Effects of nitrogen and density interactions on grain yield and nitrogen use efficiency of double-rice systems

【Objective】 Over-fertilization by farmers driven by desire for higher yields does not always contribute to increase yield, however, the over-fertilization causes fertilizer waste and negative effects on environment, meanwhile leads to low nutrient recovery efficiency and agronomic efficiency. In order to determine effects of nitrogen(N) application and transplanting density(D) on double-rice systems, a field experiment was conducted to study yield, yield components and nitrogen use efficiency of double-rice in Jiangxi province. 【Methods】 A split-plot experimental design was used with different nitrogen application amounts in the main plots and transplanting density in the sub-plots to study grain yield and N use efficiency under four N rates(N 0, 135, 180 and 225 kg/hm2, and express as N0, N135, N180 and N225) and four transplanting densities(21104, 27104, 33104 and 39104 hole/hm2, and express as D21, D27, D33 and D39) for double-rice systems, the grain yield and its components were measured, and N uptake, N use efficiency and N harvest index were analyzed at rice maturity. 【Results】 The results show that there are significant effects of the N levels and planting density on the rice yield separately. The high transplanting density contributes to increase effective panicle number per unit area, grain yield and total N uptake of shoot. The high N rate increases total N accumulation of rice, but decreases the N recovery efficiency, N partial factor productivity, N internal efficiency, N physiological efficiency and N harvest index. The agronomic efficiency of N is increased with increasing of N application rate at first and decreased after N application rate exceeding 180 kg/hm2. Compared to other treatments, there are the highest yields under the 180 kg/hm2 and 39104 hole/hm2 treatment combination, the yields are 9823.0 and 11354.7 kg/hm2 for early and late rice, and the N recovery efficiencies are 42.4% and 47.5% for early rice and late rice, respectively. The grain yield is not increased when N application rate exceeding 180 kg/hm2, while the yield is increased with increasing transplanting density in our study.【Conclusions】 The rational combination of nitrogen application and transplanting density can indeed significantly increase the effective panicle number per unit area and total N uptake of rice shoot, which further increases the rice yield and nitrogen use efficiency. The N180D39 treatment combination is recommended for double-rice systems in Jiangxi province.