Evidence of colloids as important phosphorus carriers in natural soil and stream waters in an agricultural catchment

International audience; Colloids (1‐1000 nm) are important phosphorus (P) carriers in agricultural soils. However, most studies are based on colloids from soil waters extracted in the laboratory, thus limiting the understanding of the natural transfer of colloidal P along the soil‐to‐stream continuum. Here, we conducted a field study on the colloidal P in both natural soil waters and their adjacent stream waters in an agricultural catchment (Kervidy‐Naizin, western France). Soil waters (10‐15 cm, Albeluvisol) of two riparian wetlands and the adjacent stream waters were sampled monthly during wet seasons of the 2015–2016 hydrological year (7 dates in total). Ultrafiltration at three pore sizes (5 kDa, 30 kDa and 0.45 μm) was combined with ICP‐MS to investigate variability in colloidal P concentration and its concomitant elemental composition. Results showed that colloidal P represented on average 45% and 30% of the total P (< 0.45 μm) in the soil waters and stream waters, respectively. We found that colloidal P was preferentially associated with i) organic carbon in the fine nanoparticle fraction (5‐30 kDa) and ii) Fe‐oxyhydroxides and organic carbon in the coarse colloidal fraction (30 kDa ‐ 0.45 μm). The results confirmed that colloidal P is an important component of total P in both soil waters and stream waters under field conditions, suggesting that riparian wetlands are hotspot zones for the production of colloidal P at the catchment scale which have the potential to be transported to adjacent streams. We emphasize that these findings were based on limited sampling times and further longer‐term monitoring and application of tracer or isotope methods would be necessary to better assess colloidal P variations and its transfer from soils to streams.