Poly-γ-glutamic acid differentially alters the abundances and communities of N functional genes involved in urea hydrolysis, nitrification and denitrification when applied with different nitrogen fertilizers

Poly-γ-glutamic acid (γ-PGA), a fertilizer synergist, has a significant promotional effect on nitrogen (N) fertilizer efficiency and crop production by regulating N transformation in soil. However, the underlying mechanism of its effect on soil N cycling is not well understood. The effects of γ-PGA on the soil properties, extracellular enzyme activities (EEA), abundances and community structures of N functional genes (NFGs) involved in urea (UF) hydrolysis (ureC), nitrification (ammonia oxidation archaea amoA (Arch-amoA) and bacterial amoA (Bacterial-amoA)), denitrification (narG, nirK, qnorB, nosZ) and dissimilatory N reduction (nrfA) were investigated in this study by applying γ-PGA with different N fertilizers (UF; ammonium sulfate, ASF), so as to reveal the microbial mechanism of γ-PGA on soil N transformation. The results showed that pH, electrical conductivity, NH4+-N and NO3−-N content in soil were greatly and differently affected by γ-PGA when applied with UF and ASF. γ-PGA significantly stimulated the microbial biomass N content and EEA activities, except that γ-PGA significantly inhibited the activities of urease and β-1,4-glucosidase when applied with UF. γ-PGA, with UF fertilization, significantly reduced the abundance of ureC, Arch-amoA, Bacterial-amoA and nrfA and significantly increased that of qnorB and nosZ, while γ-PGA, with ASF fertilization, significantly improved the abundance of Arch-amoA, Bacterial-amoA, qnorB and nosZ and significantly reduced that of narG and nirK. Soil pH and β-1,4-N-acetylglucosaminidase activity were the best explanatory variables for the abundance variations of NFGs. γ-PGA greatly affected the community diversity and richness of Arch-amoA, Bacterial-amoA, narG, qnorB and nosZ, and evidently changed the community composition of all NFGs and showed different effects when applied with different N fertilizers. Our results demonstrate that γ-PGA regulates soil N transformation by altering the abundance and community of NFGs and showed different alterations when applied with different N fertilizers.