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.