Soil microbial community responses to long-term nitrogen addition at different soil depths in a typical steppe

Global nitrogen (N) deposition has been influencing the structure of soil microbial communities, which play fundamental roles in modifying most biogeochemical processes. However, to date, our understanding of how long-term N deposition affects soil microbial communities, particularly those in subsurface soil, is largely incomplete. In this study, we examined soil microbial phospholipid fatty acids at different depths in the soil profile, including in topsoil (0–10 cm), midsoil (30–40 cm), and subsoil (70–100 cm), in a 10-year N addition experimental site in a semiarid steppe. We collected soil samples at four N addition treatment levels, including 0, 2, 10, and 50 g m−2 year−1, which represented control, low-N, medium-N, and high-N inputs, respectively. Our results showed that N addition remarkably shifted soil microbial communities by increasing the relative abundances of bacteria and Gram-positive (GP) bacteria, and decreasing Gram-negative (GN) bacterial relative abundance across the three soil layers by affecting soil pH and NO3−-N, particularly under the medium- and high-N addition rates. Moreover, the effects of N addition tended to increase with the N addition rate but diminished with soil depth. Soil pH, NO3−-N, and NH4+-N were the most important driving factors for changes in microbial community composition in topsoil, whereas soil total N (TN) and total carbon were important in midsoil, and TN, dissolved organic N, and soil moisture were important in subsoil. The relative abundances of bacteria and GN bacteria also decreased, whereas those of fungi and GP bacteria increased with soil depth regardless of N addition. Soil TN and pH were the most important factors in shaping the vertical distribution of soil microbial communities. Our results suggest that both N addition and soil depth may cause similar microbial community shifts, through which fungi and GP bacteria become dominant, but through different mechanisms. These results suggest that it is important to distinguish among the N-deposition effects on soil microbial communities at different soil depths when building soil system models. •N addition altered microbial communities by influencing soil pH and NO3−-N.•Subsoil had higher relative abundances of Gram-positive bacteria and fungi than topsoil.•The higher the N addition rate is, the deeper the effects on soil microbiome reach.•Soil total N and pH explained the greatest variation of vertical distribution of soil microbes.•Soil depth and N ad