Pedogenesis & Carbon Dynamics Across a Lithosequence Under Ponderosa Pine

Abstract

Three studies were completed to investigate the influence of mineral assemblage on soil organic carbon (SOC) cycling and pedogenesis in forest soils. Two studies utilized a lithosequence of four parent materials (rhyolite, granite, basalt, limestone/volcanic cinders) under Pinus ponderosa, to explicitly quantify the contribution of parent material mineral assemblage to the character of the resulting soil. The first study explored variation in pedogenesis and elemental mass loss as a product of parent material through a combination of quantitative X-ray diffraction and elemental mass balance. Results indicated significant differences in degree of soil development, profile characteristics, and mass flux according to parent material.The second study utilized the same lithosequence of soils, but focused on organic C cycling. This study explored variation in SOC content among soils of differing mineralogy and correlations among soil physiochemical variables, SOC content, soil microbial community composition and respiration rates. Metal-humus complex and Fe-oxyhydroxide content emerged as important predictors of SOC dynamics across all parent materials, showing significant correlation with both SOC content and bacterial community composition. Results indicated that within a specific ecosystem, SOC dynamics and microbial community vary predictably with soil physicochemical variables directly related to mineralogical differences among soil parent materials.The third study focused specifically on the influence of goethite and gibbsite on dissolved organic matter characteristics and microbial communities which utilize DOM as a growth substrate. Iron and aluminum oxides were selected for this study due to their wide spread occurrence in soils and their abundance of reactive surface area, qualities which enable them to have a significant effect on SOC transported through forest soils. Results indicated that exposure to goethite and gibbsite surfaces induces significant differences in DOM quality, including changes in thermal properties, molecular structure, and concentrations of P and N. Investigation of the decomposer communities indicated that exposure to goethite and gibbsite surfaces caused significant differences in microbial community structure.These investigations emphasize the important role of mineral assemblage in shaping soil characteristics and regulating the cycling of C in soils, from the molecular scale to the pedon scale.