Objectives Incorporation of rice straw and green manure to the field are important ways to improve paddy soil fertility and reduce chemical fertilizer input. Thus, assessment of the seasonal apparent balance of soil N and P in double-rice cropping system is required for farmland nutrient management.

Methods An experiment was conducted in acidic red yellow soil and alkaline purple alluvial soil of Hunan Province for 2 years. The treatments included winter fallow and no rice straw return (FRR), Chinese milk vetch incorporation into soil (MvRR), and both Chinese milk vetch and rice straw incorporation into soil (MvRR+St). Chinese milk vetch was planted during winter season and ploughed into soil before transplanting of early rice. Both early and late rice straw were incorporated to the field after rice harvest. The biomass, N and P contents of rice plant (grain, aboveground part and root), and soil physiochemical properties were analyzed.

Results  1) The biomass yield, N and P nutrient contents, N and P absorption of Chinese milk vetch, early rice and late rice in the two paddy soils were similar across all treatments ( P ＞0.05). MvRR+St significantly increased the available P content by 29.7% and 20.9% in acidic red yellow soil and alkaline purple alluvial soil, respectively, compared with MvRR (P＜0.05). 2). Under the current N and P input levels, the soil N and P balance under MvRR+St were surplus in the rotation system and early rice season, but N was deficient and P was basically balanced in late rice season. In early rice season, the N and P surplus in acidic red yellow soil were 88.06 kg/hm² and 21.26 kg/hm², respectively; however, the N and P surplus in alkaline purple alluvial soilwere 134.04 kg/hm² and 27.95 kg/hm², respectively. In late rice season, the N deficit in acidic red yellow soil and alkaline purple alluvial soil were 29.70 kg/hm²and 30.02 kg/hm², respectively, and the P surplus were 2.39 kg/hm² and 6.75 kg/hm², respectively.

Conclusions Under current chemical fertilizer rate and incorporation of Chinese milk vetch and rice straws, the annual soil N and P surplus was mainly due to the surplus in early rice season, not in late rice season, in which the soil N was in deficit and P was in basic balance. So, the N and P rate in early rice season should be reduced while the N fertilizer rate in late rice season should be increased.