Mineral Control of Soil Organic Carbon Sequestration: Examples from Temperate Conifer Forests of the Western United States

Global and regional terrestrial carbon budgets indicate temperate forest soil organic carbon (SOC) accounts for a substantial portion of overall carbon stocks. The role of forest SOC as a source or sink for atmospheric CO₂ remains unclear and depends on complex interactions between climate, soil and plant litter. It is therefore critical to understand the controls of SOC sequestration in these ecosystems and their potential role in moderating atmospheric CO₂ levels. We undertook a series of coupled field and laboratory studies focused on identifying mineralogic controls of SOC sequestration across a range of forest-types in the Western U.S. Results from these studies demonstrated that the soil mineral assemblage moderates SOC sequestration via control of: (i) partitioning of SOC into physically defined pools with varying mean residence time; (ii) cumulative SOC mineralization and mineralization rates; and (iii) SOC 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 demonstrating the importance of mineralogic controls of SOC cycling. These studies all indicate a significant role for Al, including short-range-order Al-oxydrydroxides, Al-humus complexes, and exchangeable Al, in controlling the sequestration of SOC in temperate conifer ecosystems of the Western U.S. The importance of mineralogic SOC cycle controls are likely relevant for understanding the capacity of degraded mine lands to sequester SOC.