Ammonium Diffusion as a Factor in Nitrogen Loss from Flooded Soils

The role of NH₄⁺-N diffusion in a flooded soil on nitrogen (N) loss through the nitrification-denitrification process was investigated under laboratory conditions. The distribution of applied NH₄⁺-N in both the aerobic and anaerobic soil layers of a flooded soil was experimentally determined and compared with the values obtained from theoretical equations. The total loss of NH₄⁺-N from the flooded soil system (15-cm depth) by nitrification-denitrification was equivalent to 12.43 g N/m² for a 120-day incubation period when the initial concentration of NH₄⁺-N was 44.84 g N/m². Diffusion of NH₄⁺-N from the anaerobic soil layer to the aerobic soil layer accounted for more than 50% (7.16 g N/m²) of the total NH₄⁺-N loss with the remainder being lost from NH₄⁺-N originally present in the aerobic layer. The NH₄⁺-N that diffused upward into the aerobic soil layer was nitrified to NO₃⁻-N, which readily diffused back down into the anaerobic soil layer and was subsequently denitrified. The experimental distributions of NH₄⁺-N were not in close agreement with calculated distributions in the surface aerobic soil layer, but were in close agreement in the anaerobic soil layer. It is possible that the rate constant (k) for NH₄⁺-N oxidation varied considerably with depth in the aerobic soil layer and thus resulted in the disagreement. The total NH₄⁺-N loss calculated from the experimental distributions tended to agree with the values obtained theoretically from rate constant (k) values of 3.18, 5.00, and 6.67 µg cm⁻³ day⁻¹. The first rate constant value was obtained from an independent experiment (for same soil), the second from matching the concentration of NH₄⁺-N at the aerobic-anaerobic layer interface of the theoretical and experimental distribution at 90 days after flooding, and the third from the NH₄⁺-N disappearance in the aerobic soil layer of the soil columns described in this study. These rate constants indicate that the rate of nitrification is one of the factors controlling N loss from flooded soil.