Coordinated improvement of soil nitrogen retention and root development by poly-γ-glutamic improves soybean nitrogen utilization under water deficit

Water deficit is a major constraint on crop nitrogen utilization, yet the potential of poly-γ-glutamic acid (γ-PGA) to enhance soybean nitrogen absorption and the underlying mechanisms remain poorly understood. A two-year field experiment was conducted in 2023 and 2024 using two irrigation regimes, deficit irrigation (60% crop evapotranspiration, W60) and full irrigation (100% crop evapotranspiration, W100), combined with five γ-PGA application rates: 0 (CK), 10 (T1), 20 (T2), 40 (T3), and 80 (T4) kg ha –1 . The results showed that the γ-PGA significantly increased soil NH₄⁺-N retention and reduced NH 3 volatilization under W60, with the greatest effect observed in T3. T3 also enhanced rhizosphere enzyme activities and promoted root growth, as evidenced by higher “beta” model growth parameters and improved root nutrient acquisition traits. Consequently, soybean nitrogen utilization was markedly improved, reaching the highest levels in T3. In contrast, under W100, γ-PGA exerted relatively limited effects on soil enzymes, root traits, and nitrogen utilization, with modest improvements mainly observed in T1 and T2. These responses were reflected in yield and nitrogen fertilizer productivity, which increased substantially under W60 (>40% in T3) but only slightly under W100 (>5% in T1/T2). Overall, the results demonstrate that γ-PGA, particularly at 40 kg ha –1 , enhances soybean nitrogen utilization under water deficit by improving soil nitrogen retention, stimulating rhizosphere biological activity, and promoting root development, thereby increasing yield and nitrogen fertilizer productivity.