Analysis of water and nitrogen coupling effects in paddy fields with different fertility levels

Field experiments were conducted to study the effects of different irrigation models and nitrogen application methods on rice yield，agronomic characteristics, nitrogen use efficiency (NUE) and water-saving. The results showed that compared with flooding irrigation, humid irrigation increased rice yield by 5.1%-6.5%, soil residual nitrogen by 0.4%-2.3%, and agronomic N use efficiency (AE) , recovery (RE) and physiology efficiency (PE) by 12.6%~113.4%, 0.4%-19.7%, 11.3%-92.9%，respectively. The increase extents of AE, RE and PE increased with soil background N content. Rice humid irrigation saved water by 18.0%-34.3%. Compared with the traditional model of 70:30 (70% used as basal application and 30% as topdressing), the nitrogen application model of 50:50 (50% used as basal application and 50% as topdressing) increased rice yield by 0.3%-8.9%, with more increase in flooding paddy field. The nitrogen application model of 50:50 increased NUE with different extents, but the increases were not significant in middle-N and high-N fields. In addition, there existed a negative linear correlation between NUE and soil background N content (r = -0.9919), and N loss was enhanced with the increase of soil background N. The results suggested that appropriate irrigation model and nitrogen application method could enhance NUE, increase rice yield and save water effectively. But at present, in some rice production regions with higher soil background N levels, the most important problem is to decrease application rate of N fertilizer, lower N loss, and then reduce N fertilizer pollution to environment.