Abstract
Mobilization, transformation, and bioavailability of fluvial suspended sediment-associated particulate phosphorus (PP) plays a key role in governing the surface water quality of agricultural catchment streams. Knowledge on the molecular P speciation of suspended sediment is valuable in understanding in-stream PP cycling processes. Such information enables the design of appropriate catchment management strategies in order to protect surface water quality and mitigate eutrophication. In this study, we investigated P speciation associated with fluvial suspended sediments from two geologically contrasting agricultural catchments. Sequential chemical P extractions revealed the operationally defined P fractions for the fluvial suspended sediments, with Tintern Abbey (TA) dominated by redox-sensitive P (PCBD), Al, and Fe oxyhydroxides P (PNaOH) and organic P (POrg) while Ballyboughal (BB) primarily composed of acid soluble P (PDetr), redox-sensitive P (PCBD), and loosely sorbed P (PNH4Cl). The dominant calcareous (Ca) elemental characteristic of BB suspended sediment with some concurrent iron (Fe) influences was confirmed by XRF which is consistent with the catchment soil types. Ca-P sedimentary compounds were not detected using bulk P K-edge XANES, and only P K-edge µ-XANES could confirm their presence in BB sediment. Bulk P K-edge XANES is only capable of probing the average speciation and unable to resolve Ca-P as BB spectra is dominated by organic P, which may suggest the underestimation of this P fraction by sequential chemical P extractions. Notably, µ-XANES of Ca K-edge showed consistent results with P K-edge and soil geochemical characteristics of both catchments where Ca-P bonds were detected, together with calcite in BB, while in TA, Ca-P bonds were detected but mostly as organic complexed Ca. For the TA site, Fe-P is detected using bulk P K-edge, which corresponds with its soil geochemical characteristics and sequential chemical P extraction data. Overall, P concentrations were generally lower in TA, which led to difficulties in Fe-P compound detection using µ-XANES of TA. Overall, our study showed that coupling sequential chemical P extractions with progressively more advanced spectroscopic techniques provided more detailed information on P speciation, which can play a role in mobilization, transformation, and bioavailability of fluvial sediment-associated P.
Highlights
There has been increasing awareness of the importance of fluvial suspended sediments in the transport of nutrients from agricultural catchments, which can degrade water quality and causeSoil Syst. 2020, 4, 51; doi:10.3390/soilsystems4030051 www.mdpi.com/journal/soilsystemsSoil Syst. 2020, 4, 51 eutrophication [1]
Our study showed that coupling sequential chemical P extractions with progressively more advanced spectroscopic techniques provided more detailed information on P speciation, which can play a role in mobilization, transformation, and bioavailability of fluvial sediment-associated P
The chemical speciation of fluvial suspended sediment-associated PP from two geologically contrasting agricultural sitesofwas studied by chemical extraction and bulk and two μ-X-ray absorption near-edge structure (XANES)
Summary
There has been increasing awareness of the importance of fluvial suspended sediments in the transport of nutrients from agricultural catchments, which can degrade water quality and causeSoil Syst. 2020, 4, 51; doi:10.3390/soilsystems4030051 www.mdpi.com/journal/soilsystemsSoil Syst. 2020, 4, 51 eutrophication [1]. There has been increasing awareness of the importance of fluvial suspended sediments in the transport of nutrients from agricultural catchments, which can degrade water quality and cause. A significant proportion of total phosphorus (TP) loads in agricultural streams is transported as particulate phosphorus (PP) or within the fluvial suspended sediment PP [2,3]. Suspended sediment-associated P is deposited on the riverbed channel or on floodplains and its subsequent remobilization must have an important impact on the transport, delivery pathway and fate of P species within agricultural catchments. Deposition on the river channel bed or floodplains can result in short- or long-term P storage. Remobilization of riverbed sediments coupled with bank erosion can reintroduce P to the river channel. While research has addressed the increasing recognition of the importance of sediment PP within freshwater systems, relatively little attention has been given to P in suspended and streambed sediments within agricultural catchments, with the majority of research focusing on
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