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 15 NH 4 + in intermittently aerated leachfield mesocosms over a 24‐h period. Septic tank effluent (STE) was amended with 15 NH 4 Cl to help determine N speciation and distribution in drainage water, soil, and headspace gases. Our results show that 5.7% of the 15 N was found in soil, 10.0% in drainage water, and 84.3% in the gas pool. Ammonium accounted for 41.7% of the soil 15 N pool, followed by NO 3 − (29.2%), organic N (21.7%), and microbial biomass N (7.5%). In drainage water, NO 3 − constituted ∼80% of the 15 N pool, whereas NH 4 + was absent from this pool. Nitrous oxide was the dominant form of 15 N in the gas phase 6 h after addition of 15 NH 4 + –amended STE to the mesocosms, after which its mass declined exponentially; by contrast, the mass of 15 N 2 was initially low but increased linearly with time to become the dominant form of 15 N after 24 h. Analysis based on the isotopic enrichment of 15 N 2 O and 15 N 2 indicates that nitrification contributed 98.8 and 23.1% of the 15 N 2 O flux after 6 and 24 h, respectively. Our results show that gaseous losses are the main mechanism for NH 4 + removal from wastewater in intermittently aerated soil. In addition, nitrification, which is generally not considered a significant pathway for N loss in soil‐based wastewater treatment, is an important source process for N 2 O.