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Contamination of groundwater by nitrate is a worldwide environmental issue. A better knowledge of nitrate sorption characteristics by soils contributes to efficient fertilizer use and prevents aquifer contamination. In volcanic soils, nitrate sorption is induced by variable charges due to the presence of amorphous materials and aluminum (Al) and iron (Fe) oxides. Anion transport in packed and intact columns was investigated in a Mexican Allophanic Andisol, under different permanent flow regimes in unsaturated conditions and several NO3-‐N and Br- input concentrations. In the packed columns, the NO3-‐N adsorption in the soil was nonlinear. In the intact columns, the retardation coefficient variation was directly correlated to the increase of amorphous material with depth. The presence of preferential flow in the intact columns significantly increased the mobility and velocity of nitrate moving through the columns, whereas in the packed columns, NO3-‐N fate was only affected by soil chemical composition and mineralogy. [ABSTRACT FROM AUTHOR]

Allophanic top‐ and subsoils from the Mexican and Newzealand Central Volcanic Plateau, as well as a nonallophanic sandy loam soil, were sampled to study the impact of organic matter and allophane content on 2,4‐D fate. High sorption rates were found, especially in the two topsoils from Mexico and New Zealand, with distribution coefficient (Kd) obtained from displacement experiments in packed columns equal to 7.61 and 8.43 L kg-1 respectively. 2,4‐Dichlorophenoxyacetic acid transfer through the soil columns was found to be in chemical nonequilibrium and was well predicted using a two‐site sorption model. For the two allophanic top soils, Kd obtained from batch was very different to the Kd obtained from column experiments. Either the equilibrium could not be reached in batch or the two‐site model was not able to describe the wide range of sorption sites present in the highly reactive organic matter and allophane components. [ABSTRACT FROM AUTHOR]