A new method for determining the economic optimum nitrogen application rate: Fertilizer function based on nitrogen rate and milled rice yield

Objectives We assessed the synergistic combination of density and nitrogen application rate that promotes superior yield, high-quality and high efficiency in rice.

Methods A field experiment was conducted in Huihuang Village, Longfengshan Township, Wuchang City, Heilongjiang Province from 2018 to 2019, and the tested rice cultivar was Wu you 4. The transplanting method was artificial transplanting. The main factor treatments were rice seedling density 15 holes/m² for hand-transplanting (D1) and 24 holes/m² for machine-transplanting (D2); the secondary factor was N rate (0, 75, 105, and 135 kg/hm²). The rice plant dry matter weight of shoot, rice grain yield, milled rice yield, milled rice rate, protein content, amylose content, and taste value were measured at harvest stage. The appropriate N application rates were calculated using rice grain yield and milled rice yield.

Results At both D1 and D2 densities, rice grain yield, dry matter weight of shoot, and milled rice yield showed increment trend first with the increase of N rate, and then decreased at N135 treatment. The grain yield of N105 treatment was significantly higher than that of other treatments, but the difference was not significant between N105 and N135 treatments at D1 density. The grain yield of N135 treatment was similar with that of N75 treatment, but the dry weight of shoot was significantly higher than that of N75 treatment in 2018. The protein content of rice showed an increasing trend with the increase of N rate. Compared with N0, the protein content of N135 treatment was increased by 7.58% on average, and the milled rice rate and taste value decreased by 8.81% and 10.24% on average, respectively. The N105 treatment achieved the highest N fertilizer efficiency indices, which were all significantly higher than those of N135 treatment. The rice had similar N use efficiency, agronomic efficiency and physiological efficiency of N fertilizer at both D1 and D2 densities. The rice under D2 density recorded higher rice grain yield, N accumulation and milled rice yield than those under D1 density. The N accumulation and partial productivity of N fertilizer under D2 density were 40.35% and 40.31% higher than those under D1 density. The milled rice protein content under D2 density was lower, but the milled rice rate and taste value were higher than those under D1 density. The relationship between N rate and rice yield fit a quadratic curve. The calculated economic optimum N rate, according to rice yield, was 96.4–123.7 kg/hm², and that calculated according to milled rice yield was 76.2–105.9 kg/hm². The economic benefit of the enhanced rice quality increased by 7428 yuan/hm², and extra increased benefit of 4229 yuan/hm² was achieved with higher planting density under D2.

Conclusions The dense planting (24 holes/m²) can improve grain yield, nitrogen uptake and taste value. The economic optimum nitrogen application rate determined by the nitrogen application rate and milled rice yield effect function is 76.2–105.9 kg/hm². This method is beneficial to achieve the high yield and high quality of rice and high nitrogen use efficiency while reducing nitrogen application rate.