Particulate phosphorus speciation and phosphate release in streams and ditches of Dutch agricultural lowland catchments
Popta, E.D.M. van
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Phosphorus (P) enrichment by agricultural input on land increases problems associated with eutrophication in the adjacent surface waters like ditches and streams. Phosphorus in surface water is mainly present in particulate form and its transport strongly correlates with suspended matter (SM) dynamics. Most of this particulate P (PP) is assumed to be iron-bound (Fe-P), but this is not extensively studied in agricultural lowland catchments. Moreover, PP becomes bioavailable when it releases orthophosphate (o-PO4) to the water column, which is mainly induced by the reduction of Fe-P in anoxic conditions. In this study, water samples were collected to quantify the P fractions in SM of Dutch polders and experimental set-ups were developed to examine the potential bioavailability of Fe-P in SM. The SM concentration, P content and PP speciation in streams and ditches of Dutch polders were examined in relation to spatial and temporal variance. One study area showed higher SM concentrations at the first sampling moment compared to the following moments, but no clear difference was seen in a different study area. The variation in SM concentrations of sampling locations is most likely determined by local hydrological processes rather than seasonal changes. The Fe-P fraction was found to be the main contributor (32-96%) to the total PP pool, which is in line with previous studies on this topic. Organic P was the overall second largest proportion (2%-49%). Also, a logarithmic relation between SM concentration and P content was found, where high SM concentrations had a low P content and vice versa. Seasonal variability in PP speciation was seen in the fraction contributions from one of the areas. The Fe-P was decreasing until March while Org-P was decreasing, although this occurred earlier than expected. The reduction rates from the microbially induced experiment were 7 and 12.5 µmol Fe l-1 day-1. The chemical reduction experiments showed faster rates (3.9, 12.1 and 70 µmol Fe2+ l-1 h-1), but the biological experiment is probably more representative of the field situation. The results from this study could be used to manage the P loads and eutrophication potential in agricultural-dominated catchments.