Opposite Root Morphological Responses of Chinese Cabbage to Poly-γ-glutamic Acid When Applied with Urea and Ammonium Sulphate

Poly-γ-glutamic acid (γ-PGA) significantly promotes the fertilizer N uptake efficiency of crops and evidently affects soil available N (nitrate, ammonium and glutamate) status. As an adaptive strategy to forage N, root morphology responds variably to soil available N dynamics. Detailed knowledge of how root morphology responds to γ-PGA remains unexplored. A pot trial was conducted to investigate the response of root morphological traits to γ-PGA when applied with CO(NH2)2 or (NH4)2SO4. The results showed that γ-PGA significantly improved the dry weight, total carbon and total nitrogen content of roots, and with a higher improvement with CO(NH2)2 compared to (NH4)2SO4. γ-PGA significantly increased the root length, total surface area, tips and forks with CO(NH2)2. Contrarily, γ-PGA significantly reduced root length, specific root length, specific root area, root volume, tips and forks with (NH4)2SO4, with its inhibition on root growth mainly caused by the high ammonium content in soil. In conclusion, γ-PGA gives opposite effects on root morphological traits when applied with CO(NH2)2 or (NH4)2SO4. This finding provides a new insight to reveal the promotion mechanism of γ-PGA on plant N acquisition in the rhizosphere and offers a practical reference for optimizing γ-PGA and fertilizer application management