Advancing Investigation and Physical Modeling of First-Order Fire Effects on Soils

Heating soil during intense wildland fires or slash-pile burns can alter the soil irreversibly, resulting in many significant long-term biological, chemical, physical, and hydrological effects. To better understand these long-term effects, it is necessary to improve modeling capability and prediction of the more immediate, or first-order, effects that fire can have on soils. This study uses novel and unique observational data from an experimental slash-pile burn to examine the physical processes that govern the transport of energy and mass associated with fire-related soil heating. Included in this study are the descriptions of 1) a hypothesized fire-induced air circulation within the soil, and 2) a new and significant dynamic feedback between the fire and the soil structure. The first of these two hypotheses is proposed to account for the almost instantaneous order-of-magnitude increase in soil CO2 observed during the initiation of the burn. The second results from observed changes to the thermal conductivity of the soil, thought to occur during the fire, which allow the heat pulse to penetrate deeper into the soil than would occur without this change. The first ever X-ray computed tomography images of burn area soils are consistent with a change in soil structure and a concomitant change in soil thermal conductivity. Other ways that current technology can be used to aid in improving physically-based process-level models are also suggested.

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