Nitrogen dynamics and removal within a riparian zone used as a nature-based solution for secondary-treated wastewater

Wastewater treatment plant (WWTP) effluent discharge is one of the main pressures in Mediterranean non-perennial streams because of their low dilution capacity. The concentration of nutrients present in effluents leads to the aggravation of the quality of those ecosystems. As an alternative discharge approach, a Mediterranean riparian zone has been used as a nature-based solution (NbS) to remove nitrogen from NH₄-rich effluents (3 mg/L). The effluent was discharged through an intermittent horizontal subsurface flow across a 250 m² riparian soil area located in a Mediterranean basin. The system operated during spring and summer of 2021 and 2023. Effluent application periods (wet conditions) alternated with drainage periods (dry conditions) at a 1:1 ratio.

We conducted sampling campaigns under both conditions and compared them with a control zone. We assessed the removal efficiency of NH₄ and NO₃ and the impact of the discharge on their concentrations in soil and groundwater. We also measured the N₂O soil emissions along with the expression (mRNA) of key microbial functional genes related to nitrification (archaeal and bacterial amoA) and denitrification (nirK and nosZ).

We found mean removal efficiencies of 50% for NH₄ and 23% for NO₃, similar to those reported for other NbS such as constructed wetlands. As expected, NH₄ increased in both groundwater and soil, while NO₃ decreased, with concentrations varying between wet and dry periods. Effluent application triggered a significant increase in N₂O emissions, also showed a spatial pattern across the riparian zone. The hillslope zone -where the NH₄ rich effluent was applied- presented the highest emissions mainly linked to nitrification. The near‑stream zone, characterized by higher soil moisture, had the lowest emissions, consistent with conditions favoring denitrification. Gene expression patterns confirmed the coupling between both processes. Under wet conditions, we found significant positive correlations between N₂O and archaeal amoA expression, as well as with nitrifiers/denitrifiers ratio, suggesting that N₂O production was more strongly influenced by nitrification. Moreover, we found positive correlations between amoA and nirK genes. The negative correlation between N₂O and nosZ/nirK ratio, in addition to high nosZ/nirK ratio values, indicated that wetter conditions favored complete denitrification. Nevertheless, resulting emissions were generally one order of magnitude lower than those of other NbS and like those of riparian zones.

Overall, the biogeochemical heterogeneity of riparian soils, combined with flow intermittency and the NH₄ load from wastewater, enhanced both nitrification and denitrification. This resulted in an effective system for nitrogen removal.