Vegetated canals mitigate nitrogen surplus in agricultural watersheds

Within irrigated agricultural watersheds, canal networks may play a crucial role as nitrogen (N) sink. This is due to the intertwined action of macrophytes and microbial communities occurring in the dense net of small watercourses. We hypothesize that vegetated canals may buffer relevant fractions of excess N from agriculture via microbial denitrification, and that vegetation provides multiple interfaces that greatly support the activity of bacteria. To test these hypotheses, we measured net dinitrogen (N2) fluxes in bare sediments and at the reach-scale in vegetated ditches. As study areas we selected canals subjected to diffuse N pollution, laying in a lowland sub-basin of the Po River (northern Italy). Denitrification was evaluated on the basis of changes in dissolved N2:Ar, measured by Membrane Inlet Mass Spectrometry. Complementary data were obtained via upstream-downstream inorganic N balances and intact core incubations targeting sedimentary N fluxes. Denitrification was the major pathway for N removal, with rates at the reach-scale (5-25mmolNm-2d-1) up to one order of magnitude higher than in sediment alone (3-7mmolNm-2 d-1). Results highlighted that N uptake by macrophyte stands was quantitatively small; however, aquatic vegetation provided multiple interfaces for microbial growth and N-related processes. Our data suggest that 1ha of vegetated canal may remove between 150 and 560kgNyr-1. In the study area, an average canal density of ~0.05 linear kmha-1 of agricultural land has the potential to buffer 5-17% of the excess N from agriculture (~60kgNha-1yr-1).The results of this study suggest the central role of emergent vegetation in promoting microbial N-transformation and canal self-depuration. Innovative management of the canal networks should couple hydraulic needs with the maintenance of emergent vegetation.