Methanotrophs play a vital role in the mitigation of methane emission from soils. However, the influences of cover crops incorporation on paddy soil methanotrophic community structure have not been fully understood. In this study, the impacts of two winter cover crops (Chinese milk vetch (Astragalus sinicus L.) and ryegrass (Lolium multiflorum Lam.), representing leguminous and non-leguminous cover crops, respectively) on community structure and abundance of methanotrophs were evaluated by using PCR-DGGE (polymerase chain reaction-denaturing gradient gel electrophoresis) and real-time PCR technology in a double-rice cropping system from South China. Four treatments were established in a completely randomized block design: 1) double-rice cropping without nitrogen fertilizer application, CK; 2) double-rice cropping with chemical nitrogen fertilizer application (200 kg ha–1 urea for entire double-rice season), CF; 3) Chinese milk vetch cropping followed by double-rice cultivation with Chinese milk vetch incorporation, MV; 4) ryegrass cropping followed by double-rice cultivation with ryegrass incorporation, RG. Results showed that cultivating Chinese milk vetch and ryegrass in fallow season decreased soil bulk density and increased rice yield in different extents by comparison with CK. Additionally, methanotrophic bacterial abundance and community structure changed significantly with rice growth. Methanotrophic bacterial pmoA gene copies in four treatments were higher during late-rice season (3.18×107 to 10.28×107 copies g–1 dry soil) by comparison with early-rice season (2.1×107 to 9.62×107 copies g–1 dry soil). Type I methanotrophs absolutely predominated during early-rice season. However, the advantage of type I methanotrophs kept narrowing during entire double-rice season and both types I and II methanotrophs dominated at later stage of late-rice.

      Annual ryegrass (Lolium multiflorum Lam.), a non-leguminous winter cover crop, has been adopted to absorb soil native N to minimize N loss from an intensive double rice cropping system in southern China, but a little is known about its effects on rice grain yield and rice N use efficiency. In this study, effects of ryegrass on double rice yield, N uptake and use efficiency were measured under different fertilizer N rates. A 3-year (2009–2011) field experiment arranged in a split-plot design was undertaken. Main plots were ryegrass (RG) as a winter cover crop and winter fallow (WF) without weed. Subplots were three N treatments for each rice season: 0 (N₀), 100 (N₁₀₀) and 200 kg N ha^–1 (N₂₀₀). In the 3-year experiment, RG reduced grain yield and plant N uptake for early rice (0.4–1.7 t ha^–1 for grain yield and 4.6–20.3 kg ha^–1 for N uptake) and double rice (0.6–2.0 t ha^–1 for grain yield and 6.3–27.0 kg ha^–1 for N uptake) when compared with WF among different N rates. Yield and N uptake decrease due to RG was smaller in N₁₀₀ and N₂₀₀ plots than in N₀ plots. The reduction in early rice grain yield in RG plots was associated with decrease number of panicles. Agronomic N use efficiency and fertilizer N recovery efficiency were higher in RG plots than winter fallow for early rice and double rice among different N rates and experimental years. RG tended to have little effect on grain yield, N uptake, agronomic N use efficiency, and fertilizer N recovery efficiency in the late rice season. These results suggest that ryegrass may reduce grain yield while it improves rice N use efficiency in a double rice cropping system.