Responses of functional genes involved in nitrogen cycling to green manuring in different paddy soils in south China

Aims We studied the effects of green manuring on the gene level of microorganisms associated with nitrogen (N) fixation, nitrification, and denitrification in different paddy soils. Methods Field experiments were conducted in Hunan, Jiangxi, Henan, and Fujian experimental sites in south China to investigate the responses of rice yield, soil properties, and functional genes to green manuring. The four treatments were no fertilizer application (NF), using milk vetch as green manure without chemical fertilizer (GM), winter fallow with chemical fertilizer (CF), and a combination of green manure and chemical fertilizer (GMCF). Results The rice yield and N uptake significantly increased in GMCF. The nifH gene copies significantly increased in GMCF in the Jiangxi and Henan sites. Soil SOM, TN, NH4+-N, NO3--N, AP, and AK were higher in GMCF than in the other treatments. For the nitrification process, the gene copies of ammonia-oxidizing archaea were higher than those of ammonia-oxidizing bacteria in all sites. The nirK and nosZ genes involved in denitrification were negatively correlated with soil properties. With the improvement of soil properties and rice N uptake, the denitrification process was inhibited in the GMCF treatment. The nirK gene was the most critical functional gene influencing rice yield among the tested genes. SOM was the most important factor among soil properties. Conclusions The application of green manure improved soil properties and changed the abundance of the functional genes involved in N cycling. Green manure combined with chemical fertilizer increased rice yields and N uptake and inhibited the denitrification process.