Riparian areas along springs and ditches as natural filters of pollutants from diffuse sources: some results from Northern Italy.

Over the last few decades, much interest has been focused on specific catchment areas such as soil-water or groundwater-surface water interfaces able to mitigate the flux of nutrient from terrestrial to aquatic ecosystems. Their natural buffer potential is due to two properties: a disproportionately high rate of biogeochemical activity and the position within the watersheds, so that water and nutrients necessarily passes through these ecotones. We present an overview on the effectiveness of riparian areas in nitrogen removal analysing the main results obtained from field studies conducted along small lowland springs and artificial ditches in Northern Italy. These small water bodies receiving large nutrient input are very common in the agricultural lowlands of the Po basin. The studied areas are characterised by different size (from 5 m to 35 m), vegetation (herbaceous, wooded, shrubs, etc), soils, hydrogeological settings and agriculture practises. In some case study nitrate-N concentrations in shallow groundwater from adjacent cropland declined from levels that were often 10-40 mgl-1 near the field-riparian edge to <0.5 mgl-1 in the riparian zones throughout the year. Chloride data suggested that dilution cannot account for most of this nitrate decline. The role of denitrification was investigated indirectly analysing some crucial parameters (eg. dissolved oxygen, dissolved organic carbon) and measuring the rate of potential denitrification. Our data indicated that the effective nitrate removal by denitrification did not depend on the size of riparian zones. The key function of the systems has to be found among the landscape variables (eg. slope, sediment texture) able to influence the linkages between hydrologic flow paths and patterns of electron donors and acceptors that favour denitrification in riparian areas.