Mechanisms of ammonium transformation and loss in intermittently aerated leachfield soil

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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% 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. Analysis based on the isotopic enrichment of 15N2O and 15N2 indicates that nitrification contributed 98.8 and 23.1% of the 15N2O flux after 6 and 24 h, respectively. Our results show that gaseous losses are the main mechanism for NH4+ 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 N2O.

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Journal of Environmental Quality