Climate and Topographic Controls On Soil Development Along An Environmental Gradient in the Santa Catalina Mountains, Arizona.

In the hot, dry ecosystems of the southwestern United States, examining climate and hillslope scale controls on soil development is critical to better quantifying landscape evolution and soil carbon storage. The primary objective of this study was to apply the Profile Development Index (PDI) to granitic soils spanning significant range in precipitation, temperature, and landscape position, and to evaluate the findings in conjunction with primary and secondary mineral assemblage. Soil pedons were sampled by genetic horizon from divergent and convergent landscape positions across five distinct vegetation zones spanning the Santa Catalina Mountain Critical Zone Observatory (SCM-CZO) in southern Arizona. The vegetation communities ranged from desert scrub to mixed conifer forest and represented significant shift in temperature (10-24°C) and precipitation (25-85 cm). The PDI was used to quantify field properties including rubificaiton, melanization, color paling, color lightening, total texture, structure, and dry/moist consistence. The quantified field properties were normalized and used to explore changes in soil development as a function of climate and topographic position. Primary and secondary mineral assemblages were measured using semi-quantitative x-ray diffraction (XRD) and were used to validate the PDI results. The resulting calculations confirmed a strong link to water-availability across the SCM environmental gradient where PDI values averaged ~4 (± 2.1, n=4) in the desert scrub sites compared to ~12 (±5, n=4) in the wetter mixed conifer forest ecosystem.  The secondary mineral compositions of the sites support the PDI results. The dry, low elevation field sites were predominately smectite and partially dehydrated halloysite, indicators of water-limited soil environments. In contrast, the high elevation, mixed conifer forests showed evidence of increased mineral transformation where the secondary mineral assemblage was dominated by vermiculite, chlorite, and kaolinite. When comparing PDI values locally at the hillslope scale, there was little difference in PDI between desert scrub divergent (~3) and convergent (~6) landscape positions. However, the PDI in the mixed conifer convergent positions was 2x (~17) that of adjacent divergent sites (~7), suggesting an important topographic control on water availability and soil development in high elevation forests. The results of this work document an important link between water availability and soil development, both regionally across different vegetation communities and locally, at the hillslope scale of study. The research also confirms an important link between quantified field properties calculated using the PDI approach and primary and secondary mineral assemblage along an environmental gradient.