**1. Introduction**

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 eutrophication [1]. 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 therefore 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. Similarly, remobilization of riverbed sediments coupled with bank erosion can reintroduce P to the river channel. Hence, information on P fluxes, storage, mobilization, and bioavailability within agricultural streams is required for appropriate catchment scale management policies. 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 P in soils and lentic sediments (lakes and reservoirs, etc.). Relatively few studies have specifically characterized and quantified P species within streambed sediments [4,5], and fewer still in fluvial suspended sediments [6]. Many studies have shown that in catchments with diffuse sources, episodic flooding events and surface water runoff control stream bed and suspended solid composition and fluxes over time [7–9]. In addition to P and organic matter, the concentration and form of metal complexes and their link to microbial mineralization processes are likely impacted by fluvial and stream bed sediments. For example, it has been shown that organic matter remineralization predominates P cycling in Chesapeake Bay sediments [10,11] and the formation of phosphate–Fe(III)–humic complexes significantly impact P cycling and sedimentary Fe(III) stabilization in organic matter Fe-rich lake sediments, which is rarely recognized [12,13]. Hence, the molecular forms of P and their association to metal species of fluvial and stream bed sediments are important for understanding P transformation, mobility, and the potential impact on surface water eutrophication.

Sequential chemical extractions (SCEs) have been widely used to assess and quantify P species with different binding mechanisms and bioavailability. The basis of such fractionation analyses is in the differential reactivity of solid substrates to various chemical extractants [14]. Hence, such chemical extraction methods provide only operationally defined P pools and do not directly determine P speciation [15]. Notwithstanding their widespread application, there are various limitations associated with many sequential chemical P extraction schemes, including: (1) the specificity of extracting agents for sedimentary chemical P forms is relative; (2) transformation processes during the extractions between fractions (i.e., P sorption on calcite from calcareous sediments may be extracted in the Fe-oxide CBD extractant step). For example, during extraction of organic matter-rich sediments using the original Psenner method, Al and Fe associated with humic acid complexes are often misappropriated [16–18]. In the original method, Fe-bound P is extracted using bicarbonate-buffered dithionite followed by extraction of Al-oxide-bound P using NaOH. Concurrently, within this NaOH treatment, significant organic matter-bound P is extracted. Hence an additional step involving the acidification of the NaOH treated sediments (pH ~1) precipitated humic acid (HA) associated Fe and Al resulting in a clear supernatant with precipitate containing up to 30% of the total sediment P [17]. Similarly, another SCE [19] for sedimentary P was modified for organic-rich sediments with insertion of an additional extraction step (Na2CO3) prior to the Fe-bound P focused on bicarbonate-buffered dithionite step in order to extract Fe and Al humic complexes [12,20]. Despite such limitations, SCEs are still useful to get initial estimates of sedimentary chemical P pools. However, more recently, synchrotron-based P K-edge X-ray absorption near-edge structure (XANES) spectroscopy [21,22] and solution 31P nuclear magnetic resonance (NMR) [23,24] have been applied to directly probe and distinguish different P species in terms of the inorganic form (XANES) and organic P species (NMR). More importantly, the feasibility and advantages of the combined use of these techniques in soil and sediment speciation have been demonstrated [11,12,25–27]. While such advanced spectroscopic techniques are capable of providing bulk speciation information for soil and sediment P, microclusters of concentrated P may be overlooked. Such P speciation information on microclusters of concentrated P is important, particularly for heterogeneous samples where correlations between P and metal

species are necessary to understand the transfer and transformation of P in dynamic systems [28,29]. In this study, our objectives were to: (i) couple chemical P fractionation with the bulk and P K-edge micro(μ)-XANES to show the tiered approach in studying P compositional dynamics in suspended fluvial sediments from geologically contrasting agricultural and; (ii) apply bulk and P K-edge μ-XANES to give progressively more accurate and detailed compositional information compared to chemical P fractionations; (iii) show how bulk and P K-edge μ-XANES can identify P species in detail and the correlation between P, Ca, and Fe elements from suspended solids from two geologically contrasting agricultural catchments in Ireland.
