3.3 Effects of increasing stand ages on soil properties
The soil organic C, total P concentrations, and C:N ratio in the three soil layers were significantly enhanced with increasing stand ages (Figure 4a, c, and d, P < 0.05). However, the soil C:P and N:P ratios decreased with increasing stand ages (Figure 4e and f,P < 0.05).
Increasing stand ages decreased the soil NO3-N concentrations in three soil layers (Figure 5a, P < 0.05) but had no significant change in the soil NH4+-N concentrations (Figure 5b). The soil available N concentrations in the 20–30 cm soil layer and the available P concentrations in the 0–10 cm soil layer were reduced with increasing stand ages (Figure 5c and d,P < 0.05).
The MBC and MBC:MBP ratios in the top (0–10 cm) and middle (10–20 cm) layers were reduced with increasing stand ages (Figure 6a and e,P < 0.05). The MBN and MBN:MBP in the top layer (0–10 cm) and the MBC:MBN in the middle layer (10–20 cm) displayed a declining trend with higher stand ages (Figure 6b, d, and f, P< 0.05). In contrast, increasing stand ages led to higher MBP in the top layer (0–10 cm) (Figure 6c, P < 0.05).
The AP activity decreased with increasing stand ages in the deep soil layer (20–30 cm) (Figure 7a, P < 0.05). The PhOx and NAG activities in the top layer (0–10 cm) and the CBH activity in the middle layer (10–20 cm) were enhanced with increasing stand ages (Figure 7b, c, and d, P < 0.05). The BG activity initially increased and then decreased as the stand age increased in all soil layers (Figure 7e, P < 0.05). In contrast, the peroxidase activity initially decreased and then increased as the stand age increased in the top and middle layers (Figure 7f, P < 0.05).