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

摘要

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 managementand 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, 615N) of aquatic and riparian vegetation as an indicator for determining sites of bio-geochemical N transformation. Studies in fourteen countries across Europe with a range of climatic conditions showed that nitrate (NO_3") removal by riparian buffer zones (plant uptake and denitrification) variesfrom 0 to 30 percent (%), depending on NO_3" loading to these zones and the local hydraulic gradient rather than climatic conditions. The negative relationship between in-stream NO_3" concentration and the615N 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 NO_3" along the flow path and an increased 615N in the remaining NO_3" suggest NO_3" is lost through denitrification. However, the increased 15N in the vegetation showed that plants compete with denitrification for NO_3". 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.