The effects of straw incorporation and water management on nitrogen uptake and nitrogen use efficiency in a double rice cropping system

Straw incorporation is an important measure to promote nutrients cycling in farmlands and can also improve farmland productivity. Based on the two main water management regimes in paddy production, that is intermittent irrigation and continuous flooding, the present study investigated the effects of different water management regimes on nitrogen (N) uptake and N use efficiency (NUE) in a double rice cropping system with straw incorporation in the fifth year of a long-term field experiment in Changsha county, Hunan province, China. The study had six treatments: no N fertilizer application and no straw incorporation with intermittent irrigation (CK), N fertilizer application and no straw incorporation with intermittent irrigation, N fertilizer application and low rate straw incorporation with intermittent irrigation, N fertilizer application and high rate straw incorporation with intermittent irrigation, N fertilizer application and no straw incorporation with continuous flooding, and N fertilizer application and no straw incorporation with continuous flooding. Dynamics of soil inorganic N contents, soil microbial biomass N (MBN) contents, plant N uptake and rice grain yields and N accumulation in different parts of plants in the harvest stage were measured in both early and late rice seasons in the fifth year of the field experiment. Major soil fertility properties were also measured after the late rice harvest. The results showed that straw incorporation increased soil organic carbon (SOC) and total N (TSN) as well as soil pH as compared with no straw incorporation, while continuous flooding reduced SOC, TSN and soil total phosphorus as compared intermittent irrigation. Under the same N fertilizer application rate, straw incorporation in the early rice season decreased soil inorganic N content and soil MBN content in the tillering stage, and also significantly decreased rice N uptake, grain yield as well as NUE; the NUE in the straw treatments decreased by 2.0% to 7.6% as compared with CK, and decreased with the increase of straw incorporation rate. Straw incorporation in the late rice season increased soil inorganic N content and soil MBN content in the whole crop season, and significantly increased rice N uptake, grain yield as well as NUE; the NUE in the straw treatments increased by 8.6% to 13% as compared with CK, and increased with the increase of straw incorporation rate. The two water management regimes showed no significant influences on rice N uptake, grain yield as well as NUE. Straw incorporation with continuous flooding in the early rice season caused the decrease of rice grain yield and NUE as compared with straw incorporation with intermittent irrigation. Our results indicate that in the double rice cropping system, relatively high yield and high NUE can be achieved with no straw incorporation in the early rice season and straw incorporation at a rate of 6 t/hm2 in the late rice season under the water management regime of intermittent irrigation.