Nitrogen Availability and Sorption under Alternating Waterlogged and Drying Conditions

Abstract

The effect of alternating waterlogged and drying conditions on nitrogen (N) availability and sorption was studied in three soils of contrasting texture. Air‐dry samples of each soil were treated with three levels of nitrate (0, 30, and 300 mg kg‐1; N0, N30, and N300), waterlogged for 21 days, allowed to dry at room temperature for 21 days, and rewaterlogged for an additional 21 days. Ammonium (NH4) sorption isotherms were determined using the standard batch technique for air‐dry, waterlogged (with and without ponded water), and waterlogged/dried conditions. Waterlogging reduced the Eh and increased the pH of all soils. The magnitude of change in Eh and pH was related to soil organic carbon levels and nitrate (NO3) concentration as this chemical became the principal electron acceptor following depletion of dissolved O2. Ammonium concentrations generally increased over the waterlogged periods due to mineralization of organic nitrogen and its subsequent accumulation under O2‐deficient conditions. Where NH4 concentrations decreased, losses were attributed to nitrification and denitrification reactions in aerobic and anaerobic soil zones. Denitrification was assumed to have caused the loss in NO3 over the waterlogged period. Reoxidation caused only small changes in NH4 and NO3. Rewaterlogging increased NH4 concentrations, whereas further losses in NO3 were observed in N300 only. Waterlogging reduced NH4 sorption in the krasnozem A horizon soil, and increased sorption in the wetland soil. Waterlogging did not significantly affect NH4 sorption in the remaining soils. Ammonium sorption was greatest in soils which had been waterlogged then dried. The extent of sorption depended on CEC and the relative affinity of NH4 for the sorption sites. This study has provided further evidence that inclusion of alternating waterlogged and drying conditions in the management of constructed wetlands could represent an effective method of maximizing N removal from wastewaters. Furthermore, results suggest that maximum removal of N would be achieved by optimizing contact time between the wastewater and wetland soil materials, and avoiding overland flow as much as possible.