Optimizing water and nitrogen management can enhance nitrogen heterogeneity and stimulate root foraging

The root development and soil nitrogen distribution in cotton fields under varying water and nitrogen supplies have rarely been reported in Xinjiang, Northwest China. To address this gap, a two-year field experiment was conducted during the 2019 and 2020 growing seasons to determine the impact of two irrigation amounts (I400, conventional irrigation, 400 mm; and I267, limited irrigation, 267 mm) and three N fertilizer rates (N320, 320; N272, 272; and N224, 224 kg N ha−1) on the soil NO3-N distribution, root distribution, aboveground nitrogen accumulation, and water and nitrogen use efficiency. Under the I400 condition, soil NO3-N exhibited a 30.4% higher horizontal heterogeneity in the N272 plots compared to the N320 plots. The root length and number in the N272 plots increased by 29.0% and 85.0%, respectively, compared with those in N320. Additionally, the N272 plots exhibited higher root length density and root dry weight in the 40–80 cm soil layers. These advantages of the N272I400 combination resulted in the highest seed cotton yield, nitrogen accumulation, and nitrogen use efficiency. Under the I267 condition, the N320 plots showed significantly increased yield and aboveground N content, enhanced horizontal soil NO3-N heterogeneity, and substantially accelerated root growth in deep soil compared with the N reduction plots. A quadratic correlation was observed between root length and soil NO3-N content. Root length was positively correlated with nitrogen accumulation in aboveground leaves and negatively correlated with soil NO3-N residue. These findings suggest that enhancing the development and distribution of roots promotes plant nitrogen accumulation and photosynthesis and facilitates the soil NO3-N absorption. In conclusion, nutrient heterogeneity can enhance nutrient uptake and improve plant performance, which have important implications for plant productivity and ecosystem functioning.