Heterogeneity or Optimality? Conceptual arguments and field data suggesting evolution of ecohydrological landscapes

The hydraulic properties of soils are central to the translation of precipitation into stored water to support ecosystems. Most soil hydro-physical characterization has focused on the analysis of <100 cm³ samples of soil, based on particle-size and water flow through cores of <5 cm length. These tests have been broadly predictive in disturbed agricultural soils, but often fail for natural soils without correction for field conditions poorly represented by laboratory samples. We hypothesize since water is essential to life in soil, problematic in either excess or dearth, that the ecosystem community found in natural systems should be expected to tend to alter soil properties specifically to allow sufficient infiltration to have reliable water supply in the soil, and sufficient drainage to maintain adequate oxygen content. This perspective is fundamentally different than the accepted role of climate as being the driver of soil mineral weathering, which makes no prediction that soil hydraulics might specifically adjust to accommodate their climate, or that the primary accommodation of ecosystems to climate is through rooting depth. If this is the case, then these very climatic bounds, rather than value based on texture or properties found in adjacent agricultural fields, might be the best first estimates of natural soil properties. This explains the coincidence of precipitation and transpiration identified by by Oldykop in 1911 and verified since. It also explains the rarity of overland flow in natural soils. These values may alter global climate change predictions where large expanses of natural vegetation are found. Further, it suggests that soil hydraulic properties are innately linked to their climatic setting, and may not well accommodate climatic changes. Finally, we posit that heterogeneity of soil water content, flux, and nutrients have likely been deeply addressed by soil ecosystems, leading to much less loss and greater uniformity than many studies have suggested.