4.3 Marsh degradation alters soil phosphorus availability
Our results showed that P availability in alpine wetland soils decreased
with soil depth owing to organic matter accumulation in shallow soils
and ‘surface aggregation’ of soil nutrients, particularly surface soils
(Ockenden et al., 2014; Wu et a., 2020). This result conforms with those
of previous studies on freshwater and salt wetland soils (Negassa et
al., 2020; Qu et al., 2019; Wang et al., 2008; Xu et al., 2012). Soil P
availability hardly had significant differences in the vertical
variations among differently degraded marsh soils. However, it generally
increased along the marsh degradation gradient from the RPM, reaching a
maximum in the LDM or MDM soil and subsequently decreasing to the lowest
in HDM (Figure 5a–b). This pattern can be primarily attributed to the
changes in factors affecting soil P availability, such as the
hydrological regime, vegetation presence, and external disturbance
(Duhamel et al., 2017; Ganjegunte et al., 2018; Kröger et al., 2012; Qu
et al., 2018; Wang et al., 2021a, b). As RPM evolved into marsh meadow,
the drying-rewetting and changes in vegetation community promoted the
dissolution of slowly cycling P, decomposition of extracellular polymers
and dead microorganisms containing labile monoester and diester P and
plant debris containing phytate P, and the release of colloid-bound P
through the breakdown of soil aggregates (Stevens and Tullos, 2011; Wang
et al., 2021b). Thus, soil P availability was significantly higher in
LDM than in RPM (Figure 5a–b), although the released soluble P might
have been slightly lost because of inundation (Kröger et al., 2012; Zhou
et al., 2019) in wet season. This was similar to the results previously
observed in the water-level fluctuation zones of the Three Gorges
Reservoir area, China (Wang et al., 2021b), estuarine wetlands with
different vegetation in the Yellow River Delta, China (Qu et al., 2021,
2018; Xu et al., 2012), and managed wetland cells of Mississippi, USA
(Kröger et al., 2012).
As marsh meadow continuously degraded into the meadow, the comprehensive
effects of desiccation and vegetation resulted in a significant increase
in soil P availability (Figure 5a–b) through the decrease in labile P
loss, according to Kröger et al. (2012), and transformation from slow
inorganic P and organic P to bioavailable P owing to the activation of
organic acids from microbes and plant roots and debris (Hallama et al.,
2019; Qualls and Richardson, 2000; Schelfhout et al., 2021; Wang et al.,
2021a). However, when the marsh was heavily degraded, soil microbes
significantly decreased by 39.3%–94.1% compared with the other three
types of wetlands (Pu et al., 2022), and the activities of
phosphomonoesterase, phosphodiesterase, and phytase were lower than
those in the other wetlands by 50.5%–55.9%, 44.3%–52.6%, and
34.2%–70.5% (data from unpublished results), respectively. These
results indicate that the mobilisation of soil in slowly cycling P was
inhibited by the low activities of microbes and phosphatase through
overgrazing and rodent
damage stress. Therefore, P availability in the HDM soils decreased
compared with that in RPM (Figure 5a–b), and even
P activation coefficient in
surface soil was lower than the threshold of soil P deficiency (Figure
5b). This decrease indicated a risk of P limitation via plant biomass
removal in HDM soils. Similarly, Shang et al. (2016), Teng et al. (2020)
and Zhai et al. (2022) revealed that P limited plant growth in degraded
alpine meadow and saline wetland ecosystems.