Effect of potassium rates on rice yields and potassium application efficiency in double-rice cropping system under a 5-year located experiment Studies on fertility characteristics of different productive paddy soils in double-rice copping regions of Hunan province Effects of long-term fertilization on spring season weed community in winter fallow paddy field in red soil area under double-rice cropping system Carbon efficiency of double-rice production system in Hunan Province, China. Community Structure of Methanogens from Double-rice Rhizosphere Soil as Affected by Green Manure Incorporation