Investigating Ammoniacal Nitrogen Transport in Subsurface under Alternating Dry-Wet Conditions Using Batch and Column Experiments

Nitrogen and water availability are the primary environmental factors limiting crop productivity on a global scale. Nitrogen behaviour in the subsurface is influenced by multiple factors, including continuous wetting, wetting/drying cycles, temperature, system design, and TWW quality, which are often challenging to quantify. This study examined these dynamics using batch adsorption and a laboratory-scale soil aquifer treatment system, simulated in a glass column filled with agricultural soil, to investigate the effects of synthetic ammonium solution under alternating wet and dry cycles. The study focused on ammonium removal and transformation, specifically  ammonium and nitrate, under varying wetting and drying phases. Constant-concentration synthetic wastewater was introduced, allowing analysis of how soil water content, pH, dissolved oxygen, and nitrogen concentrations impacted the geochemical properties of the soil medium. Batch adsorption experiments indicated strong alignment with Freundlich and Temkin isotherm models, suggesting heterogeneous adsorption sites and varying affinities. pH-edge experiments further revealed that ammonium adsorption was greater in alkaline conditions, indicating a pH-dependent mechanism. The column experiment continued for 52 days, studying three scenarios: (1) continuous flow, (2) alternate day wetting and drying, and (3) three days of drying followed by one day of wetting. Under drier conditions, increased ammonium transformation and sorption occur due to the formation of anoxic zones. Therefore, in the third scenario, anoxic conditions are formed, leading to a greater reduction in hydraulic conductivity. This study offers valuable insights and a strong scientific basis for the protection and management of groundwater and soil quality in agricultural areas.