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