Minerals and Fire: Controls On Soil Carbon Storage in Western Conifer Forests.

Answering the question of what controls the fate and stabilization of organic carbon in forest soils is central to understanding the role of western US ecosystems in mitigating climate change, optimizing forest management, and quantifying local and regional terrestrial carbon budgets.  Over half of forest soil C is stored belowground, stabilized by a number of separate, but interacting physical, chemical and biological mechanisms. We undertook a series of coupled field and laboratory studies focused on identifying mineral, physical, and landscape position controls on belowground C stabilization mechanisms in Western U.S.  Results from these studies demonstrate an important for role for short-range-order Fe- and Al-oxyhydroxides and Al-humus complexes in C stabilization, and that the soil mineral assemblage moderates C cycling via control on partitioning of C into physical fractions (“free”, “occluded”, “mineral”) with varying MRT and chemistry.    Measures of occluded fraction chemical composition by ¹³C-NMR indicate this fraction is 2-5 times more enriched in pyrogenic C than the bulk soil and that this fraction is on the order of ~25 to 65% charred materials.  Radiocarbon analyses of a large set of conifer soil samples from California and Arizona further indicate the occluded fraction is generally older than either the free light or mineral fraction.  In particular, soil C in convergent, water and sediment gathering portions of the landscape are enriched in long MRT charred materials.  These results indicated that in Western conifer forests drought and wildfire control belowground C storage through biochemical and physical changes in aboveground biomass, release of CO_2, ash and deposition of pyrogenic C, and indirectly via post-fire physical erosion and redistribution of C-rich sediment across the landscape.