Transformation of sedimentary and colloidal phosphorus across the land‒sea margin received effluents from agricultural and municipal activities

The transformation, transport, and release of phosphorus (P) are highly subject to anthropogenic activities and complex biochemical interactions. Its bioavailability and recalcitrance are generally regulated by P forms, arousing imposition for the accurate identification of chemical speciation. Here, P K-edge X-ray absorption near edge structure spectroscopy was exploited to determine the molecular P species on colloids, canal sludge, and coastal sediments collected in the area receiving mixed effluents from agricultural and municipal activities. Although Fe(III)-bonded P [Fe(III)–P] dominated the P inventory for colloids and sludge collected near the agricultural area, its proportion on colloids was approximately half of that on the sludge, suggesting the emerging contribution of other P species such as Al–P, Fe(II)–P, and organic-P (org-P) to carry colloidal P. Proportions of Fe(III)–P on colloids and sludge both declined seaward, yet the maximum P loss from colloids was derived from the org-P. The organic matter degradation plausibly drove the reduction of SO₄ and Fe(III), leading to the prominent increase in FeS and the conversion of org-P to authigenic P. While Al–P accounted for a comparable proportion of colloidal P inventory to that of Fe(III)–P in adjacent coastal wetlands, the proportion of Fe(II)–P showed the greatest increase on colloids in the transition from fresh-to saltwater. Collective results attest to the significance of Fe(II) and Al in partitioning colloidal and sedimentary P, which should be taken into account to improve the assessment of the P dynamics and eutrophication risk.