Abstract: 【Objective】Optimizing nitrogen (N) fertilizer rate for maize, by considering ecological and social benefits in combination with agricultural and economic benefits, is essential to promote the sustainable and green maize production in black soil region, and further to realize the multiple goals including higher yield, higher resource use efficiency, improved environment and human health.【Method】Two field experiments were conducted at San-ke-shu (SKS) and Quan-yan-gou (QYG) in typical black soil region from 2017 to 2020, to study the effects of different N fertilizer (urea) rates (0, 50, 100, 150, 200, 250, 300 kg N·hm^-2) on maize yield, N uptake and N recovery efficiency (NUE), and further to evaluate the agronomic, economic, ecological and social benefits (N-derived gross caused by N application, private benefits of removed the N fertilizer cost, ecological benefits of removed environmental pollution cost such as active nitrogen loss and greenhouse gas emission, and social benefit of removed human health harm cost caused by environmental pollution, respectively) in different N rates by using life cycle assessment and comprehensive benefits analysis. The agronomically optimal N rate (AOR), privately optimal N rate (POR), ecologically optimal N rate (EOR) and socially optimal N rate (SOR) were calculated to evaluate the integrated benefits and determine the optimal N fertilizer rate for maize in black soil region. 【Result】Maize grain yields were significantly affected by N rates, which increased continuously with increasing N input and reached the yield plateau in 200 kg N·hm^-2 treatment at both two experimental sites. Under this N rate, the average yields were 10.3 and 11.1 t·hm^-2 at SKS and QYG across four experimental years, respectively. The N uptake of maize plants also showed increased trends with increasing N rates, and the highest value in 200 kg N·hm^-2 treatment at SKS and QYG (151.9, 161.8 kg N·hm^-2, respectively). The NUE of maize showed the highest values in 100 kg N·hm^-2 treatment at both two experimental sites, the averages were 70.3% and 72.2%, respectively; and then, which decreased with increasing N rates. Based on 4-year results, the ecological and social costs caused by N fertilizer input increased exponentially with the increase of N application rate. The N-derived gross, private benefits, ecological benefits and social benefits of maize increased firstly and then decreased with the increase of N application rate. Based on the curve fitting calculation, the AOR, POR, EOR and SOR were estimated as 236, 225, 215 and 211 kg N·hm^-2 at SKS, respectively, and which were 245, 235, 225 and 221 kg N·hm^-2 at QYG, respectively. Under AOR condition, maize yields of 10.6 and 11.4 t·hm^-2, the Nr losses of 44.4 and 46.8 kg N·hm^-2 were obtained at SKS and QYG, respectively, while their ecological benefits were 8 786 and 10 271 yuan/hm², and social benefits were 8 351 and 9 822 yuan/hm², respectively. Compared with AOR, by reducing N inputs by 8.8% and 7.9% at SKS and QYG, respectively, EOR increased partial factor productivity from applied N by 9.1% and 8.1%, respectively, while reducing Nr losses by 11.7% and 11.0%, respectively. Compared with EOR, SOR further reduced N inputs by 10.6% and 9.6% at SKS and QYG, respectively, thus reduced Nr losses by 14.0% and 13.1%, respectively, while increasing social benefits by 124 and 119 yuan/hm², respectively. 【Conclusion】Based on the comprehensive consideration with ecological and social benefits, the optimal N fertilizer rate was determined as 210-220 kg N·hm^-2 for maize with yield of 10.5-12.0 t·hm^-2, it is suggested that the optimal application of ecological or social benefits should be recommended for maize nutrient management in black soil area which could synergistically achieve the multiple goals for higher yields and NUE, improved ecological environment and human health.  

Key words: black soil region, maize, optimal N rate, ecological benefit, social benefit, life cycle assessment