Speciation of Phosphorus from Suspended Sediment Studied by Bulk and Micro-XANES

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.

Challenges and Successes in Identifying the Transfer and Transformation of Phosphorus from Soils to Open Waters and Sediments

“…In this special issue, we invited research and review articles that address the topic of soil P processes involving transfer and transformation across the landscape, either presenting novel research methods or synergy among non-traditional research fields; a review of existing successes and failures with underlying causes; or data-driven recommendations on the various approaches necessary to mitigate P loss and achieve the tangible goal of improving water quality. This volume contains eight original research articles [6][7][8][9][10][11][12][13] and two review articles [14,15].…”

“…General contribution papers covered the various aspects of basic-applied research on mineral-P interaction and how these reactions impact P mobilization, bioavailability, transfer [7], and speciation of P in different soil matrices using advanced analytical methods. Some of these methods included the application of XANES [13] and field-based research related to stream bank legacy nutrients [8]; natural and anthropogenic eutrophication, and its relationship to climate change [12]; and the evaluation of the impact of P due to (i) grazing systems [11], (ii) weathering and vegetation [6], and soil and manure management practices [10]. Together, these contributions improved our current understanding of the reactions and processes that impact P concentration, speciation, cycling, loss, and transfer from agroecosystems.…”