Effects of Poly-γ-Glutamic Acid Molecular Weight on Lettuce Growth, Soil Properties, and Bacterial Community Structure

Poly-γ-glutamic acid (γ-PGA) can regulate soil physicochemical properties and enhance crop yield. However, the effect of γ-PGA molecular weight (Mw) on plant growth remains unclear. In this study, we investigated the effects of γ-PGAs with low (70–100 kDa), high (700–1100 kDa), and ultra-high (>3000 kDa) Mws on lettuce growth and soil properties. The results showed that γ-PGA application reduced the infiltration rate of red soil. In pot experiments, γ-PGAs with different Mws at 0.1% promoted lettuce growth, and blade length and width increased with increasing Mw. However, the excessive application of ultra-high Mw γ-PGA inhibited lettuce growth. Soil chemical properties revealed that γ-PGA treatments significantly increased soil ammonium nitrogen and available potassium content. Furthermore, bacterial community structure analysis indicated that adding γ-PGA reduced bacterial diversity and richness, particularly under low and high Mw γ-PGA treatments, while increasing the relative abundance of beneficial plant-associated bacteria, including Proteobacteria and Acidobacteriota. Overall, ultra-high Mw γ-PGA exhibited the strongest effects on soil water retention and nutrient regulation, whereas low application rate was more favorable for plant growth. These findings can provide insights into the agricultural application of γ-PGA.