Phosphorus Fertilization Overrides Intercropping-Induced Shifts in Microbial Stoichiometry to Increase Forage Yield

Legume–grass intercropping and phosphorus (P) fertilization are recognized strategies for enhancing forage productivity, but their interactive effects on soil microbial processes and plant phosphorus nutrition in a semi-arid climate remain poorly understood. We conducted a field experiment with common vetch (Vicia sativa) and oat (Avena sativa) under two monocultures and three intercropping treatments (legume–grass ratios of 1:3, 2:3, and 1:1), combined with three P fertilization rates (0, 60, and 120 kg P ha−1). The results showed that common vetch/oat intercropping with moderate legume–grass proportions (1:3 and 2:3) significantly outyielded monocultures across all P levels and exhibited a stronger net biodiversity effect than the 1:1 intercropping at P fertilization. Plant P concentration was primarily increased by P fertilization. Crucially, all intercropping treatments showed a significantly lower microbial biomass carbon/phosphorous ratio than the monoculture in the absence of P fertilization. However, this difference disappeared when P was applied, indicating P fertilization overrode the intercropping-induced stoichiometric shift. Correlation analyses further showed that forage yield and plant P uptake were positively linked to microbial biomass P and negatively to the microbial biomass carbon/phosphorous ratio. Together, our findings reveal that a common vetch/oat intercropping system combined with P fertilization may improve the nutrient use efficiency through microbial pathway. This improvement in nutrient use efficiency leads to higher nutrient uptake by plants, thereby causing more rapid soil reserve depletion.