Transformation of phosphorus forms and its regulation on phosphorus availability across differently degraded marsh soils

Soil phosphorus (P) is a vital nutrient that controls wetland productivity and ecological functions. However, the effects of soil P forms on P availability during wetland degradation are relatively unknown. We selected differently degraded marshes, including non-degraded marsh (NDM), lightly degraded marsh (LDM), moderately degraded marsh (MDM), and heavily degraded marsh (HDM), to research the changes in soil P forms by sequential chemical extraction and its regulation on P availability in the Zoige Plateau, China. The results showed that compared with the 0-30 cm soil layers of NDM, the main changes in total P concentration were a significant increase of 31.6%-44.2% in LDM and MDM, and the available P concentration increased by 28.7%-76.2% in LDM and MDM but decreased by 4.0%-47.5% in HDM with a lower P activation coefficient. Marsh degradation increased the concentration of soil dicalcium phosphates, P occluded in iron hydroxides, and organic P from the entire profile by 46.7%-137.7%, 31.7%-49.3%, and 78.1%-82.1%, respectively, while it decreased that of iron oxide surface adsorbed P and apatite P by 17.2%-32.2% and 15.3%-27.8%, respectively. Soil available P was mainly related to organic P and P non-occluded in iron oxide minerals, which might also be a non-negligible direct source of available P. The transformation from apatite P to organic P was an important regulatory mechanism of P availability in soils during marsh degradation. This study revealed the transformation trajectory of soil P forms owing to soil desiccation accompanied by plant community changes and overgrazing during alpine marsh degradation, and the risk of P limitation in HDMs with low soil available P. For degraded marsh restoration, some measures for improving P availability should be implemented in the future, such as grazing exclusion and the application of organic fertilizer.