## 3 Results
### Correlation and Multiple Regression of Soil Chemical Properties and the Gene Copy Number
Figure 1: Gene abundance of nitrogen cycling genes influenced by decades of long-term compost and NPK + compost amendment. The abundance of nitrification-related genes (**A**) as quantified with archaeal ammonia monooxygenase gene (**Aamo**) and bacterial monooxygenase gene (**Bamo**), and denitrification-related genes (**B**) as quantified with nitrite reductase gene (**nirK**) and nitrous oxide reductase genes (**nosZI** and **nosZII**). Same letters in the plot indicate non-significant difference (_p_ > 0.05; Tukey’s test). The standard error is denoted in the error bars. with the dissolved organic carbon (DOC). The bacterial _amoA_, _nosZI_, and _nosZII_ were all significantly and positively correlated with SOM, TN, and K, with additional correlations for each gene. The _nirK_ gene abundance did not show correlation with any edaphic factor as well as to the other genes included in the study (Table 3). The multiple regression analysis pointed to a more specific soil chemical property that may directly influence the gene abundance (Table 4 and Figure 2). For archeal _amoA_, DOC is the only significant factor that influences its gene abundance while total nitrogen is the most important factor influencing bacterial _amoA_. The _nirK_ gene abundance is significantly influenced by P\({}_{2}\)O\({}_{5}\), OM, and pH. For the abundance of denitrification-related genes, _nosZI_ is mainly affected by TN, P\({}_{2}\)O\({}_{5}\), and pH, while _nosZII_ is significantly affected by TN. It is noteworthy that, during the multiple regression analysis, SOM, TN, P, and K always showed a collinear relationship. \begin{table}
\begin{tabular}{l l l l l l l l l l l l l} \hline \hline
**Gene** & **Variable** & **Coeff.** & **S.E.** & **S. Coeff.** & **Ranking** & **T-Value** & _p-Value** & **VIF** & **F-Value** & _p-Value** & **R\({}^{2}\)** \\ \hline _AamoA_ & Constant & 13.398 & 0.460 & & & 0.000 & 0.000 & & 4.940 & 0.05 & 0.622 \\  & DOC & 0.012 & 0.004 & 0.766 & 1 & 3.031 & 0.023 & 1.013 & & & & \\  & NH\({}_{4}\)-N & \(-\)0.020 & 0.042 & \(-\)0.122 & 2 & \(-\)0.481 & 0.648 & 1.013 & & & & \\ \hline _BamoA_ & Constant & 11.460 & 1.114 & & & 10.289 & 0.000 & & 5.772 & 0.040 & 0.658 \\  & TN & 23.734 & 7.130 & 0.986 & 1 & 3.329 & 0.016 & 1.541 & & & & \\  & DOC & \(-\)0.015 & 0.010 & \(-\)0.422 & 2 & \(-\)1.423 & 0.205 & 1.541 & & & & \\ \hline _nirK_ & Constant & 42.819 & 1.973 & & & 21.698 & 0.000 & & 52.239 & 0.001 & 0.981 \\  & NO\({}_{3}\)-N & \(-\)0.014 & 0.015 & \(-\)0.191 & 4 & \(-\)0.971 & 0.387 & 8.231 & & & & \\  & P\({}_{2}\)O\({}_{5}\) & 0.022 & 0.002 & 1.841 & 1 & 10.788 & 0.000 & 6.200 & & & & \\  & pH & \(-\)2.984 & 0.270 & \(-\)1.344 & 3 & \(-\)11.036 & 0.000 & 3.157 & & & & \\  & OM & \(-\)2.523 & 0.335 & \(-\)1.620 & 2 & \(-\)7.522 & 0.002 & 9.874 & & & & \\ \hline _nosZI_ & Constant & 11.105 & 0.962 & & & 11.546 & 0.000 & & 60.173 & 0.000 & 0.973 \\  & TN & 19.759 & 1.760 & 1.6000 & 1 & 11.225 & 0.000 & 3.769 & & & & \\  & P\({}_{2}\)O\({}_{5}\) & \(-\)0.007 & 0.001 & \(-\)0.886 & 2 & \(-\)5.863 & 0.002 & 4.233 & & & & \\  & pH & 0.660 & 0.140 & 0.428 & 3 & 4.698 & 0.005 & 1.538 & & & & \\ \hline _nosZII_ & Constant & 16.8 & 0.347 & & & 48.460 & 0.000 & & 10.515 & 0.021 & 0.913 \\  & TN & 13.697 & 3.145 & 1.817 & 1 & 4.355 & 0.012 & 8.017 & & & & \\  & NO\({}_{3}\)—N & \(-\)0.029 & 0.012 & \(-\)0.906 & 2 & \(-\)2.332 & 0.080 & 6.948 & & & & \\  & DOC & \(-\)0.002 & 0.002 & \(-\)0.211 & 4 & \(-\)1.118 & 0.326 & 1.640 & & & & \\  & NH\({}_{4}\)-N & 0.025 & 0.022 & 0.219 & 3 & 1.145 & 0.316 & 1.678 & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Relationships between soil edaphic factors and gene copy numbers of nitrogen cycling genes as shown through multiple regression analyses.