Importance of Near and In-stream Zones in Small Agricultural Catchments to Buffer Diffuse Nitrogen Pollution

Riparian ecosystems play an important role in removing nitrogen (N) from water and improving water quality downstream. However, factors influencing N retention in these water conservation areas are not fully understood, hindering effective management and their potential use for N removal. The objective of this paper is to evaluate the N buffering capacity of landscape features, including riparian zones, using isotopic signatures of nitrogen-15 (delta ¹⁵N, δ¹⁵N) of aquatic and riparian vegetation as an indicator for determining sites of biogeochemical N transformation. Studies in fourteen countries across Europe with a range of climatic conditions showed that nitrate (NO₃^-) removal by riparian buffer zones (plant uptake and denitrification) varies from 0 to 30 percent (%), depending on NO₃^- loading to these zones and the local hydraulic gradient rather than climatic conditions. The negative relationship between in-stream NO₃^- concentration and the δ¹⁵N of diatoms in water showed that the ¹⁵N natural abundance in diatoms can be used as a proxy for in-stream denitrification and point source pollution of N. In flood plains, the percentage reduction in NO₃^- along the flow path and an increased δ¹⁵N in the remaining NO₃^- suggest NO₃^- is lost through denitrification. However, the increased ¹⁵N in the vegetation showed that plants compete with denitrification for NO₃^- . The isotopic signature of ¹⁵N is an important tool to assess in-stream, riparian zone and flood plain denitrification thus allowing us to improve management practices for reducing the fluxes from croplands to streams. (author)