Effects of nitrogen application rate and irrigation quota on yield and water and nitrogen utilization of post-spring wheat multiple cropping oilseed rape in Yellow River Irrigation Area

The effects of successive oilseed rape cropping on the absorption and utilization of residual nitrogen and water, and nitrogen use efficiency in spring wheat stubble was studied to provide a theoretical basis for the effective utilization of residual nitrogen in post-spring wheat soil and for the prevention and control of agricultural non-point source pollution. A field trial was conducted to investigate the effects of nitrogen application (conventional application, 270 kg∙hm⁻²; reduced application, 202.5 kg∙hm⁻²; and no application, 0 kg∙hm⁻²) and irrigation quota (conventional irrigation, 400 mm; 20% water-saving irrigation, 320 mm; and 40% water-saving irrigation, 240 mm) on the yield and nitrogen uptake of succession oilseed rape crops, as well as the dynamic changes involving soil moisture and mineral nitrogen in the 0–100 cm layer. Nitrogen balance analysis was also conducted for both seasons. The results revealed that residual nitrogen in spring wheat stubble had a notable effect on the yield and nitrogen uptake of successive oilseed rape, and the residual effect of nitrogen fertilizer was positively correlated with the nitrogen applied to spring wheat stubble. When the nitrogen fertilizer application rate in spring wheat stubble was 270 kg·hm⁻², the yield and nitrogen uptake of succession oilseed rape were the highest, reaching 6640 kg·hm⁻² and 25.7 kg·hm⁻², respectively, which were 11.8%–43.5% and 14.8%–58.8% higher than those under reduced or no nitrogen application. Irrigation quota had no substantial effect on oilseed rape yield but had a significant effect on nitrogen uptake. Nitrogen uptake of oilseed rape under conventional nitrogen application was increased by 9.6%−10.2% compared with the water-saving treatments. Compared with before planting, mineral nitrogen levels in the 0–100 cm soil layer under nitrogen application treatment after oilseed rape harvesting decreased by 18.8–96.1 kg·hm⁻², indicating that succession oilseed rape cropping has an absorption capacity for residual nitrogen. The mineral nitrogen content of soil decreased by 96.1 kg·hm⁻² under conventional nitrogen application and conventional irrigation treatment compared with no or reduced nitrogen application. After the succession oilseed rape was crushed and turned over and returned to the field after winter freezing and thawing, the mineral nitrogen in the 0−100 cm soil layer increased by 86.1 to 171.8 kg·hm⁻². This increase was positively correlated with the nitrogen application rate in the spring wheat season. Conventional nitrogen application combined with conventional irrigation had a small effect on soil water storage, and the residual effect of nitrogen fertilizer significantly improved irrigation water use efficiency, water use efficiency, and precipitation productivity of oilseed rape. Conventional nitrogen application combined with a 20% water-saving treatment had the highest irrigation water use efficiency and precipitation productivity, whereas conventional nitrogen application combined with a 40% water-saving treatment had the highest oilseed rape water use efficiency. Under these experimental conditions, the cumulative utilization rate of nitrogen fertilizer was the highest at 89.8% with reduced nitrogen combined with a 20% water-saving treatment. Nitrogen application rates of 270 kg·hm⁻² and an irrigation quota of 320−400 mm considerably improved the yield, nitrogen uptake, water use efficiency, irrigation water use efficiency, and precipitation productivity of succession oilseed rape crops and reduced soil mineral nitrogen content under nitrogen application treatment. Turning over and returning the succession of oilseed rape to the field significantly increased soil mineral nitrogen content after winter freezing and thawing.