Mechanisms of Ammonium Transformation and Loss in Intermittently Aerated Leachfield Soil.

Optimization of N removal in soil-based wastewater treatment systems requires an understanding of the microbial processes involved in N transformations. We examined the fate of ¹⁵NH₄ in intermittently aerated leachfield mesocosms over a 24-h period. Septic tank effluent (STE) was amended with ¹⁵NH₄Cl to help determine N speciation and distribution in drainage water, soil, and headspace gases. Our results show that 5.7% of the ¹⁵N was found in soil, 10.0% in drainage water, and 84.3% was in the gas pool. Ammonium accounted for 41.7% of the soil ¹⁵N pool, followed by NO₃ (29.2%), organic N (21.7%), and microbial biomass N (7.5%). In drainage water, NO₃ constituted ~ 80% of the ¹⁵N pool, whereas NH₄ was absent from this pool. Nitrous oxide was the dominant form of ¹⁵N in the gas phase 6 h after addition of ¹⁵NH₄-amended STE to the mesocosms, after which its mass declined exponentially; by contrast, the mass of ¹⁵N₂ was initially low but increased linearly with time to become the dominant form of ¹⁵N after 24 h. Our results show that gaseous losses are the main mechanism for NH₄-N removal from wastewater in intermittently aerated soil, with N₂O production as important pathway for N losses.