## 3 Results
### Temporal Variation in Soil Enzyme Activities after Afforestation  
The changes in soil enzyme activities in response to time, stand age, and soil depth are shown in Figure 3. The activities of catalase, sucrase, urease, and alkaline phosphatase varied significantly (\(p<0.01\)) with the plant growing season (Figure 3, Table 3). The activities of these four enzymes were highest in the summer (June or August), especially for sucrase and alkaline phosphatase. Catalase and urease activities were lowest in October, and sucrase and alkaline phosphatase activities were lowest in April (\(p<0.01\)). In addition, the CV of catalase, sucrase, urease, and alkaline phosphatase activities among the different plant growing seasons in each plot were 0.08-0.16, 0.19-0.52, 0.13-0.19, and 0.33-0.39, respectively; this indicated that alkaline phosphatase and sucrase were more sensitive to temporal variation compared with the other two enzymes. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & **Source** & **Df** & **Sum of Squares** & **Mean Square** & **F value** & **Pr > F** \\ \hline \multirow{3}{*}{**DOC**} & Model & 79 & 776313.47 & 9826.75 & 130.46 & \textless{}0.0001 \\  & Error & 64 & 4820.66 & 75.32 & & \\  & Total & 143 & 781134.13 & & & \\ \hline \multirow{3}{*}{**DON**} & Model & 79 & 20312.92 & 257.13 & 108.86 & \textless{}0.0001 \\  & Error & 64 & 151.16 & 2.36 & & \\  & Total & 143 & 20464.08 & & & \\ \hline \multirow{3}{*}{**AP**} & Model & 79 & 224.20 & 2.84 & 8.52 & \textless{}0.0001 \\  & Error & 64 & 21.32 & 0.33 & & \\ \cline{1-1}  & Total & 143 & 245.52 & & & \\ \hline \multirow{3}{*}{**SWC**} & Model & 79 & 869.46 & 11.01 & 3717.37 & \textless{}0.0001 \\ \cline{1-1}  & Error & 64 & 0.19 & 0.01 & & \\ \cline{1-1}  & Total & 143 & 869.65 & & & \\ \hline \hline \end{tabular} DOC, DON, AP, and SWC represent soil dissolved organic carbon, dissolved organic nitrogen, available phosphorus, and water content, respectively.  
\end{table} 
Table 2: ANOVA tables of available nutrients and water content. Factors include years (April, June, August, and October), lands (CK40, CK30, CK15, and SC) and soil depth (0–10, 10–20, and 20–30 cm). \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & **Source** & **DF** & **Sum of Squares** & **Mean Square** & **F value** & **Pr > F** \\ \hline \multirow{3}{*}{**Catalase**} & Model & 79 & 8.70 & 0.11 & 25.64 & \textless{}0.0001 \\  & Error & 64 & 0.27 & 0.00 & & \\  & Total & 143 & 8.97 & & & \\ \hline \multirow{3}{*}{**Sucrase**} & Model & 79 & 5645.89 & 71.47 & 49.65 & \textless{}0.0001 \\  & Error & 64 & 92.13 & 1.44 & & \\ \cline{1-1}  & Total & 143 & 5738.02 & & & \\ \hline \multirow{3}{*}{**Urease**} & Model & 79 & 3.57 & 0.05 & 65.90 & \textless{}0.0001 \\  & Error & 64 & 0.04 & 0.00 & & \\ \cline{1-1}  & Total & 143 & 3.62 & & & \\ \hline \multirow{3}{*}{**Alkaline phosphatase**} & Model & 79 & 187.66 & 2.38 & 304.84 & \textless{}0.0001 \\ \cline{1-1}  & Error & 64 & 0.50 & 0.01 & & \\ \cline{1-1}  & Total & 143 & 188.16 & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: ANOVA tables of soil enzyme activities. Factors include years (April, June, August, and October), lands (CK40, CK30, CK15, and SC) and soil depth (0–10, 10–20, and 20–30 cm). Moreover, the soil enzyme activities significantly (\(p<0.01\)) increased after chronosequences of 40, 30, and 20 years of CK (Figure 3, Table S2). Specifically, the catalase, sucrase, urease, and alkaline phosphatase activities increased by 37.51% to 1000.33% in CK40, 22.79% to 738.76% in CK30, and 21.45% to 506.79% in CK20, indicating an increasing trend in soil enzyme activities with an increase in stand ages. The increments of sucrase activity in each stand age were the largest, followed by alkaline phosphatase activity. For the CV analysis in terms of land-use change, the CV of soil enzymes were ranked as alkaline phosphatase (0.37) \(>\) sucrase (0.35) \(>\) urease (0.16) \(>\) catalase (0.11), implying that soil alkaline phosphatase and sucrase were more sensitive to afforestation and stand age.