An ounce of prevention is worth a pound of cure: Managing macrophytes for nitrate mitigation in irrigated agricultural watersheds

Although ubiquitous elements of agricultural landscapes, the interest on ditches and canals as effective filters to buffer nitrate pollution has been raised only recently. The aim of the present study was to investigate the importance of in-ditch denitrification supported by emergent aquatic vegetation in the context of N budget in agricultural lands of a worldwide hotspot of nitrate contamination and eutrophication, i.e. the lowlands of the Po River basin (Northern Italy). The effectiveness of N abatement in the ditch network (>18,500 km) was evaluated by extrapolating up to the watershed reach-scale denitrification rates measured in a wide range of environmental conditions. Scenarios of variable extents of vegetation maintenance were simulated (25%, 50% and 90%), and compared to the current situation when the natural development occurs in only 5% of the ditch network length, subjected to mechanical mowing in summer. Along the typical range of nitrate availability in the Po River lowlands waterways (0.5–8 mg N L−1), the current N removal performed by the ditch network was estimated in 3300–4900 t N yr−1, accounting for at most 11% of the N excess from agriculture. The predicted nitrate mitigation potential would increase up to 4000–33,600 t N yr−1 in case of vegetation maintenance in 90% of the total ditch length. Moreover, a further significant enhancement (57% on average) of this key ecosystem function would be achieved by postponing the mowing of vegetation at the end of the growing season. The simulated outcomes suggest that vegetated ditches may offer new agricultural landscape management opportunities for effectively decreasing nitrate loads in surface waters, with potential improved water quality at the watershed level and in the coastal zones. In conclusion, ditches and canals may act as metabolic regulators and providers of ecosystem services if conservative management practices of in-stream vegetation are properly implemented and coupled to hydraulic needs.