| PROJECT AIM
This PhD project aims at explaining observed mixed success of in-stream mitigation measures for reducing nutrient concentrations in Sweden by evaluating their structural and functional properties. Eutrophication is caused mainly by high nutrient losses from agricultural soils to receiving waters. Mitigation measures are tools for reducing these losses by eliminating nutrient leaching and soil erosion or by capturing nutrients dissolved or suspended in stream water by restoring agricultural ditches. Restored agricultural ditches resemble structurally natural streams with good self-cleaning capacity for reducing nutrient and sediment concentrations. However, there is a general lack of studies evaluating the long-term effectiveness of restored agricultural ditches and existing studies show their mixed success. We hypothesise that this mixed success results from limited understanding of the hydrochemical and biogeochemical processes controlling nutrient and sediment transport and transformation in agricultural streams. Our project aims at advancing this scientific understanding and by doing this, improving the effectiveness of future mitigation measures.
METHODOLOGY
Our main objective is to evaluate structural and functional properties of agricultural ditches in different stages of restoration, from a traditional ditch to a flow-through wetland. Our main hypothesis is that structural properties of ditches control their capacity for reducing nutrients and sediment concentrations and restored ditches are better in reducing eutrophication compared to traditional ditches.
The project is divided into 4 work packages running in parallel for 4 years. WP1 collates information on structural properties of restored agricultural ditches in Sweden (e.g. cross-sectional area, width and gradient of terraces, vegetation cover, location in relation to critical source areas) to select 12 study locations. WP2-3 compares hydrochemical and biogeochemical functional properties of restored vs. unrestored ditches (e.g. water and nutrient retention, denitrification rate). WP4 shows how sediment and vegetation removal enhances nutrient retention.
PRELIMINARY RESULTS
This project builds on our ongoing work (SLF O-16-23-640, Carl Tryggers CTS 17:59, ÅForsk 17-373, Jordbruksverket 16742-3, FORMAS 2018-00890) evaluating the effectiveness of mitigation measures in Sweden, which shows that restored ditches can be sources of pollution if their structural properties are not optimised to local catchment conditions.
IMPORTANCE
The project will generate new knowledge of links between hydro- and biogeochemical processes controlling eutrophication in agricultural streams, which is needed to inform water managers and stakeholders designing mitigation measures in Sweden to maximise their cost-effectiveness. As such the project addresses an important challenge of water quality management, which is one of the key focus areas of Stiftelsen Oscar och Lili Lamms Minne. Our ambition is that knowledge generated here will improve effectiveness of the mitigation measures and their uptake by Swedish farmers in the short-term and will help to move a step closer to achieving Swedish and global water management goals: Zero Eutrophication and UN Clean Water.
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