Long-term urea fertilization alters the composition and increases the abundance of soil ureolytic bacterial communities in an upland soil

Urea is a widely used nitrogen (N) fertilizer in agriculture, but considerable amounts of urea are lost through ammonia volatilization. Soil microbes are major urease producers; however, the impact of urea application on the soil ureolytic microbial community is poorly understood. In this study, the urease activity and the abundance and composition of the ureolytic bacterial community in soil (30 cm deep) under long-term urea application (four treatments: 0, 200, 400 and 600 kg N ha−1yr−1) were investigated by quantitative polymerase chain reaction and high-throughput sequencing of the ureC gene. Urease activity and ureC abundance decreased with the soil depth and increased with urea fertilization. The ureC/16S rRNA gene ratio slightly varied in the different treatments, and the ureC gene abundance was significantly and positively correlated with urease activity only in surface soil (0–10 cm), despite the greater impact of urea application on the ureolytic bacterial community structure observed in deeper soil layers (10–20 and 20–30 cm). The diversity of the ureolytic bacterial community was higher in upper soil layers than deeper ones and decreased with the urea application rate. These results suggest that long-term intensive urea fertilization may increase the risk of N loss through ammonia volatilization and increase the risk of soil degradation due to the collapse of soil microbial diversity.