The Effect of Charcoal on Soil Hydrologic Properties Under Natural and Elevated Concentrations

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Soil charcoal has been shown to play a key role in the dynamics of nutrients in soils [e.g. Liang et al., 2006], but our understanding of the effects of charcoal on soil physical and chemical properties is quite limited. Particularly absent are both data and a conceptual framework to explain observed charcoal alteration of soil hydrologic properties. Understanding the effects of charcoal on soil is increasingly important because humans are changing soil charcoal content both unintentionally and intentionally, the first driven by fire management practices and land use change, and the second by intentional amendment of soil with charcoal to improve crop performance and sequester carbon. We hypothesize that charcoal affects soil hydrologic properties through alteration of soil grain size, charge density, and hydrophobicity. We predict that grain size will change mechanical interactions (e.g., pore size, tortuosity), while charge density will change physico-chemical interactions which impact strength, soil water potential, and flow pathways. Hydrophobicity is a function of both surface chemistry and surface area (controlled partly by grain size and porosity), and will respond with changes in these properties. Charcoal grain size decreases and surface charge density increases with in situ weathering so the impacts will be vary significantly with time as charcoal is incorporated into the soil clay fraction, which itself can be altered by the release of inorganic materials during charcoal weathering. We are conducting a series of experiments designed to determine the mechanisms through which charcoal affects hydrologically-relevant physical and chemical properties of soils. We are performing experiments with natural and laboratory soils mixed with charcoal, measuring (1) soil hydrophobicity, (2) surface and bulk chemistry of soils and charcoals, (3) saturated permeability, (4) soil water potential, and (5) soil tensile strength. Our ultimate goal is to develop a conceptual model of the soil-charcoal-water system.