Synergistic Effects of Mineralization Degree and Sodium Adsorption Ratio on the Rhizosphere Bacterial Community and Soil Nutrients of Upland Cotton Under Saline Water Irrigation

In global drought-prone cotton-growing (Gossypium hirsutum L.) areas, saline water irrigation has become a key strategy to alleviate the shortage of freshwater resources. Against this backdrop, the synergistic effect of mineralization degree (MD) and sodium adsorption ratio (SAR) on the rhizosphere microecological regulation mechanism remains unclear. To address this issue, this study constructed an experimental framework of the interaction between MD and SAR, aiming to explore their effects on the bacterial community structure in the rhizosphere of cotton and the soil environment. The soil type in the study area is saline–sodic sandy loam. In the experimental design, three MD levels (3 g/L, 5 g/L, 7 g/L) were set, and under each mineralization condition, three SAR levels (10 (mmol/L)1/2, 15 (mmol/L)1/2, 20 (mmol/L)1/2) were arranged. In addition, local freshwater irrigation was used as the control group (CG), resulting in a total of 10 treatment schemes. The aim of this study was to investigate the effects of varying levels of irrigation water MD and SAR on the structure of bacterial communities in cotton rhizosphere soil and the soil environment. The results indicated that saline water irrigation could enhance the diversity and richness of the bacterial community in the rhizosphere soil of cotton and alter its community structure. Under treatment with the MD of 3 g/L and the SAR of 10 (mmol/L)1/2, the diversity and richness of the bacterial community in the cotton rhizosphere reached their peak levels. Compared with the CG, the Chao1 index significantly increased by 260 units, while the Shannon index increased by 0.464. When the MD does not exceed 5 g/L, reducing SAR can enhance the diversity and network stability of the rhizosphere bacterial community, thereby synergistically promoting the accumulation of soil nutrients. The key soil environmental factors driving changes in the rhizosphere bacterial community structure mainly include soil moisture content, total nitrogen, nitrate nitrogen, and total organic carbon. The concentrations of total nitrogen, nitrate nitrogen, available phosphorus, and available potassium significantly increased by 19.66%, 26.10%, 89.41%, and 49.76% respectively (p < 0.05). This study provides a theoretical basis for sustainable irrigation and microbial regulation strategies in saline–alkali cotton fields at the theoretical level, and offers a new perspective for revealing the mutual feedback mechanism between bacterial community assembly and soil environment under saline conditions. From a practical perspective, this research offers valuable hands-on experience for optimizing agricultural ecological management in saline–alkali sandy loam soils, thereby contributing to the sustainable development of agriculture on such lands.