Integrated assessment of water-nitrogen management for winter oilseed rape production in Northwest China

With global warming, the winter oilseed rape planting area in Northwest China is gradually increasing. It is vital to address the problems faced by winter oilseed rape cultivation in this region, such as low winter temperature, water shortage and low resources utilization efficiency. To explore the coupling effects of supplementary irrigation level and nitrogen (N) application rate on winter oilseed rape growth, water and N use efficiency, and determine the multi-objective optimal water-N management strategies, a two-year field experiment was conducted (2020–2021 and 2021–2022). The experiments included three supplementary irrigation levels (I0: rainfed; I1: supplementary irrigation at overwintering stage; I2: supplementary irrigation at overwintering and budding stages) and five N application rates (N0: 0 kg/hm2; N1: 120 kg/hm2; N2: 200 kg/hm2; N3: 280 kg/hm2; N4: 360 kg/hm2). The results showed that supplementary irrigation at overwintering and budding stages (I2) can improve the physiological growth indexes, N accumulation and utilization, main quality indexes, and yield of winter oilseed rape. Under I2, LAI, aboveground biomass, chlorophyll content, water use efficiency, N accumulation, N agronomy efficiency and yield of winter oilseed rape first increased and then decreased as N application rate increased, and all reached maximized at medium levels of N application rate (N2 or N3). Under the same irrigation level, the oil content of grain decreased as N application rate increased, while the protein and thioside acid content increased. Based on the TOPSIS with Nash equilibrium-combination weighting method, the multi-objective optimal strategies were determined to be the combination of supplementary irrigation at overwintering and budding stages (I2) and N application rate of 200 kg/hm2 (N2). This method overcomes the limitations of subjective and objective weighting methods, and made the evaluation results more reasonable. The finding can provide a reference for the application of water and N application in winter oilseed rape production in semi-arid areas of Northwest China and other regions with similar environments.