Interactive Effects of Mulching Width and Irrigation Management on Cotton Growth and Dynamic Changes in Soil Factors in Arid Regions

Mulching and irrigation are key practices for improving cotton yield and soil conditions, especially in Xinjiang, China. This study investigated the combined effects of mulching width and irrigation depth on cotton growth and rhizosphere microorganisms. Two mulching widths—conventional (M1) and ultra-wide (M2)—and three irrigation depths, 0.8 ETc (W1), 1.0 ETc (W2), and 1.2 ETc (W3), were tested. The impacts on cotton growth, soil environment, and rhizosphere microbial communities were analyzed. Results showed that under the same irrigation depth, M2 significantly increased soil moisture and reduced salt accumulation. Soil temperature under M2 was higher than M1, with increases of 0.55 °C and 1.65 °C during the budding and flowering–boll stages. M2 also increased root length (3.52–10.72%) and root surface area (5.8–7.51%). The beneficial fungus Cladosporium was enriched, while the pathogen Fusarium was suppressed under M2. With the same mulching width, increasing irrigation improved soil moisture, reduced electrical conductivity, and decreased soil temperature. Root diameter and volume increased by 7.67–47% and 9.43–10.36%, respectively. Mulching width and irrigation depth significantly affected bacterial α-diversity. M2W3 showed the highest microbial richness and functional diversity. This study offers guidance for efficient cotton cultivation in southern Xinjiang.