4.4 Regulation of soil phosphorus forms on available phosphorus under marsh degradation
Organic P can directly or indirectly transform into available P by mineralization, and some labile organic P, such as nucleic acids, phospholipids, and mononucleotides, might be directly available forms of P for plant uptake (Gatiboni et al., 2021; Tiessen et al., 1984). Thus, soil organic P not only acted as a dominant P form of regulating available P, but also was an important direct source of available P (Figures 6 a–b), because Zoige wetland soils had higher organic P concentration compared with other wetlands (e.g. estuarine wetland) soils (Cheesman et al., 2010; Li et al., 2018; Shao et al., 2019; Zhang et al., 2015). The moderate labile non-occluded P, similar to labile Ca2-P, had a high regulation on available P, which was accorded to the reports from Gama-Rodrigues et al. (2014) and Hou et al. (2016), and also supported by a previous result exhibiting the significant correlations between available P and Al-P in albic-bleached meadow soils (Yang et al., 2013). Meanwhile, non-occluded P is easy to be mobilized into labile P (e.g. Ca2-P) due to the activation of organic acids and phosphate-dissolving bacteria (Almeida et al., 2020; Susilowati et al., 2019), and indirectly regulate available P (Hou et al., 2016). Hence, soil non-occluded P was the second P form of regulating available P (Figures 6 a–b). Soil available P was primarily related to organic and non-occluded P that might also be non-negligible direct source of available P in alpine wetland ecosystem. However, to confirm organic and non-occluded P considered as the direct source of available P, further examination of the contribution of soil organic and non-occluded P to plant P uptake is required in future studies.
Marsh degradation significantly influenced soil available P through the transformation from soil Ca10-P to organic and non-occluded P, especially organic P (Figure 6a–b). This can be ascribed to the differences in hydrography, vegetation, and grazing between RPM and degraded marshes. For degraded marshes, soil organic P increased (Figures 3a–d and 4a–b) via plant uptake of bioavailable inorganic P and a high litter input in LDM and MDM with drying-rewetting and/or dense vegetation (Table 1 and 2); an increase in input of livestock excreta in HDM with overgrazing. Moreover, the risk of P limitation occurred with low available P in HDM. Some studies confirmed that the frequent drying-rewetting and organic acids from litter and/or organic fertiliser (e.g., livestock excreta) might induce direct and indirect transformation from apatite and organic P to labile and moderately labile inorganic P (Hallama et al., 2019; Schelfhout et al., 2021; Wang et al., 2021b), organic acids could dissolve occluded P into non-occluded P (Gatiboni et al., 2021; Touhami et al., 2020; Wang et al., 2016b), and organic and occluded P might be potential sources of available P in P-deficient soils (Turner et al., 2014; Yu et al., 2019). Additionally, the loss of carbon pools owing to marsh degradation may also result in the release of moderate labile inorganic P such as Fe-P and Al-P from organic matter (Yuan et al., 2015; Zhang et al., 2015). In this study, an increase in organic acids associated with a mobilisation of phosphate-dissolving bacteria and a loss of organic carbon (Gatiboni et al., 2021, Almeida et al., 2020; Susilowati et al., 2019; Pu et al., 2020) can improve the activation from soil Ca10-P to organic P, non-occluded P and Ca2-P under marsh degradation. Therefore, the transformation from Ca10-P to organic P was an important regulation mechanism of P availability in soils during marsh degradation on the Zoige plateau.