To mitigate excess of nitrate-N (NO-N) derived from agricultural activity, constructed wetlands (CWs) are created to simulate natural removal mechanisms. Irrigated agricultural drainage water is commonly characterized by an organic carbon/nitrogen (C/N) imbalance, thus, C limitation constrains heterotrophic denitrification, the main biotic process implicated in NO-N removal in wetlands. We studied a pilot plant with three series (169 m) of hybrid CWs over the first two years of functioning to examine: i) the effect of adding different C-rich substrates (natural soil vs. biochar) to gravel on NO-N removal in a subsurface flow (Phase I), ii) the role of a second phase with a horizontal surface flow (Phase II) as a source of dissolved organic C (DOC), and its effect in a consecutive horizontal subsurface flow (Phase III) on NO-N removal, and iii) the contribution of each phase to global NO-N removal. Our results showed that the addition of a C-rich substrate to gravel had a positive effect on NO-N removal in Phase I, with mean efficiencies of 40% and 17% for soil and biochar addition, respectively, compared to only gravel (0.75%). In Phase II, the algae growth turned into a DOC concentration increase, but it did not enhance NO-N removal in Phase III. In series with C-rich substrate addition, the largest contribution to NO-N removal was found in Phase I. However, in series with only gravel, Phase II was the most effective on NO-N removal. Contribution of Phase III to NO-N removal was almost negligible.
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