Water-nitrogen coupling promotes efficient resource utilization by optimizing cotton root morphology under salt stress

Optimal root morphology is essential for crops to acquire soil resources and adapt to rhizosphere adversity. Effective water and nitrogen management strategies can regulate root growth to enhance resource utilization in arid saline-alkali regions. However, the response characteristics of crop roots morphology and nutrient utilization to water-nitrogen coupling under various salinity gradients remain incompletely understood. Understanding this coupling's regulation mechanisms in diverse rhizosphere environments is vital for sustainable agriculture in saline-alkali regions globally. We conducted a two-year field experiment in Xinjiang, China, treating cotton fields with differing salt gradients (7.67-11.53 dS.m-1) and varying water (60 %-80 %-100 %ETc) and nitrogen (75 %-100 %-125 %Nck) levels. Using the 15N isotope labeling method and structural equation model, we analyzed and quantified the interplay of water-nitrogen coupling on root morphology and resource utilization under salinity stress. Our results confirmed that water and nitrogen applications significantly improved root morphology in saline-alkali soil, water promotes root elongation slightly more than nitrogen, with root length density increasing by 14.63 % and 8.45 %, respectively. However, soil salinity significantly inhibited root morphology optimization, resulting in an average root length density reduction of 19.08 %. The response of nitrogen content in cotton organs to water was slightly stronger than that to nitrogen. Salt stress primarily inhibited urea nitrogen absorption, resulting in a 28.94 % decrease in total nitrogen uptake and a 17.31 % decrease in nitrogen utilization efficiency. A structural equation model was developed to understand the regulatory effects of water-nitrogen coupling on cotton growth under salinity stress. The model revealed that the positive influence of water-nitrogen inputs on efficiency indexes was mainly achieved by optimizing root morphology, with an impact coefficient of 0.66 for the control variables-root-efficiency evaluation. Water had a greater positive effect on cotton than nitrogen, with load coefficient of 0.20 and 0.15, respectively. Therefore, this study provides a theoretical basis for crops to adapt to adverse conditions by aligning root system construction with biomass allocation strategies.