Effects of milk vetch (Astragalus sinicus) as winter green manure on rice yield and rate of fertilizer application in rice paddies in south China

In this paper, we collected data from previous investigations on milk vetch (Astragalus sinicus) as winter green manure, and co-incorporation of milk vetch and rice straw in rice paddy fields in south China to study the effects of milk vetch green manure on rice yield and chemical fertilizer application, and the mechanisms of plant nutrition and microbial ecology. Thus, a total of 930 datasets obtained from 11 joint experiments conducted from 2008 to 2019 were analyzed to address this objective. The results showed that incorporation of milk vetch together with application of 100% and 80% of conventional amounts of chemical fertilizer significantly increased rice yield by 6.53% and 4.15%, respectively. The incorporation of milk vetch into paddy fields also prevented decrease in the rice yield at 40% reduction in conventional chemical fertilizer application. The positive effects of milk vetch incorporation on rice yield and chemical fertilizer application increased with the planting years of milk vetch. For instance, a 7-year results of 5 joint experiments showed that milk vetch plants combined with 40% reduction of conventional chemical fertilizer increased rice yield by 0.87% in the first year, compared with conventional fertilization, and the increase rate reached 3.98% in the 7th year. Co-incorporation of milk vetch and rice straw was also an important management practice in rice-planting regions, and widely promoted in south China. A total of 342 datasets obtained from 7 joint experiments conducted from 2016 to 2019 showed that co-incorporation of milk vetch and rice straw increased rice yield by 11.71%, compared with rice straw incorporation alone. We also analyzed the data of rice yield composition, rice nutrient absorption, soil fertility and microbial ecology to understand the mechanisms underlying the increase in rice yield and reduction in chemical fertilizer due to milk vetch green manure. Winter planting of milk vetch increased the effective panicle number and number of grains per panicle, thus optimized rice yield composition. Compared with conventional fertilization, milk vetch combined with reduced chemical fertilizer increased rice N absorption by 6.4%–6.9%, and increased N use efficiency by 6.6%–31.1%. Planting of milk vetch increased soil carbon (C) and nitrogen (N) pool, promoted soil labile organic C contents and enzymatic activities in C transformation, increased soil available nutrients contents and improved soil structure. For example, incorporation of milk vetch combined with 20% and 40% of conventional chemical fertilizer reduction increased soil organic matter by 3.95% and 4.15%, respectively, and soil total N by 1.22% and 1.74%, respectively. The results also showed milk vetch incorporation regulated soil microbes and N transformation. Incorporation of milk vetch influenced soil nutrient cycling by promoting the growth of soil microorganisms and enzymatic activities associated with soil microorganisms, and changing the community structures and functional microbes. Milk vetch combined with reduced amount of chemical fertilizer increased the abundance of azotobacter, and the N fixation process of milk vetch could be optimized through reasonable regulation methods. Responses of nitrification to winter green manuring varied a lot in different soil types. For example, in alkaline paddy soil, winter green manuring inhibited nitrification potential thus reduced the risk of nitrate leaching, and the community changes in ammonia oxidizers was the important mechanisms. Overall, the incorporation of milk vetch as winter green manure has showed reliable effects in increasing rice yield and reducing chemical fertilizer application. The mechanism study also serves as an important reference for further studies on impacts of green manures in rice fields.