Location, Location, Location: Desert Microsites and Soil Property Development.

It is now well established that feedbacks exist between soil properties, ecological functioning, and climate.  In arid alluvial fan deposits, common to the southwestern United States, these complex feedbacks are strongly influenced by microsite, specifically interspaces and sub-canopies locations. Of interest is how these microsite locations influence the two-way coupling of pedological development and ecosystem functions and whether we can predict the strength of these feedback and processes using knowledge of soil systems today. At field sites in both the Mojave and Sonoran Deserts, including a well-defined soil chronosequence, high spatial resolution infiltrometer measurements were taken along transects radiating from canopies of perennial shrubs. We augmented these measurements with ground-penetrating radar (GPR), laboratory analyses, and (in some cases) soil trenches. The results, taken together with studies by others, allow us to correlate biotic versus abiotic processes, against the heterogeneity of soil properties and ecosystem function. Although results showed higher saturated conductivity under canopies regardless of surface age, the largest differences were observed on older, developed soils. Bulk density, soil structure grade, and silt and clay content increased significantly radially from the canopy. Organic content decreased toward interspaces. Trends in soil properties, from canopies to interspaces, were found to be predictable to a distance of 1.35 +/- 0.32 times the canopy radius, regardless of the size or genus of the shrub. Pedologically, these differences (75% of the variance) can be explained by sand content, soil structure grade, mean-particle diameter, and soil organic material.  These trends in reduced hydraulic conductivity and increased water holding capacity in near-surface soil have profound impacts on ability of the canopy to expand—most studies have shown contraction of canopy volumes and ecosystem production in older soils with vesicular horizons and prevalence of surface clasts, both characteristics of desert pavements. GPR and observations in soil trenches highlight soil structural truncation in deeper soils below canopies. In drying climates, without meaningful downward translocation of salts and dust, these features are preserved, essentially forever. However, today’s soil is clearly overprinted by changing climates, availability of source material, and extreme precipitation and flooding events leading to pavement erosion.  This synthesis will illustrate the importance of microsite location when considering complex feedbacks that result through currently-observed, time-dependent processes of soil pedogenesis in arid regions of the desert southwest.