Modeling Oxygen Diffusion in Aggregated Soils: I. Anaerobiosis inside the Aggregates

Abstract Denitrification may appear in anaerobic regions of soil aggregates. In order to evaluate the dependence of anaerobiosis on aggregate shape, on contact with other aggregates, and on surface occlusion by water, a numerical method was used to simulate O 2 diffusion and consumption in three‐dimensional models of water‐saturated aggregates. The aggregates were assumed to be homogeneous in regard to porosity, gas diffusion, and microbial respiration. After equilibration, the calculated anaerobic fractions of simulated aggregates were compared with those obtained for homogeneous spherical ones having the same unblocked external surface area/total aggregate volume ratio. Anaerobiosis of unoccluded aggregates may be predicted by the spherical model with an uncertainty less than ≈5%. When there are occlusions, anaerobiosis may still be predicted by the spherical model for aggregates with a radius greater than some critical radius, but the uncertainty may be as high as 100%. The critical radius corresponds to the radius of the smallest homogeneous spherical aggregate that has an anaerobic center when there is no occlusion of its external surface. For smaller aggregates, we proposed an empirical reduction of the anaerobic fraction predicted by the spherical model in order to predict their actual anaerobic fraction.