Urea fertilization decreases soil bacterial diversity, but improves microbial biomass, respiration, and N-cycling potential in a semiarid grassland

In this study, changes in plant diversity and aboveground biomass, soil chemical properties, microbial biomass and respiration, microbial composition, and microbial N-cycling potential (represented by the abundance of genes involved in N reaction) were studied after 3 years of urea fertilization (0, 25, 50, and 100 kg N ha−1 year−1) in a semiarid grassland in China. The microbial composition and N-cycling genes were determined using metagenome sequencing. Urea fertilization significantly decreased soil bacterial diversity, possibly through its negative effect on plant diversity, whereas it increased fungal diversity, and microbial biomass and respiration through enhancing aboveground biomass production with increases in the C input into the soil. However, above the threshold N rate of 50 kg N ha−1 year−1, microbial biomass and respiration decreased probably because of a strong N inhibitory effect on aboveground biomass. Further, urea fertilization increased the gene abundances of narH, nrfA, nirB, and napA, which are involved in dissimilatory nitrate reduction, and those of nifH and nifD, involved in N2 fixation, gdh, involved in organic N decomposition, and glnA, involved in glutamine synthesis and ureC. These findings suggested that urea addition has a positive effect on N-turnover potential. Burkholderiales and Rhizobiales play an important role in soil N cycling. Changes in plant community (diversity and biomass) were responsible for the shift in microbial diversity, biomass, and respiration, whereas alterations in inorganic N levels (exchangeable NH4+ and NO3−) potentially affected microbial N cycling. Our results show that N-mediated environmental stresses can play an important role in microbial diversity and functions, which appear to be mediated largely by plant–soil interactions.