Row Ratio in Faba Bean–Oat Intercropping Enhances System Productivity by Regulating Rhizosphere Bacterial Communities and Root Traits in the Qinghai Lake Region

The biological mechanisms through which faba bean–oat intercropping coordinates interspecific competition, belowground responses, and system productivity remain poorly understood in alpine environments. A field experiment was conducted in the alpine region surrounding Qinghai Lake, with six treatments comprising faba bean monoculture (F), oat monoculture (O), and four faba bean–oat intercropping patterns with row ratios of 1:1 (FO), 2:1 (F2O), 2:2 (F2O2), and 1:2 (FO2). Agronomic traits, root morphology, soil physicochemical properties, and rhizosphere bacterial communities were measured to clarify above- and below-ground coordinated responses of the two crops to varying row ratios. The 2:1 faba bean–oat intercropping yielded the optimal performance, with a land equivalent ratio of 1.479 and a 29.3% higher dry matter yield (6795.4 kg ha−1) than faba bean monoculture, as well as the highest system crude protein production. Dominant oat compensatory growth primarily boosted system productivity. Compared with the corresponding monocultures, the 2:1 intercropping treatment increased the total root length, root surface area, and root tip number of faba bean by 23.9%, 33.9%, and 54.3%, respectively, while increasing the total root length of oat by 110.3%. Under this treatment, soil alkali-hydrolyzable nitrogen reached 167.17 mg kg−1, 39.9% higher than that under oat monoculture, whereas soil available phosphorus and catalase activity increased by 13.2% and 54.4%, respectively. Rhizosphere bacterial communities exhibited host-specific responses, with stronger treatment-associated differentiation in the oat rhizosphere and enrichment of Bacteroidia, Chitinophagales, and Chitinophagaceae under F2O. Partial least squares path modeling (PLS-PM) further revealed crop-specific pathways linking row configuration, belowground properties, bacterial α-diversity, and crop performance. Collectively, these findings demonstrate that the productivity advantage of faba bean–oat intercropping at a 2:1 row ratio is jointly mediated by competitive compensation of oat, enhanced root proliferation, improved soil nitrogen and phosphorus availability, and restructuring of rhizosphere bacterial communities.