The role of bed sediments for fractionation of phosphorus in remediated ditches in Sweden

Sweden’s environmental quality objectives “A varied agricultural landscape” and “Zero eutrophication”, aims to ensure food production while reducing negative impact from fertilizers on the environment. The input ceilings of nutrient concentrations in the Baltic Sea are still critical. A great part of the nutrient pressures on lakes and seas that can be seen today are historically caused by agriculture and have turned into legacy P (phosphorus). However, there are several management practices to improve the nutrient use efficiency in agriculture, e.g., improved drainage, buffer zones, wetlands, or two-stage ditches. A two-stage ditch is a ditch design with terraces, that aims to improve water quality and increase nutrient retention within the landscape. The terraces should even out high and low flow to reduce flooding and stabilize the ditch to reduce erosion. According to the latest estimations, the N (nitrogen) pressures in the Baltic Sea have slightly improved, but not the P pressures. Yet, the goals of nutrient concentrations and inputs in the Baltic Sea are still not achieved. P has two main pathways, either leaching in dissolved form, or absorbed to soil particles or incorporated in organic material. The P concentrations absorbed to particles are substantially higher than the P concentrations in dissolved form. There are also different types of bonds and bond strengths between P and soil particles. The aim of this study was to investigate P dynamics in sediments of two-stage ditches, focusing on how hydrology, water chemistry and catchment properties affects P stocks. In the study, sediments from 10 two-stage ditches in Sweden were sampled to analyse the P fractions: loosely bound P, Fe-P, Al-P, organic bound P and Ca-P, with a chemical extraction method. The P fractions and TP (total phosphorus) were then tested to see if any changes in concentration within and between the sampled two-stage ditches could be found. The P fractions and TP concentrations were also investigated with clay content, pH, DO (dissolved oxygen), BFI (baseflow index) and EPC0 to see if there were correlations that could explain observed patterns. The results showed no significant changes for any P fractions or TP concentrations within same two-stage ditch or between the two-stage ditches. There were no significant correlations between the P fractions and TP and the characteristics of the catchment nor chemical and hydrological factors. However, patterns show that HCl-P (Ca-P), BD-P (Fe-P) and TP concentrations seem to be higher within the sediments of the channel rather than in the sediments of the terraces. HCl-P also seems to reside to a higher extent downstream within the sediments of the channels. Higher P concentrations residing in the channels could indicate that two-stage ditches as a nutrient mitigation measure works as a P sink. Although there are risks of having large amount of P stored in sediments if any extreme weather events would occur and predicted future climate change is suggesting more frequent but intense weather events. Also, larger concentrations of P within stream sediments indicates that further research of P in sediments are needed to fully understand the P dynamics to tackle eutrophication problems and further evaluate two-stage ditches as a nutrient mitigation measure. There are possible explanations to the varied results. E.g., features of the landscape, mineralogy, past or current agricultural practices, location of drainage pipes and dams, or legacy P could have affected the results. Keywords: Phosphorus fractionation, agricultural stream, sediment, two-stage ditch, eutrophicatio