## 4 Discussion  
### Effects of Chronic Nitrogen Enrichment on Microbial Residues and Amino Sugar Accumulation  
Amino sugars are usually derived from the turnover of microbial residues in soils, as plants cannot produce a large amount of AS [34; 35]. Previous studies reported that amino sugars decreased significantly with N enrichment due to calcium or magnesium deficiency resulting from soil acidification, N saturation, or altered carbon availability [19; 20]. Thus, \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Parameter** & **pH** & **MBC** & **MBN** & **APB** & **DOC** & **SOC** \\ \hline GalN & \(-\)0.937 & 0.959 \({}^{\star}\) & 0.934 \({}^{\star}\) & 0.913 & 0.953 \({}^{\star}\) & 0.933 \({}^{\star}\) \\ GluN & \(-\)0.967 \({}^{\star}\) & 0.994 \({}^{\star}\) & 0.976 \({}^{\star}\) & 0.956 \({}^{\star}\) & 0.980 \({}^{\star}\) & 0.983 \({}^{\star}\) \\ MurN & \(-\)0.996 \({}^{\star}\) & 0.993 \({}^{\star}\) & 0.995 \({}^{\star}\) & 0.989 \({}^{\star}\) & 0.998 \({}^{\star}\) & 0.988 \({}^{\star}\) \\ TAS & \(-\)0.963 \({}^{\star}\) & 0.987 \({}^{\star}\) & 0.967 \({}^{\star}\) & 0.948 & 0.976 \({}^{\star}\) & 0.971 \({}^{\star}\) \\ BR & \(-\)0.996 \({}^{\star}\) & 0.994 \({}^{\star}\) & 0.995 \({}^{\star}\) & 0.989 \({}^{\star}\) & 0.994 \({}^{\star}\) & 0.989 \({}^{\star}\) \\ FR & \(-\)0.939 & 0.962 \({}^{\star}\) & 0.936 & 0.915 & 0.954 \({}^{\star}\) & 0.937 \({}^{\star}\) \\ TMR & \(-\)0.974 \({}^{\star}\) & 0.996 \({}^{\star}\) & 0.981 \({}^{\star}\) & 0.964 \({}^{\star}\) & 0.985 \({}^{\star}\) & 0.985 \({}^{\star}\) \\ \hline \hline \end{tabular}  
* GalN = galactosamine; GluN = glucosamine; MurN = muramic acid; TAS = total among sugar; BR = bacterial residues; FR = fungal residues; TMR = total microbial residues; pH = soil reaction; MBC = microbial biomass carbon; MBN = microbial biomass nitrogen; APB = aboveground biomass; DOC = dissolved organic carbon; SOC = soil organic carbon. \({}^{\star}\) means significant difference at 5% probability level; \({}^{\star}\) means significant difference at 1% probability level.  
\end{table} 
Table 4: Pearson’s correlation analysis between microbial amino sugar components and soil biochemical properties in 2018. Figure 6: Principal components analysis (PCA) based on microbial residues, amino sugars, and soil biochemical properties after 6 years of nitrogen enrichment (No-N, low-N, medium-N, and high-N). N enrichment decreases microbial AS accumulation in acidic soils. [PERSON] et al. [24] and [PERSON] et al. [25] reported that nitrogen enrichment had an insignificant effect on AS concentrations in soil. However, the results of the present study showed that chronic N enrichment increased microbial residues and amino sugar concentrations in the soil (Figures 1 and 2), which is consistent with previous studies [17; 26; 36]. The discrepancy between the previous studies [24; 25] and our study may be attributed to differences in soil pH. The pH of the soil used in the present study is above 8, while that of the previous studies was less than 5. [PERSON] et al. [25] documented that N enrichment lowered the pH of the acidic soil, which hindered microbial activities and production. Soils with low pH suppress the growth and activity of microbes, especially bacteria [22; 37], thereby reducing amino sugar accumulation in N-fertil soils [25; 38]. In the present study, N enrichment reduced soil pH close to the neutral range (Table 2), which might have facilitated microbial growth and activity; consequently, this stimulated microbial residues and amino sugar accumulation. Soils with a neutral pH or a near-neutral range promote microbial growth and activity [22; 23; 39]. In addition, N enrichment stimulated microbial residues and AS probably due to its high microbial biomass, dissolved organic C, and SOC concentrations in the soil. Microbial residues and AS showed a positive correlation with microbial biomass, dissolved organic carbon, and SOC (Table 4).