Phosphate fertilizer is essential for crop production but poses environmental risks in agriculture. The agronomic and environmental thresholds for phosphorus application in Southwest China's purple soils (Luvic Xerosols) remain poorly defined. We combined soil phosphorus fractionation, root phenotyping, and microbial community analysis (16S rRNA and ITS amplicon sequencing) to explore soil-microbe-plant interactions in a 12-year field experiment with five P application rates (0, 37.5, 75, 112.5 and 150 kg P2O5 ha-1 yr-1). Results showed that beyond 75 kg ha-1, the medium-soluble phosphorus pool increased significantly while stable phosphorus decreased. Fungal diversity was more sensitive to high phosphorus than bacterial diversity. Maize yield plateaued at 75-150 kg ha-1, mainly due to increased grain weight and optimized root architecture. An environmental risk threshold was identified at 83.54 kg ha-1 and an optimal yield threshold at 84.65 kg ha-1, enabling high yield with low environmental risk via microbially mediated phosphorus activation. Therefore, this research reveals that the phosphorus input threshold can provide a basis for reducing phosphorus application, regulating phosphorus components, and maintaining microbial diversity and network complexity in purple soil dryland farming systems, thereby ensuring maize yield.