4.2 Marsh degradation influences the transformation of soil
phosphorus forms
The most remarkable changes in P forms due to marsh degradation was
an increase in organic P
concentration and proportion, and organic P became the predominant P
form in surface soils (Figures. 3a–d and 4a–b) in Zoige, which is
consistent with the results of the Min River Estuary, China (Zhang et
al., 2015). However, some studies have revealed that the concentration
and proportion of organic P in wetland soils decreased because
cultivation and drainage stimulated the transformation from organic P to
inorganic P and increased organic P loss (Negassa et al., 2020;
Schlichting et al., 2002; Wang et al., 2006).
Negassa et al. (2020) demonstrated
that the proportion of organic P was higher in long-term rewetted
peatlands than in drained peatlands. This result was closely associated
with marsh desiccation, combined with the changes in the plant community
or overgrazing. The alpine marsh experienced light or moderate
degradation, and the dense plant communities of hygrophytes and
mesophytes, such as Carex , Blysmus sinocompressus, andKobresia tibetica (Table 2), increased plant coverage,
aboveground biomass (Table 2), and biodiversity and richness owing to
water table drawdown (Zeng et al., 2021) and drying-rewetting (Wang et
al., 2021b). Consequently, plant roots might take up more bioavailable
inorganic P and then return more organic P (Huang et al., 2015b) via a
high litter input compared to RPM, particularly in surface soil (Table
2). Although livestock foraged some plant shoots during grazing, this
effect was very minor and hardly altered the above increase in soil
organic P because of the limitation of flooding or excess water during
the outdoor grazing season (wet season) in LDM and MDM. Alpine marshes
are heavily degraded, and more livestock excreta can compensate for the
decrease in organic P owing to livestock foraging and an increase in the
bioaccumulation of organic P in surface soils during overgrazing.
Furthermore, the improvement in
aerobic
environments in degraded marshes can enhance the microbial activity and
decomposition rate of litter and livestock excreta (Gyaneshwar et al.,
2002; Jones et al., 2018; Prado and Airoldi, 1999; Qian et al., 2010),
which can induce labile organic P migration from the upper to lower
layers of natural soil profiles through soil porewater to further
increase the organic P concentration in subsurface soils (Pan et al.,
2021).
Ca-P was not only the dominant inorganic P form but also the majority of
total P for the whole soil profile in this study (Figures 3a–d and
4a–b) because the soil was derived from the calcareous parent material
of lacustrine sediment, where the concentration of calcium carbonate
equivalent ranged from 61.3 to 240.6 g kg−1 (data
from an investigation of soil series survey in Sichuan Province, China).
For Ca-P forms, a remarkable change was observed in the concentration
and proportion of available Ca2-P increased under marsh
degradation, whereas those of unavailable Ca10-P
decreased (Figures 3a–d and 4a–b). This is because marsh desiccation
accompanied by the transition of the plant community or overgrazing may
enhance the activity and/or quantity of
phosphate-dissolving bacteria,
such as Pseudomonas , Bacillus, and Erwini (Liu et
al., 2015; Susilowati et al., 2019) and increase the organic acids from
plants, microbes, and/or livestock excreta (Almeida et al., 2020;
Gyaneshwar et al., 2002; Miao et al., 2013; Nuryana et al., 2019).
Improved phosphate-dissolving bacteria and organic acids can further
stimulate the transformation of Ca10-P to other labile
inorganic P forms such as Ca2-P, Ca8-P,
and Al-P (Almeida et al., 2020; Susilowati et al., 2019). Similarly,
Jamil et al. (2016) revealed that
Calcisols with greater biological activity under sugarcane crop cover
had a high concentration of Ca2-P in the estuary plains
of Pakistan. However, the input of external P fertiliser from
surrounding pasture uplands resulted in an increasing trend of exchange
P (primarily Ca2-P) with increasing hydroperiod in
isolated Lake Okeechobee in Florida, USA (Cheesman et al. 2010), which
was contrasting with our result, owing to a significant loss of soluble
P (e.g. Ca2-P) and lack of external P in RPM with
perennial flooding. Another remarkable change in inorganic P forms was
that marsh degradation decreased the concentration and proportion of
soil Fe-P but increased those of soil occluded P similar to the changes
in Al-P or Fe-Al-P during soil drying (Bai et al., 2019; Zhang et al.,
2015). This is primarily because soil Fe2+ is oxidised
and transformed into colloidal ferric hydroxide and amorphous hydrated
iron oxide (Ding et al., 2016; Wang et al., 2021b), which can adsorb and
immobilise more phosphate during soil desiccation, resulting in the
transformation from Fe-P to occluded P (Sah and Mikkelsen, 1986, 1989).
In addition, marsh degradation did not alter the vertical variation of
all forms of P. However, the vertical variation differed among the
different forms (Figures 3a–d and 4a–b). The concentration and
proportion of Ca10-P evidently increased with increasing
soil depth because of the downward trend of soil minerals from the
surface to the bottom soil, whereas the concentrations of other
inorganic P forms and organic P generally decreased with depth because
of ‘surface aggregation’ from plants and microbes (Meyerson et al. 2000;
Reddy et al. 1999; Wu et a., 2020). These results are supported by those
of previous studies on wetland soils (Luo et al., 2021; Wang et al.,
2008; Yuan et al., 2015; Zhang et al., 2020).
In summary, the differences in P forms among marsh soils with different
degrees of degradation likely indicates that marsh degradation primarily
induced the transformation from recalcitrant Ca10-P to
organic P and labile Ca2-P via litter return or
livestock excreta combined with marsh desiccation.