The role of soil properties in regulating non-equilibrium macropore flow and solute transport in agricultural topsoils

摘要

Dual-permeability models can account for the strong influence of soil macropores on contaminant transport, but their predictive application is hampered, by the difficulty in estimating a priori values for the rate coefficient controlling lateral massexchange between the two flow domains. Our aim was to investigate the possibility of estimating the mass transfer coefficient in the dual-permeability model MACRO from fundamental soil properties. To this end, we calibrated MACRO against transient chloride leaching tests carried out on 33 undisturbed soil columns taken from the topsoils of three agricultural fields, characterized by a wide range of texture and organic matter content. The global search algorithm SUFI was used to derive optimum values ofthe mass transfer coefficient in each column. A Monte Carlo procedure was carried out on two columns to investigate the stability of the estimates in the presence of potential errors in fixed parameters. Despite such errors, c. 50% of the variation in mass transfer for this data set could be explained by two fundamental soil properties: the geometric mean particle size and the organic matter content. Soils of coarser texture and larger organic matter content were characterized by stronger lateral mass exchange and therefore weaker macropore flow. Harrowing and a 6-year grass ley also reduced the extent of non-equilibrium transport. Our results suggest that a robust functional description of the effects of soil structure on chemical transport is possible for predictive management applications of dual-permeability models across larger areas (i.e. mapping leaching risks at field, farm and catchment scales).