Complementary Root Niches Improve Stratified Phosphorus Acquisition and Use Efficiency in Perennial Legume–Grass Mixtures Under Low-P Conditions

Root foraging efficiency under low-phosphorus (P) conditions is a critical constraint for the sustainable development of artificial grasslands. However, the mechanisms by which legume–grass mixtures with complementary deep and shallow root systems regulate soil P availability remain unclear. Based on the hypothesis that root niche differentiation drives stratified soil P utilization, a two-year pot experiment was conducted on these intercropped forages with chernozem soil from 77th Regiment, Zhaosu, Yili, Xinjiang, China. The experiment included monocultures and two mixed-sowing combinations (Medicago sativa/Bromus inermis (AB) and Trifolium pratense/Bromus inermis (CB)) with two sowing patterns (intra-row and inter-row mixed sowing). The results indicated the following: (1) Relative to the CB–intra-treatment, the AB combination increased aboveground and belowground biomass by 39.2% and 138.5%, respectively, and enhanced phosphorus accumulation by 42.2% (inter-row) and 46.7% (intra-row). The land equivalent ratio (LER) of all mixed-sowing treatments was greater than 1, exhibiting stable overyielding performance. (2) Mixed-sowing significantly regulated vertical soil P distribution and rhizosphere P enrichment. The AB combination exhibited significantly superior root phenotypes, associated with deep rhizosphere P content, suggesting a differentiated soil-layer P capture pattern relative to the CB combination. (3) Root volume was the primary factor influencing P use efficiency (PUE), and the mixed-sowing system exhibited a pronounced stratified regulation characteristic: in topsoil, PUE appeared to be synergistically regulated by roots and rhizosphere P; in subsoil, PUE relied primarily on root volume to alleviate low-P stress, achieving efficient deep P utilization. (4) This study proposed a potential topsoil rhizosphere P supplementation threshold range of 4.47–4.899 mg·kg−1. Mixed-sowing forage grasses achieved stratified P utilization in deep and shallow soil layers through root niche differentiation, offering potential quantitative references and practical guidance for efficient establishment and sustainable P management of artificial grasslands in low-P habitats.