Supplementary irrigation and varying nitrogen fertilizer rate mediate grain yield, soil-maize nitrogen accumulation and metabolism

Inappropriate management of irrigation and nitrogen (N) fertilization can lead to underground water pollution, and soil degradation as well as increase the cost of crop production particularly in humid areas of China. In this context, the present experiment was designed to determine the appropriate irrigation amount and N fertilization rate for higher crop production, enzymatic activities, and N content. A two years field experiment was conducted during 2020–2021, to assess the effects of two irrigation condition (rainfed and supplementary irrigation) and five N fertilization rates (i.e. 0, 150, 200, 250 and 300 kg ha⁻1, thereafter referred as N0, N150, N200, N250, and N300) on aboveground dry biomass, soil-maize N accumulation, N-metabolisms related enzymes, grain yield and economic efficiency. Supplementary irrigation, N fertilization, and their interaction significantly affected the above measured parameters. For instance, N250 treatment with supplementary irrigation significantly increased grain N content, aboveground dry biomass, and N metabolisms enzymes (nitrate reductase, glutamine synthetase and glutamate dehydrogenase). Compared to N250 treatment, N300 resulted 0.4 % lower grain yield under supplementary irrigation. Similarly, the interaction of supplementary irrigation and N250 treatment enhanced the soil total N and nitrate-N content compared to other treatments. Supplementary irrigation significantly increased grain yield by 13.6 % compared to rainfed condition. More importantly, both N use efficiency, irrigation water use efficiency and the cost-benefit ratio were reduced when N fertilization rate exceed to N300 treatment of the supplemented irrigation plot. Compared to other N fertilization rates, the N250 treatment resulted in a maximum net benefit of 10,082 CNY ha⁻1 under supplementary irrigation condition. Therefore, we suggest that 250 kg N ha⁻1 with supplementary irrigation is the best choice to improve N use efficiency, grain yield and provide a maximum economic benefit of maize crop. These finding may facilitate the scientific basis for irrigation and N fertilization management of maize crop in humid areas of China. •This study highlighted the effect of irrigation level and N fertilization rate on maize in southern China.•Supplemental irrigation promotes N use and reduces N runoff losses.•The net benefits from different combinations of water and nitrogen were evaluated.•The N250 treatment resulted in a maximum net benefit under supple