Modeling Coupled Nitrification/Denitrification in SoilAggregates Irrigated with Reclaimed Effluent

摘要

Anaerobic conditions may arise in soil aggregates as a result of respiratory activity, whenO2 demand exceeds the supply. Irrigation with reclaimed effluent may further affect Ntransformationsin aggregates due to elevated BOD, supply of NH4-N and elevated pH. A model ofreactive, multi-species diffusion is developed to describe N transformations in spherical aggregates,emphasizing the incremental effects of irrigation with reclaimed wastewater. Kinetic models ofmineralization, nitrification, and denitrification are employed to facilitate the investigation of couplednitrification denitrification (CND) subject to oxygen availability. Aggregate size and soil respirationrate are identified as the most influential parameters governing the existence and extent of anaerobicvolume in aggregates, whereas the contribution of BOD and NH4 oxidation is secondary.With increasing anaerobic volume the effective nitrification rate of aggregates decreases, until theircontribution is negligible, committing the bulk of nitrification to aggregates that are significantlysmaller than 0.25 cm. Assuming nitrification is the main source for nitrate in the soil, denitrificationefficiency will peak in medium sized aggregates, where aerobic and anaerobic conditions coexist,sustaining CND.The nitrite formation rate is significantly enhanced in small aggregates of effluent irrigated soils,compared to freshwater irrigation. The difference disappears with increasing aggregate size due thecoupling between nitrite formation under nitrification and its consumption during denitirifcation.Diffusive constraints may introduce significant differences between apparent and gross rates of Ntransformations(e.g., ammonium oxidation, di-nitrogen formation), which can be explained andevaluated by the proposed model.