Evaluation of cotton N nutrition status based on critical N dilution curve, N uptake and residual under different drip fertigation regimes in Southern Xinjiang of China

Abstract Irrigation and fertilization are the two most important measures to ensure the agricultural development in arid regions. However, there is still a lack of reasonable irrigation and fertilization strategy in Xinjiang, the largest cotton producing area in China, which causes a waste of resources and a reduction in cotton yield and pollutes the environment. This study investigated the effects of four irrigation amounts (W0.6: 60% ETc, W0.8: 80% ETc, W1.0: 100% ETc and W1.2: 120% ETc, where ETc is the crop evapotranspiration) and four nitrogen rates (N250-250 kg N ha-1, N300-300 kg N ha-1, N350-350 kg N ha-1 and N400-400 kg N ha-1) on seed cotton yield, nitrogen nutrition status and soil nitrogen residue, so as to explore the optimal water-nitrogen coupling strategy to increase cotton production and water-fertilizer use efficiency while reducing environmental pollution. The nitrogen dilution curves were built to assess the nitrogen nutrition status of cotton. The results showed that increasing irrigation amount increased dry matter and seed cotton yield, but decreased harvest index. Nitrogen application enhanced the nitrogen concentration and accumulation in cotton. Irrigation decreased the nitrogen concentration, but increased the nitrogen accumulation by increasing the dry matter. The highest total plant nitrogen accumulation was obtained under W1.2N400, 223.88 kg ha-1 in 2018 and 242.77 kg ha-1 in 2019. With the increase of irrigation amount, agronomic nitrogen utilization efficiency (aNUE) first increased and then decreased. Increasing irrigation amount increased nitrogen recovery efficiency (NRE), but reduced physiological nitrogen utilization efficiency (pNUE). pNUE and aNUE decreased with the increasing nitrogen rate, while NRE first increased and then decreased. Soil NO 3 − -N content increased with the increase of nitrogen rate. Irrigation reduced NO 3 − -N content in the surface soil (0–30 cm), while increased that in the deeper soil (40–80 cm). The optimal nitrogen nutrition status was found under W1.0N300, followed by W1.0N350. Furthermore, the optimal dry matter, seed cotton yield, NRE and total nitrogen accumulation were obtained under W1.0N350, which was considered as the optimal combination of irrigation amount and nitrogen rate in this study.