Enzyme C: N: P stoichiometry
The mean soil ecoenzymatic C: N, C: P ratios increased from the uncultivated to afforested lands to cropland at both two soil depths (Fig. 4 a, b). The mean soil ecoenzymatic N: P ratios were significantly higher in the cropland than other soil types at both two soil depths (Fig. 4c). There was no significant difference in soil ecoenzymatic N: P ratios between the uncultivated land and afforested lands at top soil, while the soil ecoenzymatic N: P ratios were significantly lower in the afforested lands than uncultivated land at deep soil (Fig. 4c). The mean soil ecoenzymatic C: N, C: P, N: P ratios were significantly higher in the top soil than deep soil in the cropland and afforested land except ecoenzymatic N: P ratio in the cropland, but there were no significant differences in soil ecoenzymatic C: N, C: P, N: P ratios between top soil and deep soil in the uncultivated land (Fig. 4). Notably, all the soil ecoenzymatic C: N, C: P and N: P ratios were less than 1 (Fig. 4), and mean ecosystem ratios of C: N: P hydrolyzing enzyme activities [i.e. the ratio of (BG): ln (NAG + LAP): ln (AP)] ranged from 1: 1.06: 1.12 to 1: 2.20: 2.50 in all land use types (Table 3).
Key factors affecting soil enzyme activities and C: N: P stoichiometry
The Pearson correlation analysis showed that two N- hydrolyzing enzymes (NAG, LAP) activities were significantly related to soil N and C contents [SON, SOC, LN, RN, C: N ratio of organic soil, C: N ratio of labile pool, C: N ratio of recalcitrant soil, RIN, NH4+-N, NO3--N), they were also significantly positively correlated with soil total PL FAs and negatively correlated with the F: B ratios (Table 1). The RDA showed that there was a significant relationship between the soil enzyme activities and environment factors (F = 117, p = 0.002, Fig. 5). The abiotic and biotic factors (i.e., SON, SOC, C: N ratio of organic soil, RIN, NH4+-N, NO3--N, total PLFAs, F: B ratios, moisture, temperature, pH) together explained 96.13% of the total variability in the soil enzyme activities (axis 1, 96.62%; axis 2, 1.51%) (Fig. 5a).The environmental factors (i.e., C: N ratio of organic soil, C: N ratio of labile pool, C: N ratio of recalcitrant pool, RIN, NH4+-N, NO3--N, total PLFAs, F: B ratios, moisture, temperature and pH) explained 96.13% of the total variability in the soil enzyme activities (axis 1, 96.46%; axis 2, 1.49%) (Fig. 5b).
Soil enzyme C: N ratio and soil enzyme C: P ratio had a positive relationship with SON, SOC, LN, RN, C: N ratio of recalcitrant pool, and total PLFAs had a negative relationship with RIN, NH4+-N, F: B ratios, pH. Soil enzyme N: P ratios were positively correlated with NO3--N, soil moisture and were negatively correlated with F: B ratios and pH (Table 1).