Dual nature of soil structure: The unity of aggregates and pores

Soil is a hierarchical, self-organizing, and emergent system that supports plant and microbial growth, enables carbon sequestration, facilitates water fluxes, and provide habitat for microorganisms, all of which depend on soil structure. Recent debates have generally reduced soil functioning to geometry and topology of soil solids and pores and denied the existence and role of soil aggregates and hierarchy of solids. Here we argue that soil structure has a dual nature that essentially boils down to the interlocking of pores and solids in groupings of specific complexity and dynamics called aggregates. By comparing their architectural, chemical, and energetic parameters, we conclude that aggregates have a much higher information density than pores. Therefore, aggregates (as unity of solids and pores) perform much broader range of functions compared to pores alone, especially in long-term. A set of soil functions corresponding to each level of the soil structure hierarchy depends on aggregate type (macroaggregates, water-stable aggregates, microaggregates, and elementary soil particles) determined by their specific binding energy, dynamics, and lifetime. The introduced here energy-based concept justifies the hierarchy of soil structure, and is the base for the soil structuring and carbon stabilization processes in their most general form. We understand the soil structure implying the energy-based approach: each hierarchy level corresponds to specific bonding strength of mineral and organic particles forming aggregates. Aggregate formation is a bottom-up process because the energy binding elementary soil particles and microaggregates is orders of magnitude higher than that gluing macroaggregates. The duality of soil structure is manifested not only in the relationship between pores and solids in aggregates, but also in the interactions and competition between the biological and non-biological processes that aggregate and disaggregate the structure. The view of the pore space as a transport pathway and habitat for soil living phase and plant roots, the solid-pore interface as a setting for physico-chemical and biological transformations, and aggregates as a result of these phenomena, provides a context for mechanistic understanding and process-based modeling of soil functions and health.