The Role of Variable-Charge Minerals in Deep Soil Carbon Storage in a Pacific Northwest Andisol

Soil is the most important long-term sink for carbon (C) in terrestrial ecosystems, containing more C than plant biomass and the atmosphere combined. However, soil has historically been under-represented in C cycling literature, especially in regards to information about subsurface (>1.0 m) layers and processes. Previous research has indicated that Andisols with large quantities of noncrystalline, variable-charge minerals, including allophane, imogolite, and ferrihydrite, contain more C both in total and at depth than other soil types in the Pacific Northwest. The electrostatic charge of variable-charge soils depends on pH and is sometimes net positive, particularly in acid conditions, such as those commonly developed under the coniferous forests of the Pacific Northwest. However, even soils with a net negative charge may contain a mixture of negative and positive exchange sites and can hold some nutrient anions through the anion exchange capacity. The most abundant organic functional groups, including carboxylic and phenolic groups, are anionic in nature, and soil positive charge may play an important role in binding and stabilizing soil organic matter and sequestering C. To increase our understanding of the role of variable-charge minerals in soil organic matter stabilization in deep soils, samples were taken to a depth of 3 m at the Fall River Long-Term Soil Productivity Site in western Washington. This site has a deep, well-drained soil with few rocks, which developed from weathered basalt and is classified as an Andisol of the Boistfort Series. Analysis of soil charge characteristics over a pH range allowed for the determination of anion exchange capacity and point of zero net charge at 8 depth intervals. These results, along with total carbon analysis and C-14 dating at each depth interval, are used to evaluate the importance of the anion exchange capacity as a mechanism for storing carbon at depth in variable-charge soils.