Geologic Control of Soil Carbon Stabilization – a Dynamic Interaction Among Physical, Chemical and Biological Processes.

The role of forest soil carbon (SOC) as a source or sink for atmospheric CO₂ remains unclear and depends on complex interactions among climate, geology, and plant litter.  We undertook a series of coupled field and laboratory studies focused on identifying mineralogic controls of carbon sequestration across a range of geologic materials and forest-types in the Western U.S.  Results from these studies demonstrate that the soil mineral assemblage moderates carbon sequestration via control of: (i) partitioning of carbon into physical fractions with varying mean residence time; (ii) carbon mineralization and mineralization rates; and (iii) carbon response to increased temperature and litter additions.  In particular, soils enriched in short-range-order Fe- and Al-oxyhydroxides and Al-humus complexes exhibited greater partitioning of SOC into stable pools and significantly reduced SOC mineralization relative to soils dominated by crystalline minerals.  Ongoing research in Pinus ponderosa ecosystems of Arizona indicates variation in both the microbial community and SOC dynamics with increasing soil acidity and Al availability, further indicating the importance of mineralogic controls of SOC cycling.  These studies all demonstrate a significant role for Al, including short-range-order Al-oxydrydroxides, Al-humus complexes, and exchangeable Al, in control of stabilization of SOC in temperate conifer ecosystems of the Western U.S.  Geology and soil mineralogy are therefore fundamental controls of forest soil carbon sequestration and must be considered in landscape and regional scale models of soil carbon sequestration.