Rhizobia Inoculation Enhances Soybean Productivity Through Optimization of Root and Leaf Traits in the Huang‐Huai‐Hai Plain

ABSTRACT Rhizobia inoculation is an effective strategy to enhance soybean yield for ensuring food and feed security, yet its synergistic effects with nitrogen (N) application and the physiological mechanisms remain unclear in the Huang‐Huai‐Hai Plain, a major high‐quality soybean production area. Thus, a two‐year, two‐site split‐plot experiment was conducted with and without rhizobia inoculation as main plots, and seven N application rates (0, 30, 36, 42, 48, 54, and 60 kg N ha −1 ; denoted as N0, N30, N36, N42, N48, N54, and N60) as subplots, to evaluate their effects on root and nodule traits, SPAD value, yield components, seed yield, and potential yield‐enhancing mechanisms. Results showed that inoculation improved yield across all N rates compared to non‐inoculation, with larger increases under lower N inputs (37.1%, 18.5%, 14.3%, 14.1%, 9.9%, 6.5%, and 4.9% from N0 to N60, respectively). Meanwhile, yield increases with rising N application were smaller under inoculation than non‐inoculation, indicating its potential to reduce reliance on synthetic N inputs. Furthermore, compared to non‐inoculation at N60, rhizobia inoculation combined with N60 further increased yield by 4.9% and net returns by 6.8%, and similar productivity was maintained even with a 20% reduction in N (N48). This may be attributed to the fact that, compared to non‐inoculation, rhizobia inoculation partially mitigated the inhibitory effects of increasing N application on nodule number (13.9%–26.7%), weight (11.6%–24.0%), and nitrogenase activity (31.6%–55.0%), while further enhancing root weight (20.1%–30.3%), length (16.0%–41.2%), surface area (15.6%–45.9%), and SPAD value (9.6%–18.2%). These changes collectively promoted aboveground biomass accumulation (4.5%–16.2%) and seeds per plant (8.0%–21.5%), which are key determinants of yield formation. In summary, enhanced root and leaf development under inoculation combined with N fertilization was the main driver of yield improvement. Rhizobia inoculation with 60 kg N ha −1 is recommended to further increase productivity, or with 48 kg N ha −1 to reduce N input and environmental risk while maintaining yield. This study provides theoretical and empirical support for optimizing soybean cultivation toward green, efficient, and sustainable production in the region.