Moderate P Fertilizer Promotes Cucumber Yields and Modulates Bacterial Community in the Wheat Cover Crop System

The over-reliance on phosphorus (P) fertilizers in intensive agriculture has led to the depletion of phosphate resources and soil health deterioration, with continuous cropping systems further exacerbating these issues. However, strategies to reduce phosphorus inputs and simultaneously maintain soil health in the wheat cover crop system remain underexplored. With the aim to address this gap, a pot trial was conducted with five P application levels: 100%, 75%, 50%, 25%, and 0% of the conventional application amount (17.42 g·superphosphate·pot−1). For each P level, a corresponding no cover crop treatment was set up. The results demonstrated that wheat cover crop enhanced cucumber growth compared to not applying wheat cover crop, and it also stimulated the proliferation of plant growth-promoting bacteria. More importantly, in wheat cover crop systems, 50% of the conventional P fertilizer rate illustrated the best performance, including the highest value of dry biomass, yield, and soil enzyme activity. This treatment further enriched the beneficial microbial taxa, such as Burkholderiaceae, Rhodobacteriaceae, LWQ8, and Alkalinaceae, optimizing microbial community structure and plant-microbe interactions (p < 0.05). Thus, reducing phosphorus fertilizer to 8.71 g·pot−1 was optimal for achieving soil functions and crop productivity in this study, highlighting the importance of optimizing P fertilizer in cover crop systems.