Bioturbation-Driven Particle Transport in Surface Soil

Macrofauna-induced bioturbation of soil is dominated by earthworms, among other invertebrates, in most grassland and forest soil. First observed by Darwin, bioturbation drives particle mixing in the upper surface layers, leading to beneficial results to agricultural soils, including enhanced porosity, water permeability, and aeration and improved organic matter and nutrient distributions. Applied pesticides and other chemicals residing on surface soils are transported downward into the soil column by a random mixing of particles. This physical particle diffusion, which conceptually mimics the random mixing of molecular species in fluids, is treated as a Fickian chemical flux mechanism. Using this mechanism, for the mobility rate, while extending the soil-water advection-dispersion model to particles, yields a theoretical approach for obtaining the biodiffusion coefficient (Db). The Db is a numerical soil parameter reflecting biology-induced particle movement and differs significantly from the conventional physical and chemical diffusion coefficients. It is a kinetic parameter with units of square centimeters per year and when used with the bulk density gradient quantifies the soil particle mobility rate within the bioturbated surface layer. Field measurements on soil turnover rates and mixing depth from the literature, including Darwin’s work, were used to produce Db data sets for earthworms, ants, termites, and so on. The highly variable coefficients necessitate lognormal statistics to summarize the findings. However, the average Db values for the three invertebrates were 2.12, 0.39, and 0.75 cm2 year−1, respectively, and surprisingly similar. The need for more field and laboratory data, process-based and species-specific theoretical models, and chemical-based soil Db are discussed.