Effects of Reduced Nitrogen Fertilization Combined with Biofertilizer Application on Cotton Growth Under Saline Water Drip Irrigation

Freshwater scarcity limits agricultural production in southern Xinjiang, China, while saline groundwater utilized for direct irrigation adversely affects soils and crops. Excessive nitrogen fertilizer is often applied to compensate for these adverse effects, potentially jeopardizing soil environmental quality. A two-year field experiment was conducted to assess the impact of decreased nitrogen application on cotton growth, nitrogen use efficiency, and yield under different irrigation water salinity levels, with the addition of biofertilizer. The experiment was undertaken on drip-irrigated cotton fields in southern Xinjiang, China, during 2021 and 2022. Three salinity concentrations of irrigation water were quantified: W1 (1 g L−1), W2 (3 g L−1), and W3 (7 g L−1). Under all three salinity levels, conventional fertilization (F1) served as the control, and F0, a no-nitrogen treatment, was also utilized. A total of 18 treatments were assessed using four nitrogen fertilizer application rates in conjunction with biofertilizer: no nitrogen (B0), 100% conventional nitrogen rate (B1), 85% conventional nitrogen rate (B2), and 70% conventional nitrogen rate (B3). The findings showed that adding biofertilizer considerably increased cotton output under both freshwater and brackish water irrigation regimes when compared to traditional nitrogen fertilization. In just two years, the yield of seed cotton grew by 6.15–10.56% (W1) and 6.49–11.81% (W2). In 2021, lint yield climbed by 11.79% (W1), and in two years, it increased by 6.69–15.51% (W2). Although internal nitrogen use efficiency (iNUE) initially rose and subsequently fell with escalating nitrogen rates, the application of lower nitrogen combined with biofertilizer significantly enhanced agronomic nitrogen use efficiency (aNUE) and diminished soil nitrogen residue. Recommended nitrogen application rates for cotton, utilizing 1200 kg ha−1 of biofertilizer, were established for diverse irrigation water qualities to achieve optimal nitrogen reduction, maximum iNUE, and peak yield: 283.21–322.95 kg ha−1 under freshwater irrigation (W1), 281.00–328.14 kg ha−1 under brackish water (W2) irrigation, and ≥326.28 kg ha−1 under saline irrigation (W3). These findings recommend the optimization of fertilizers across various irrigation conditions and facilitate the efficient utilization of saline water resources.