Vertical Patterns and Responses of Soil Microbial Functional Groups to Chronic Nitrogen Addition and Their Linkages to Nutrient Dynamics in a Typical Steppe

Despite the pivotal importance of soil microbial communities in regulating the terrestrial carbon (C) cycle and ecosystem functioning, their vertical patterns and their responses to long-term nitrogen (N) deposition, in terms of microbial biomass and relative abundance, still remain unclear, particularly for fungi versus (vs.) bacteria and Gram-positive (GP) vs. -negative (GN) bacteria, where they traditionally have contrasting biological characteristics. In this study, we investigated the responses of GP and GN bacteria, total bacteria (TB), and fungi (TF) and their relationships with four nutrient ratios to 10 years of N addition with three N addition rates (control, N2, and N10 representing 0, 2, and 10 g m−2yr−1, respectively) across a 100 cm soil profile (0-10-20-30-40-70-100 cm) in a semiarid grassland, both in terms of absolute biomass and relative abundance represented by the amount and percentage of phospholipid fatty acids (PLFAs) (e.g., GP bacterial PLFAs: total PLFAS) of specific microbial groups, respectively. Our results showed that N addition has significant negative effects on the biomass of these four microbial groups across the whole soil profile, while N addition significantly decreased the relative abundance of GN bacteria and increased those of TF and GP bacteria. Additionally, N addition significantly decreased the biomass of TF and GP bacteria in 0–10 cm soil, and of TB and GN bacteria in 0–10, 40–70, and 70–100 cm soils, with the significant increase or decrease mainly occurring at the N10 rate. Our results also showed that the biomass of these four microbial groups decreased with soil depth regardless of N addition, but that of TB and GN bacteria decreased faster than that of TF and GP bacteria did, and further caused the relative abundance of TF and GP bacteria to increase with soil depth. Significant Pearson correlations among the biomass and relative abundance of these microbial taxa with soil pH, total phosphorus, NH4+-N, and NO3−-N were mainly found in 0–10 cm soil. C:N and NH4+-N: NO3−-N ratios increased with increasing soil depth, while C: phosphorus (P) and N:P ratios decreased. N addition significantly increased C:P and NH4+-N: NO3−-N ratios and decreased N:P ratios, especially under the N10 rate. Among these four nutrient ratios, N:P significantly explained the highest variations in F:B (76%) and GP:GN (38%) across the whole soil profile. Together, these results underline the different responses of soil microbial relative abundance and biomass along the soil profile under long-term N addition. They also suggest a relationship between soil microbial diversity and biomass relationships, and how this links to nutrient ratios in the subsoil needs to be investigated in future.