Abstract
This study was performed in the Vansjø-Hobøl catchment in SE Norway, as part of research related to the transport of phosphorus to the eutrophic lake Vansjø, the largest lake in the catchment. This study is part of The Research Council of Norway (RCN) project EUTROPIA, which aims to improve understanding of geochemical and hydrological processes governing the mobilization of nutrients with special emphasis on phosphorus compounds. This work has examined sampling and fractionating methods for better understanding of phosphorus processes in the catchment, which will further contribute to the studies by other participants in the project.
Considerable research has been conducted on P leaching mechanisms from agricultural land, but little is know about the significance and mechanisms controlling P leaching from forests. Focus of this research has therefore been on forested catchments, comprising 85% of the catchment.
The lack of positive response to the considerable abatement actions is likely due to changes in environmental pressures, such as climate, land use change and long-range transported pollutants. This monitoring study indicates that the decrease in Al leaching, from acid sensitive forested catchments, due to reduced S deposition may be an important factor governing the flux of P to the lake Vansjø. Aluminium ions are known to be a strong precipitating agent of P.
The monitoring also showed considerable concentrations of dissolved organic matter (DOM) from forested watersheds. The flux of DOM is known to have increased considerably since the 80ties. With this increase there has likely also been an increase in the flux to the lake of P bound to DOM.
The organic P fraction was shown to be a major contributor of the total P to the stream from the catchment. Also it was found that larger than expected amounts of Al was leaching from the soils.
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Research on UV exposure to DOM was also conducted to determine the effect on the release of orthophosphate. UV exposure experiments showed up to 17% release of the phosphorus bound to DOM during the first period of UV exposure, indicating a P- fraction, which may easily be released to water. However, the UV decomposition process is complex and more research is needed.
A laboratory study showed that the DGT sampler (Diffusive Gradient in Thin films) specially designed for orthophosphate (P-DGT), also collected small organic molecules with phosphate groups, using the model compound AMP (adenosine monophosphate). The diffusion coefficient of AMP was calculated to 3.3 × 10-6 cm2/s (22 °C), compared to 5.5 × 10-6 for orthophosphate, which agrees with larger molecules having slower diffusion rates. With the P-DGT it was possible to collect LMW organic phosphorus compounds as well as orthophosphate in water from the catchment. LMW organic phosphorus fraction was shown to contribute 6.5% of the total P collected by P-DGTs from an agricultural site. The P-DGT sampler can thus be used to get more information on this fraction in water containing dissolved organic phosphorus (DOP).