1. Introduction
Phosphorus (P) plays a crucial role in maintaining microbial activity
and plant growth (Cui et al., 2019a; Fan et al., 2021; Huang et al.,
2015a; Oelmann et al., 2017; Sitters et al., 2019) and is anticipated to
replace nitrogen as a limiting nutrient in natural ecosystems (e.g.
wetland ecosystems) (Chen et al., 2020; Du et al., 2020; Turner et al.,
2018; Vitousek et al., 2010). However, elevated P concentrations in
soils and sediments are critical environmental problems (e.g.
eutrophication) (Cheesman et al., 2012; Zhang et al., 2020; Zhou et al.,
2019). In wetland ecosystems, P limitation risk and eutrophication
primarily depend on labile P concentrations (e.g. soluble phosphate)
that can be transformed into other P forms and then back again via
biological or geochemical processes (Gao et al., 2019; Hu et al., 2022;
Vitousek et al., 2010). Moreover, soil P transformation is strongly
linked to carbon (C) cycles via the effect of the microbial community
(Luo et al., 2021; Wang et al., 2021a; Zhai et al., 2022). The
concentrations of P and the composition of P forms ultimately affect
wetland ecological functions such as water conservation,
CO2 fixation, and climate regulation and health (Bai et
al., 2020; Cui et al., 2019a; Liu et al., 2020; Sorrell et al., 2011).
Therefore, soil P forms have received considerable attention in wetland
ecosystems (Cheesman et al.; 2012; Dunne et al., 2011; Hamdan et al.,
2012; Luo et al., 2021; Qu et al., 2021; Zhang et al., 2020).
The forms of P in soils generally exhibit evident variations owing to
the differences and changes in the pedogenic environments of wetlands,
such as parent material, hydrothermal, and vegetation conditions
(Cheesman et al., 2010, 2012; Cui et al., 2019b; Negassa et al., 2020;
Qu et al., 2021; Wang et al., 2016a). Presently, climate change, rodent
harm, and human disturbances such as drainage, overgrazing, and
aquaculture have led to the degradation of more than half of wetlands
worldwide to some degree (Huisman et al., 2017; Nguyen et al., 2016; Ren
et al., 2019; Zuquette et al., 2020), which has completely altered the
conditions of hydrology, salinity, vegetation, and soil characteristics
in some wetland ecosystems (Cheng et al., 2020; Li et al., 2022; Zeng et
al., 2021; Zhao et al., 2017a). This has further affected soil P
accumulation and its forms via P transformation processes such as
sorption/desorption, precipitation/dissolution,
immobilisation/mineralisation, and weathering (Augusto et al., 2017;
Barrow, 2015; Khosa et al., 2021; Qu et al., 2021; Smith et al., 2021).
For example, cultivation and drainage stimulated the transformation of
organic P to inorganic P and decreased organic P in freshwater wetlands
of the Sanjiang Plain region, China (Wang et al., 2006), and in peatland
wetlands of Mecklenburg-West Pomerania and Saxony-Anhalt, Germany
(Negassa et al., 2020; Schlichting et al., 2002), resulting in reduced P
accumulation. Grazing increased soil P accumulation in wetlands within
the dairy pasture of the Okeechobee Basin wetlands in the United States
(Dunne et al., 2011). However, existing studies have primarily focused
on the varying effects of P forms in wetlands with low elevations, such
as estuaries and coastal areas (Cheesman et al.; 2012; Dunne et al.,
2011; Hu et al., 2021; Zhang et al., 2020), and minimal studies
regarding this varying effect have been conducted in alpine wetlands
where environmental factors are complex and shifting (Li et al., 2022;
Wang et al., 2022; Wu et al., 2021).
The changes in P accumulation and forms would further alter P
availability in wetland soils (Hu et al., 2021; Huang et al., 2015a;
Wang et al., 2021b). For example, Huang et al. (2015a) observed that the
exotic invasive plant Spartina alterniflora significantly
increased the concentrations of P extracted by 1.0 mol
L−1 NH4Cl,
bicarbonate/dithionite-extracted
P, and P extracted by 0.5 mol L−1 NaOH in the Yancheng
wetland of eastern China, leading to an increase in available P.
Similarly, Hu et al. (2021) observed that an increase in wetland
salinity might significantly enhance labile P release owing to Fe-bound
P reduction in the Min River estuary wetland, China. Furthermore, the
primary forms of P in soils that regulate P availability differ owing to
the differences in environmental conditions such as vegetation
(Gama-Rodrigues et al., 2014; Hou et al., 2016). For example, some
studies have revealed that the major sources of the available P pool are
organic P, oxalate-extractable P, and iron-bound P in tropical forest,
tropical acid farmland, and temperate meadow soils, respectively
(Gama-Rodrigues et al., 2014; Melese et al., 2015; Yang et al., 2013).
In addition, wetland ecosystems have unique environmental
characteristics, such as perennial waterlogging and anaerobic
conditions, and multiple functions compared with terrestrial ecosystems
(Ouyang and Lee, 2020; Shen et al., 2019). However, limited information
is available on the main P forms that regulate available P in different
wetland soils, particularly in alpine wetlands.
Marsh wetlands on the Zoige Plateau with alpine and fragile environments
are located on the eastern edge
of
the Qinghai–Tibet Plateau, China, and have undergone degradation to
different degrees because of natural threats and human activities over
the past 60 years (Li et al., 2015, 2019; Shen et al., 2019). Several
studies have revealed that alpine marsh degradation has decreased soil C
sink function by increasing C emission fluxes in the Zoige Plateau (Ma
et al., 2016; Pu et al., 2020; Zhou et al., 2020). Furthermore, total P
and available P significantly affect the concentration of organic C in
the Zoige peatland soils (Luo et al., 2021). However, it remains unclear
whether marsh degradation impacts P accumulation and availability in
soils, further inducing P limitation of primary productivity of alpine
wetland ecosystems. Solving this question would contribute to
effectively assessing soil P supply and further implementing the
measures of soil P regulation to improve the C neutrality potential and
promote the ecological restoration of degraded alpine marshes.
Therefore, we hypothesised that marsh degradation would have a pronounce
effect on the accumulation and transformation of soil P owing to the
desiccation accompanied with plant community and overgrazing, further
influencing soil P availability. To test this hypothesis, this study
selected Zoige marsh wetlands with different degradation degrees and
aimed to (1) quantify the changes in soil P and its forms for exploring
the characteristics of P accumulation and transformation during marsh
degradation, (2) determine how marsh degradation influences soil P
availability, and (3) elucidate the regulation of soil P forms on
available P under marsh degradation.
2. Materials and
methods