Structure formation and its consequences for gas and water transport in unsaturated arable and forest soils

The aim of this paper is to clarify the effect of soil aggregation on physical and chemical properties of structured soils both on a bulk soil scale, for single aggregates, as well as for homogenized material. Aggregate formation and aggregate strength depend on swelling and shrinkage processes and on biological activity and kinds of organic exudates as well as on the intensity, number and time of swelling and drying events. Such aggregates are more dense than the bulk soil. The intra-aggregate pore distribution consists not only of finer pores, but these are also more tortuous. Therefore, water fluxes in aggregated soils are mostly multidimensional and the corresponding water fluxes in the intra-aggregate pore system are much smaller. Furthermore, ion transport both by mass flow and diffusion are delayed, whereby the length of the follow path in such tortuous finer pores further retards chemical exchange processes. The rearrangement of particles by aggregate formation also induces an increased apparent thermal diffusivity as compared with the homogenized material. The aggregate formation also affects the aeration and the gaseous composition in the intra-aggregate pore space. Depending on the kind and intensity of aggregation, the intra-aggregate pores can be completely anoxic, while the inter-aggregate pores are already completely aerated. The possibility to predict physical properties on these various scales depends on the rigidity of the pore system. In general, this rigidity depends on the above-mentioned physical and chemical processes both with respect to intensity and frequency. Thus, aggregates like subangular block or plates are the most rigid systems, while physical processes in the seedbed cannot be modelled using existing approaches due to the low strength and consequent instability of the pore system.