Background and aims <p>Phosphorus (P) fertilizer is essential for improving crop yield, especially in arid agricultural regions. While P application is known to influence rhizosphere microbial communities, the effects of excessive P application on microbial diversity and functionality—particularly those microbes involved in plant growth—remain poorly understood.</p> Methods <p>We conducted a field experiment with four P addition levels (0, 44, 87, and 131&#xa0;kg P ha⁻¹&#xa0;yr⁻¹) in the arid region of China. The effects of P fertilization on soil bacterial communities, rhizosphere microbial functions, co-occurrence patterns, and cotton growth were examined.</p> Results <p>The highest seed cotton yield (5224&#xa0;kg&#xa0;ha⁻¹) and rhizosphere microbial diversity occurred at 87&#xa0;kg P ha⁻¹&#xa0;yr⁻¹. This level significantly increased both microbial taxonomic and functional diversity, while a decline in diversity was observed at the 131&#xa0;kg P ha⁻¹&#xa0;yr⁻¹, suggesting a threshold effect. Network analysis of microbial ecological associations corroborated these patterns. Notably, rhizosphere microbial functional diversity, rather than taxonomic diversity, exhibited a significant correlation with cotton yield. Multivariate correlation analysis and structural equation modeling established that P addition directly affected soil properties and indirectly caused shifts in the abundance of beneficial microbial taxa linked with improved root growth and yield. Although 87&#xa0;kg P ha⁻¹&#xa0;yr⁻¹ represented moderate input that balanced rhizosphere function and plant growth, it still exceeded cotton’s removal.</p> Conclusions <p>This study underscores the intricate interplay between P application, rhizosphere microbial ecology, and plant growth, revealing that excessive P input disrupted these patterns without further yield benefits.</p>