COS 53-5
Climate and parent material controls on soil formation in the Gallatin Valley, MT
The Langohr Campground lithosequence compares soils on gneiss, sandstone, shale, limestone, and volcanic rocks within a 2.6 km transect ~16 km south of Bozeman, Montana. In this short distance, vegetation fluctuates between grassland meadows and lodgepole pine (Pinus contorta) forests. A state factor approach was used to gain insights into soil processes: all other state factors (mean annual precipitation 76 cm, mean annual air temperature -1.1⁰C, west aspects, 15 to 20% slopes, est. soil age ~20 ky) are held constant except lithology. The lithosequence seeks to determine the importance of rock to soil formation. Further north, the Bridger Climosequence studies soils on Cambrian to Carboniferous aged limestone in the Bridger Range. Nine climosequence sites range from ~1400 to 2900 meters above mean sea level across a 35 km transect. Modeled effective precipitation ranges from 35 to 120 cm suggesting a range of climates. The climosequence asks how climate impacts soil formation in the Gallatin Valley and the Bridger Range. All 14 profiles were characterized by morphologic horizon to bedrock for field and laboratory measurements, including immobile elements to quantify chemical depletion fractions (CDF), or the fraction of total mass loss that can be attributed to chemical weathering (versus physical erosion).
Results/Conclusions
In the lithosequence, soil textures and coarse fragment content varied most: soils over gneiss and sandstone showed coarser textures and higher coarse fragments (up to 80%) compared to the fewer coarse fragments (<5%) and clayey textures of the other soil profiles. Geochemical differences were also apparent: CDF ranged from ~0.2 for the basaltic andesite soil to ~0.9 for the limestone soil. We conclude that underlying rock strongly influences soil properties, but in variable ways across this lithosequence. This influence appears to be both direct and indirect: while chemical weathering of rock leads to compositional changes in overlying soil horizons, it also produces coarse fragments that appear to greatly influence soil hydrology. Across the climosequence, CDF averaged 79%, consistent with the weatherability of limestone. CDF ranged from an anomalously low value of ~0.3 at 2100 m to much higher values of ~0.98 at both 2300 m and 2600 m. These high CDF at high elevations is surprising given the expectation that physical erosion associated with freezing and thawing would play a greater role in soils at these elevations. Additional mineralogic analyses may account for these discrepancies.