning(RSB) in situ, 3) Green Manure(GM) planting without rice straw return, 4) rice straw mulching with green manure planting (RSM+GM), and 5) a control (CK1) with no rice straw returning and 6) a control (CK0) with no rice straw returning and chemical fertilizer application. Soil samples (at depths of 0-20 cm) were taken and analyzed after rice was harvested in each of the years. The results showed that as compared with CK0 and CK1, RSB decreased the soil Stable Organic Carbon (SOC) content significantly (P<0.05) only in 2013. Conversely, the other treatments, RSM, GM and RSM+GM increased soil Active Organic Carbon(AOC) content, Carbon Pool Index(CPI), Activity (A), Activity Index (AI) and CPMI significantly (P<0.05). Additionally, the treatments of RSM and RSM+GM alsoincreased soil total organic carbon (TOC) content significantly (P<0.05). Most of these indices showed the largest increase in RSM+GM treatment, followed by RSM treatment, which was significantly higher than GM treatment. Compared with CK0 and CK1, RSM, GM and RSM+GM treatments also increased soil catalase, urease and invertase activities. The RSM+GM treatment significantly (P<0.05) increased soil urease activity in 2011, 2012 and 2013, and improved soil catalase and invertase activity in 2012 and 2013, and proved to be superior to other treatments. Compared with CK1, the grain yields of rice in GM and RSM+GM treatments were significantly (P<0.05) increased by 6.88%-11.77% and 6.00%-13.40%, respectively, across the three years. However, no significant difference was observed among the grain yields by RSM, RSB and CK1 treatments. Pearson's correlation analysis indicated that rice yield was significantly (P<0.01) positively correlated with TOC, AOC, CMPI and soil catalase, urease and invertase activities. These results indicated that the co-incorporation of green manure planting and rice straw in winter fallow season could improve soil fertility and increase crop yield, but the former was more conducive to the accumulation of soil organic carbon pool, while the latter was more conducive to the improvement of crop yield. For better soil quality, higher crop yield and the efficient utilization of crop straw, the co-incorporation of green manure planting and rice straw was recommended for the mono-rice cropping system in ecological condition similar to those in the location of this study..