**4. Conclusions**

The chemical speciation of fluvial suspended sediment-associated PP from two geologically contrasting agricultural sites was studied by chemical extraction and bulk and μ-XANES. It was demonstrated that the microanalysis of P K-edge, combined with the μ-XANES of Ca and Fe K-edge, is critical to achieving a detailed characterization of the PP species. The limestone nature of BB was confirmed by XRF, which is consistent with the soil type at the BB site. The chemical confirmation of Ca-P-containing compounds in BB sediments was revealed only by P K-edge μ-XANES and not by its bulk spectrum, as the bulk XANES was only capable of probing the average speciation. Using bulk P K-edge XANES, it was difficult to resolve Ca-P peaks as the BB spectra is dominated by organic P. For the TA location, Fe-P containing compounds were detected by P K-edge XANES, which correlates with its soil geochemical characteristics and the sequential chemical P extraction data. Notably, the P concentration is generally lower in TA, which makes it difficult to detect Fe-P in the μ-XANES of TA. μ-XANES of Ca K-edge was consistent with the P K-edge and the geochemical characteristics of the study sites (i.e., both Ca-P associated with apatite and calcite detected in BB; TA has mostly organic Ca). A significant amount of organic P was detected in these sediments using P K-edge XANES which suggested the sequential chemical P extractions may have underestimated its importance. Overall, our study provided additional, detailed information on the P speciation associated with fluvial sediments by coupling sequential chemical P extractions with progressively more advanced spectroscopy, including XRF, bulk-XANES, and μ-XANES of multiple elements. This additional knowledge contributes to our understanding of the geochemical processes governing P mobilization, bioavailability, and transformation to potentially inform improved agricultural catchment management policies to protect the water quality of associated rivers and streams.

**Author Contributions:** D.O.; L.G. and Y.H. designed the study. Q.Z.; M.W.; D.O. collected the samples and performed most data measurement and analyses. Q.X. and Y.H. collected the XRF and XANES data. Y.H. and D.O. did most of the writing with inputs from all. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work has been funded by the Natural Sciences and Engineering Research Council of Canada (NSERC) in collaboration with the Irish EPA (EPA), the Irish Department of Agriculture, Food and the Marine (DAFM) and WaterWorks Joint Call (WTWPJ 506072-2016).

**Acknowledgments:** We would like to acknowledge the support of numerous staff and scientists at the Canadian Light Source, which is supported by Canadian Foundation for Innovation, NSERC and the University of Saskatchewan. In addition, we acknowledge the support from the technical staff of the Environmental Engineering Research laboratory in Trinity College Dublin, Ireland.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
