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).