Nitrogen transformations in different types of soil treatment areas receiving domestic wastewater

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Removal of N within the soil treatment area (STA) of onsite wastewater treatment systems is attributed to heterotrophic denitrification, with N lost to the atmosphere as N2. However, the evidence supporting heterotrophic denitrification as the sole process for N removal is scant. We used 15NH4+ to follow N transformations in intact soil mesocosms representing a conventional STA receiving anoxic, C-rich wastewater, and two shallow-placed STAs receiving partially oxygenated, low-C wastewater. Nitrogen losses in the gas phase took place almost exclusively as 15N2 in all STA types. We observed 102-103 times higher flux of N2 than N2O in all STAs, as well as net production of 15N2 and 15N2O near the infiltrative surface and at greater depths in the soil profile. In situ net production of 15NH4+ suggested internal recycling of inorganic N in all STAs. The constraints imposed by differences in availability of electron donors and acceptors and soil physicochemical parameters in different STAs, point to autotrophic N removal processes (e.g., anaerobic ammonia oxidation, autotrophic denitrification) as playing an important role in N removal. Our results suggest that N removal occurs at all depths of the STA, with losses due to both autotrophic and heterotrophic processes. Optimization of autotrophic N removal processes in the STA warrants further research efforts and may provide improved N removal.

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Ecological Engineering