4. Discussion
4.1 Marsh degradation increases soil phosphorus accumulation
In this study, the P accumulation (total P concentration) in all soil profiles ranged from 183.0 to 1433.8 mg kg−1, with an average value of 683.3 mg kg−1, indicating slight P enrichment according to the threshold of 500 mg kg−1(DeBusk et al., 2001). Similar to estuaries or coastal, depressional, and riparian wetlands (Huang et al., 2015a; Wang et al., 2006, 2013; Zhang et al., 2015), the P accumulation in all soils exhibited a decreasing trend with increasing soil depth (Figure 3a–d) because plant detritus was deposited, which led to P enrichment in surface soils (Meyerson et al. 2000; Reddy et al. 1999); however, no significant difference was observed in vertical variations among differently degraded marshes.
Marsh degradation primarily increased soil P accumulation at 0–30 cm depth (Figure 3 a-d) owing to the comprehensive effects of hydrothermal environments, input of organic materials, and human activities. In RPM, long-term floods and highly anaerobic environments may stimulate the reductive dissolution of binding partners of phosphate (Kjaergaard et al., 2021) and increase soluble P concentrations in the overlying water (Dunne et al., 2011; Kröger et al., 2012; Qu et al., 2019; Zhou et al., 2019), further resulting in a significant loss of soluble P via runoff and leakage. However, marsh desiccation can greatly decrease the loss of soluble P, even under light or mediated grazing. Hence, the accumulation of shallow soil P in LDM and MDM significantly increased owing to marsh desiccation and a higher litter (approximate to aboveground biomass) input compared with RPM (p < 0.05, Figure 3. a–c), which was inconsistent with the results that wetland degradation due to cultivation activities decreased soil P accumulation in the Sanjiang Plain, China (Wang et al., 2006) and Saxong-Anhalt, Germany (Schlichting et al., 2002). This might be because cultivation decreases the amount of soil P returned owing to crop absorption (Schlichting et al., 2002; Wang et al., 2006).
As meadows continued to degrade into desertified meadows, soil P accumulation in HDM was significantly lower than that in LDM and MDM. Two facts could explain this: (1) decrease in litter input and root biomass (Table 2) reduced the amount of soil P returned owing to overgrazing and severe rodent damage; and (2) decrease in vegetation coverage, soil clay, and organic matter (Tables 1 and 2) weakened the ability of soil fixation and increased P loss of dissolved P via runoff and leakage (Cui et al., 2018). Similarly, previous results demonstrated that the replacement of the native mangrove community by the invasive plant Phragmites australis reduced the ability of soil to adsorb immobilised P in the Minjiang River estuarine wetland, China (Wang et al., 2016a), and cultivations reduced P accumulation in the Sanjiang Plain wetlands, China (Wang et al., 2006). However, desiccation and excreta from bovines and sheep may supplement a certain amount of P to offset the partial loss of P. Thus, soil P accumulation in HDM was slightly higher than that in RPM, which is consistent with the result of Dunne et al. (2011) that soil P accumulation increased because of the input of organic fertiliser (cattle manure) in the Okeechobee Basin wetlands in the United States.