Denitrification activity and denitrifying population dynamic in the soil of a wooded riparian strip

The EU “Nitrates Directive” (Directive 91/676/EEC) and the WFD (Water Framework Directive 2000/60/EEC) introduced a series of measures designed to reduce and prevent water pollution caused or induced by nitrates from agricultural sources. Riparian zones, located at the interface between terrestrial human activities and waterways, play a key role as a buffer system protecting aquatic ecosystems from excessive nitrogen loads. There are several mechanisms through which excess nitrogen is removed in riparian buffer zones: some act as temporary sinks, for instance soil storage, assimilation and retention by plants and microbes, while the denitrification process permanently removes nitrogen from the soil in a gaseous form. Within the lower plan of Venice Lagoon watershed, a newly afforested riparian buffer, irrigated with freshwater from the Zero River, was realized; inside this afforested area, a pilot experimental scale system was established. The experimental forest buffer received almost continuous sub-surface water flow with the aim of enhancing nitrate removal through denitrification. The objectives of this research were to verify the potential capacity of this buffer system in removing the excess of nitrogen from river water and to increase knowledge on the processes there involved, with particular emphasis on denitrification. To achieve these objectives the following specific activities were performed: (i) the quantification of the combined nitrogen removal rates from water which flows through the woody buffer area; (ii) the measure of the denitrification process in soils, both “in situ” conditions (DNT) and potential denitrification (DEA) and its relationship with the main environmental limiting factors (hydrology, soil, climate, vegetation); (iii) a specific study on denitrifying community, focused on nirK gene, in the soil of the riparian buffer compared to that of a neighbouring agricultural area. The main results of this work demonstrates that a buffer strip 15 meters wide can remove an excess of nitrate not only at concentrations typical of freshwater bodies (less than 5 mg/L N-NO3), but also with higher peaks (until 25 mg/L), with a reduced effectiveness during colder seasons. It was also demonstrated that microbial denitrification plays a key role in nitrogen removal and that higher denitrification rates were reached in the soil layer often saturated by the perched aquifer. As expected, the potential denitrification rates generally decrease with soil depth, depending on the distribution of the microbial population. In general, organic carbon availability resulted as the most limiting factor. Moreover, it was established that in the riparian buffer under study both denitrification potential and the nirK-type denitrifying community distribution significantly differ from a contiguous agricultural soil. Even if there is still a limited understanding of the relationships between denitrification activity and denitrifying community structure and/or abundance we observed that denitrifying community composition could affect potential denitrification in soils characterized by different management practices (i.e. riparian forested soils and agricultural soils)