3.3 Soil phosphorus availability
The available P concentration and P activation coefficient first
decreased and then slightly increased in the LDM soil but gradually
decreased in other soils as the soil depth increased (Figure 5a–b).
Compared with RPM soil, the available P concentration was
17.0%–234.0% and 0.0%–85.1% higher in all soil layers from the LDM
and MDM, respectively, with a significant difference at the 10–30 cm
and 60–100 cm layers in the LDM and the 0–20 cm layers in the MDM
(p < 0.05) (Figure 5a); it decreased by 4.0%–30.7%
at the 0–40 cm depth of HDM but hardly changed at the 60–100 cm depth
(Figure 5a). The variations in P activation coefficient between the RPM
and degraded marsh soils exhibited a trend similar to that of available
P (Figure 5a–b). Furthermore, the P activation coefficient in the
surface (0–10 cm) soil was lower than the threshold of soil P
deficiency of 2.0% (Li and Zhang, 1994).
3.4 Soil
available phosphorus related to phosphorus forms
The conceptual structural equation model in Figure 2 was successfully
fitted to our data, and the goodness-of-fit metrics were satisfied
(χ2/df was approximately 1, NFI > 0.95,
CFI > 0.99, and RMSEA < 0.08; Figure 6a). All P
forms explained 79% of the variations in soil available P. The
non-occluded Ca8-P and Al-P exhibited a strong effect on
the variations in soil available P, whereas the effect of non-occluded
Fe-P was so minor that it did not exhibit in the fitting structural
equation mode (Figure 6a).
Soil organic P had a strong total effect (0.784) on the variations in
available P and was the most important positive factor influencing the
variations in available P, followed by non-occluded P (0.578), occluded
P (0.447), Ca2-P (0.409) and Ca10-P
(-0.093) (Figure 6b). This indicated that the variations of available P
were mainly related to organic and non-occluded P in alpine wetland
soils. The direct effect of soil non-occluded and organic P on available
P was slightly less than that of Ca2-P (Figure 6b),
suggesting that soil non-occluded
and organic P also might be non-negligible direct source of available P.
Marsh degradation exhibited a high direct effect (0.455) on the
variations in Ca10-P. Meanwhile, Ca10-P
presented a high indirect effect (-0.402) on the variations in available
P through the effect of organic and non-occluded P in soils (Figure
6a–b). These revealed that marsh degradation significantly influenced
soil P availability via the transformation from Ca10-P
to organic and non-occluded P.