Effect of poly(γ-glutamic acid) on microbial community and nitrogen pools of soil

Given the numerous environmental problems caused by chemical fertilizer overuse, agricultural practices are shifting toward the development of environmentally friendly nitrogen (N) fertilizers. In this study, a pot experiment was conducted to investigate the effect of poly(gamma-glutamic acid) (gamma-PGA), an effective fertilizer synergist, on rapeseed (Brassica napus cv. Qinyou No. 9) productivity, soil N pools, and soil enzymes. Seven treatments were implemented: T-1, a check without urea; T-2, a check with urea only; T-3, urea mixed with glutamic acid; T-4, urea mixed with 3 mg of gamma-PGA per kilogram of soil; T-5, urea mixed with 10 mg of gamma-PGA per kilogram of soil; T-6, urea coated with gamma-PGA (0.9%, m/m); and T-7, urea coated with gamma-PGA (3.1%, m/m). The results demonstrated that the application of gamma-PGA increased the above-ground fresh weight of rapeseed by 7.57-9.26% and the grain yield per pot by 6.46-10.98% compared with T-2. The rapeseed grain yield significantly increased from 15.94 +/- 0.55 g pot(-1) in T-2 to 17.69 +/- 0.78 g pot(-1) in T-7. With the increase in pod numbers and seed numbers per pod, the seed yield was significantly improved after gamma-PGA application. Culturable soil microbial count and microbial diversity were increased by gamma-PGA application. In addition, the effects of application method of gamma-PGA have a more important role on the soil microflora than its amount. The reduced loss of N can be attributed to the capacity of gamma-PGA to increase the "NH4+ turnover pool." More mineral N in the soil was immobilized by clays, soil microbes, and gamma-PGA at the early growth stage, and the immobilized N was then released at the later growth stage for plant growth. Moreover, urease activity also increased with gamma-PGA application. These findings indicated that the urea coated with gamma-PGA could be a better choice for agricultural applications.