Phosphorus-driven rhizobial community assembly underpins superior nitrogen fixation efficiency in high-oil soybean

Phosphorus (P) supply plays a critical role in regulating symbiotic nitrogen (N) acquisition in soybeans, yet the mechanisms underlying varietal differences between high-oil and non-high-oil varieties remain poorly understood. This study investigated the varietal-specific mechanisms of phosphorus supply intensity on plant nitrogen acquisition via rhizobial community restructuring using two high-oil (Kenong 18, Kenong 39) and two non-high-oil varieties (Heihe 43, Longken 310) under five phosphorus levels (0, 35, 70, 105, 140 kg·hm− 2). The results showed that high-oil varieties exhibited superior growth performance and nitrogen acquisition efficiency at 105 kg·hm− 2 phosphorus supply, with increases of 20.0% in plant height, 4.1% in shoot dry weight and 18.0% in root dry weight versus controls. Nodule number, dry weight and haemoglobin content increased by 83.0%, 30.0% and 33.0%, respectively, in high-oil genotypes. Enhanced nitrogen metabolism was evidenced by significantly elevated GOGAT/GS activities (9.3–17.1%) and leaf total nitrogen content. Crucially, under optimal phosphorus conditions, high-oil varieties enriched specific nitrogen-fixing rhizobia, such as Bradyrhizobium sp. 173_3_module and Rhizobium sp., and exhibited stronger correlations between community structure and soil available phosphorus (AP), along with a predicted greater potential for nitrogen acquisition and aerobic chemoheterotrophy. This study demonstrates that optimal phosphorus supply enhances symbiotic nitrogen acquisition efficiency in high-oil soybeans by driving the assembly of more specialized rhizobial communities, providing microbial mechanistic insights for varietal-specific phosphorus management in soybean cultivation.