Spatial Heterogeneity in Redox Processes Governing Soil Biogeochemical Cycles.

Soils are a complex physical assemblage of mineral, organic, and biological matter. As a consequence, biogeochemical cycles within soils are governed by a combination of chemical and biological factors, as constrained by physical aspects of the system, that are effectively inseparable.  Macropores, with low tortuosity and high continuity, in combination with micropores, with high tortuosity and discontinuity, results in non-equilibrium flow of water (preferential flow).  Solutes (e.g. nutrients and dissolved organic C) and gas (e.g. oxygen, CO₂, and methane) migrates via advection through macropores, in contrast to diffusion away from the flow channels into the soil matrix.  As a consequence of diffusive constraints within the soil matrix, a diversity of operative respiratory pathways transpire, leading to denitrification and methonogenesis, for example, being detected in seemingly aerated soil conditions.  Further, biological activity along major flow channels are typically higher compared to bulk soil matrix (ped interior) as a consequence of better accessibility to oxygen, nutrients, and substrates.  As a consequence, the biogeochemical cycling of elements ranging from carbon through uranium within soils is critically dependent on the spatial variation in redox process.