Lacustrine wetland in an agricultural catchment: nitrogen removal and related biogeochemical processes

The role of specific catchment areas, such as the soil-river or lake interfaces, in removing or buffering the flux of N from terrestrial to aquatic ecosystems is globally recog- nized but the extreme variability of microbiological and hy- drological processes make it difficult to predict the extent to which different wetlands function as buffer systems. In this paper we evaluate the degree to which biogeochemical pro- cesses in a lacustrine wetland are responsible for the nitrate removal from ground waters feeding Candia Lake (Northern Italy). A transect of 18 piezometers was installed perpendic- ular to the shoreline, in a sub-unit formed by 80 m of poplar plantation, close to a crop field and 30 m of reed swamp. The chemical analysis revealed a drastic NO 3 -N ground water depletion from the crop field to the lake, with concentrations decreasing from 15-18 mg N/l to the detection limit within the reeds. Patterns of Cl , SO 2 , O2, NO 2 -N, HCO 3 and DOC suggest that the metabolic activity of bacterial commu- nities, based on the differential use of electron donors and ac- ceptors in redox reactions is the key function of this system. The significant inverse relationship found between NO 3 -N and HCO 3 is a valuable indicator of the denitrification ac- tivity. The pluviometric regime, the temperature, the organic carbon availability and the hydrogeomorphic properties are the main environmental factors affecting the N transforma- tions in the studied lacustrine ecosystem.