On the use of solute water fronts to measure nitrate adsorption in a Red Ferrosol

Abstract

Nitrogen leaching in soils depends on the mobility of nitrate with, and relative to, water. This mobility is affected by anion adsorption at soil surfaces. Our work examines nitrate (NO3−) adsorption during non‐steady, unsaturated absorption of NO3− solution by columns of dry Red Ferrosol (Inceptic Eutrudox), approximating solute transport in unsaturated field soils. The experiments, in which no water flows past the end of the column, show that the application of conventional water and reactive solute flow equations is valid. The profiles of water and NO3−, measured at different horizontal infiltration times, coalesce on to unique curves when plotted as a function of the Boltzmann variable, λ. Anion adsorption delays the NO3− advance relative to that of water. Pragmatically, we use Freundlich and Langmuir equations as convenient mathematical descriptions of how the NO3− is partitioned between the soluble and adsorbed concentrations and conclude that the latter is appropriate for our dataset. We advocate the use of the Langmuir equation (non‐linear form) to obtain the fitting parameters to avoid the need to use ratios of small values, which results in large errors. Absorption of a solution into unsaturated columns of soil provides a powerful method for exploring adsorption: the experiment implicitly tests the water flow equation, the salt flow equation and solute equilibrium between the water‐soluble and adsorbed phases.