Nitrogen Transformation during Miscible Displacement of Ammonium Nitrate Solution through the Root Zone of Maize

Two NH₄NO₃ solution pulses, each containing 400 mg of NH⁺₄-N and 402 mg of NO⁻₃-N were sprayed and leached separately through a limed and an unlimed 3 m x 3 m field plot with standing maize crop. The loss of NH₃ by volatilization appeared to be negligible in this acid soil. The movement and adsorption of NH⁺₄-N were confined within 0.5 m soil depth under both the conditions. The apparent rate constants of nitrification and nitrate reduction were assessed by curve fitting of the breakthrough curves to known analytical solution of convective diffusion equations governing transport of biodegradable species. The rate of nitrate reduction was about 0.28 x 10⁻⁴ s⁻¹ (0.1 h⁻¹) within the active root zone of maize and drastically reduced at or below the soil depth of 0.5 m. In presence of maize, liming of the soil increased the nitrification rate constant by 1.5 times at a soil depth of 0.15m and about 3 times at a soil depth of 0.5 m in comparison to unlimed soil. On the other hand, the rate of nitrification appeared to decrease exponentially with depth under both limed and unlimed conditions.