# Effects of Simulated Nitrogen Deposition on the Bacterial Community of Urban Green Spaces
## 4 Discussion  
### Effects of Soil Chemical Properties on the Soil Bacterial Diversity and Community Composition  
To clarify the global interactions between urban soils and their inhabiting microbes, we deemed necessary to understand the specific ones occurring between urban green space soil and hosted bacteria, in order to seize the effects of N enrichment on the whole urban ecosystem functioning. Sequencing analyses revealed that Proteobacteria and Acidobacteria were the dominant phyla in all samples. When considering the time and dose-related patterns, Proteobacteria specifically increased following N enrichment, while Acidobacteria, Chloroflexi, Bacteroidetes, and Nitrospirae exhibited the opposite trend. Therefore, the response of the bacterial community to N deposition appears to vary substantially among different phyla. The deposition of N as a nutrient may trigger both direct and indirect effects on microbial communities and diversity [40; 41]. These can affect the soil nutrient pool, along with environmental factors changes. N deposition could affect soil bacteria by changing soil inorganic N content. In addition to soil N content, pH is one of the critical factors affecting soil microbial diversity [42; 43], because N could indirectly affect soil bacterial community by soil acidification [44; 45]. It is, in fact, widely accepted that soil acidification can directly influence soil bacterial community composition [43; 46]. In this sense, most of the prior studies indicated that soil pH was more important than nutrients in shaping bacterial community structure [47; 48; 49; 50]. In our experiments, we observed that the response of urban green space bacteria to N enrichment appears related mainly to NH\({}_{4}\)\({}^{+}\)-N and to its effects on soil pH (Figure 5). The soil dissolved inorganic N (NH\({}_{4}\)\({}^{+}\)-N and NO\({}_{3}\)\({}^{-}\)-N were generally correlated with the N treatment gradient, which was accompanied by a corresponding decrease in soil pH. The reason for pH decreases as a consequence of ammonium-based fertilization is mainly to be seen in the fact that when ammonium undergoes nitrification it leaves net H\({}^{+}\) ions into the circulating solution. Bacterial sensitivity to acidic conditions is widespread and mainly due to the scarce attitude to maintain neutrality within the cytoplasm, to avoid amino acid charged groups depolarization. From the statistical point of view, significant differences resulted in soil pH, NH\({}_{4}\)\({}^{+}\)-N and NO\({}_{3}\)\({}^{-}\)-N among different N treatments after 12 months of experimentation (Table 1). Considering the different forms of N, both NH\({}_{4}\)\({}^{+}\)-N and NO\({}_{3}\)\({}^{-}\)-N can cause soil acidification [51]. Our data showed that soil bacterial community composition was closely related to the soil chemical parameters. In fact, some of the parameters did correlate and they overlapped in their explanatory power under N enrichment. The correlations of phylum-level abundances with soil pH and NH\({}_{4}\)\({}^{+}\)-N were observed for most of the dominant phyla (Figure 5). In particular, our results suggest that the bacterial community structure has \begin{table}
\begin{tabular}{c c c} \hline \hline
**Soil Properties** & \(\mathbf{r}\) & \(\mathbf{p}\) \\ \hline pH & 0.2273 & 0.004 \\ NH\({}_{4}\)\({}^{+}\)-N & 0.1799 & 0.037 \\ NO\({}_{3}\)\({}^{-}\)-N & \(-\)0.1457 & 0.986 \\ TN & 0.0066 & 0.401 \\ TC & \(-\)0.0815 & 0.848 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Correlations among the overall bacterial community structure and soil proprieties according to the Mantel test. a significant relationship with soil pH and NH\({}_{4}\)+-N (Table 2). Soil mineral N availability was supposedly also tightly connected to the bacterial community structure as other studies reported [50; 52]. Besides, [PERSON] et al. pointed out that ammonium nitrogen content was a dominant predictor of bacterial community composition in acidic soil with exogenous nitrogen enrichment [53].