Reactions in Unsaturated Soils: On the Dynamic Role of Heterogeneity‐Controlled Mixing

Abstract Soils are complex systems where different physical, chemical and biological processes occurring simultaneously and interact in a non‐linear way. Additionally, the high degree of soil heterogeneity poses significant challenges in studying soil reactivity due to its high impact on mixing. In this study we evaluate the effect of soil heterogeneity, hydraulic structure, reaction rate and initial distribution of reactants on effective reaction rates at the plot scale. We tackle this, by explicitly simulating hypothetical biomolecular soil reaction (A + B C) for different degrees of heterogeneity, different hydraulic structures, different reaction rates and finally different initial distribution of the reactants. Results are evaluated in terms of effective reaction rates at the plot scale. The simulation results reveal that mixing conditions control reactions in unsaturated soils. Non‐ideal reactivity due to mixing‐limited conditions is not only a consequence of the simple presence of heterogeneity or even of its intensity. Instead, it results from ( at least ): the characteristics of heterogeneity, the Péclet number, the Damköhler number and the spatial distribution of the reactants. Therefore, reaction rates estimated under a certain set of flow and heterogeneity conditions can't be simply transposed to any other setting.