Long-term nitrogen fertilization impacts soil fungal and bacterial community structures in a dryland soil of Loess Plateau in China

Nitrogen (N) fertilization is a key factor that affects soil biogeochemical properties and microbial community structures across ecosystems. However, we know less about the responses of soil microbial community dynamics and biogeochemical properties to long-term N fertilization practices in a loess dryland soil in Northwest China. We sampled dryland soils at the 0-20-cm soil layer from a long-term field experiment (initiated in 2004) conducted in Northwest China, which has three treatments (i.e., N0 (control), N160 (160 kg N ha(-1) year(-1)), and N320 (320 kg N ha(-1) year(-1)). We determined soil biogeochemical properties and used 454 pyrosequencing of internal transcribed spacer (ITS) regions (for fungi) and V1-V3 regions of 16S rRNA (for bacteria) genes to explore soil microbial communities. Increased concentrations of soil organic carbon (SOC), total N (TN), nitrate N and microbial biomass N, and decreased soil pH were found in N-fertilized treatments and probably due to N fertilizer additions. However, N fertilization had no significant effect on soil microbial biomass C, available phosphorus, available potassium, and electrical conductivity. N fertilization did not affect the fungal species number, but it decreased the abundance of phylum Zygomycota and increased genus Fusarium. The predominant bacterial phyla in N0 were Bacteroidetes, Firmicutes, and Fusobacteria, but in N160 and N320 were Acidobacteria, Actinobacteria, and Proteobacteria. At the bacterial genus level, N fertilization decreased the relative abundances of Bacteroides, Fusobacterium, and Faecalibacterium and increased other genera (e.g., Bacillus, Lactococcus, Rhodoplanes, Steroidobacter). However, there was no difference in community structure between N160 and N320. The bacterial community structure had close correlation to soil properties SOC, TN, and pH, while no correlations were observed between fungal community structure and these soil properties, which indicated that soil bacterial community structure was easily changed by soil properties as affected by N fertilization. N fertilization changed soil biogeochemical properties and thereby altered soil fungal community compositions and bacterial community structure, but it did not impact the fungal diversity. These results demonstrated that a reasonable rate of N fertilizer applied to wheat field can improve soil fertility and microbial communities.