Effects of nitrogen application on yield, nitrogen use efficiency and economic benefit of double-cropping rice by mechanical transplanting

Objectives Machine-transplanted seedling has become one of the important ways for decreasing labor cost in rice cultivation. Studying the optimized N application is crucial for achieving high N use efficiency and stable and high yield of double-cropping rice.

Methods Field experiments were conducted on a double-cropping rice using mechanical transplanting. Under the same nitrogen application rate and with no- nitrogen fertilizer as control (CK), six ratios of N fertilizer amount in basal: tillering: earring were designed as: 8:2:0 (N_8:2:0), 7:2:1 (N_7:2:1), 6:2:2 (N_6:2:2), 5:2:3 (N_5:2:3), 4:2:4 (N_4:2:4), 3:2:5 (N_3:2:5). The rice yield, and N absorption and use efficiency were investigated, the relationships among them were discussed.

Results With the N ratio of basal: tillering: panicle of 6:2:2 for early rice, and 5:2:3 for late rice, the rice had more reasonable population structure, higher effective panicle number, more grain number per panicle, higher seed-setting rate and 1000-grain weight, and the optimal coordination of each factor. The N ratio in panicle fertilizer had a significantly negative correlation with the effective panicle number, but a positive correlation with the grain number per panicle, and had a parabolic relation with the seed-setting rate. For early and late rice, the N contents of both grain and straw and N accumulation amount in all N fertilizer treatments increased compared with those of CK treatment, where in the N contents of both grain and straw took on a rising trend with the increase of panicle fertilizer proportion, while the N accumulation amount first went up and then dropped down. Whether machine-transplanted early rice (N 180 kg/hm²) or late rice (N 195 kg/hm²), along with the increase of panicle fertilizer proportion, nitrogen contribution rate (NCR), nitrogen agronomic efficiency (NAE), nitrogen recovery efficiency (NRE), nitrogen partial factor productivity (PFP_N) and economic benefit all increased first and then decreased, each reaching up to a maximum for early rice in the N_6∶2∶2 treatment and for late rice in the N_5∶2∶3 treatment, respectively. However, the physiological nitrogen efficiency (NPE) went down for early and late rice with the increase of panicle fertilizer proportion.

Conclusions Under the same amounts of total N application and allocated proportion for tillering fertilizer, the N ratio of basal to tillering to panicle fertilizer N of 6:2:2 for early rice and 5:2:3 for late rice contributed to high yield of rice and high N utilization efficiency, and could be used as an ideal mode of nitrogen fertilizer application.