Catastrophic burning of conifer forests during wildfires frequently results in surface temperatures in excess of 600 degrees C. This presents an extreme and unique environment with consequences for weathering of rocks and minerals in the soil. Changes in mineral and rock weathering affect erosion, water infiltration, and nutrient availability and are important impacts of wildfire. This research will address how mineral constituents of the soil react to the extreme heat of forest fires, and how this impacts the physical properties of the soil The research compares weathering characteristics of soil components (minerals and particles) from recently burned sites with weathering characteristics of soils in unburned areas, using the Hayman (Colorado) and Rodeo-Chedeski (Arizona) fires of summer 2002 as case studies. About 25 sites from each fire will be surveyed in areas varying in topography and degree of fire impact. A field survey will assess (1) environmental impacts (erosion, vegetation loss); (2) soil profiles; (3) soil physical properties; and (4) weathering of rock outcrops and cobbles. Samples will be taken throughout the soil profile for laboratory analysis. Laboratory analysis will include: (1) particle size analysis; (2) backscatter electron microscopy to assess mechanical and chemical weathering of soil particles and minerals at small scales; (3) inductively-coupled plasma optical emission spectroscopy to assess the soil's chemical composition and nutrient depletion due to leaching, (4) x-ray diffraction to identify clay minerals and post-fire alteration of these minerals; (5) water loss on heating to identify the amount of water held at the molecular level; and (6) tests using high temperature (oven) exposure of unfired soil samples in order to simulate and model fire conditions. It is expected that severe fires will have a significant impact on weathering of soils including alteration and disintegration of minerals, aggregation and cementing of soil particles, and release and leaching of elements and nutrients from the soil.
A better understanding of soil weathering processes will lead to more comprehensive information for management of the soil and the ecosystem, and aid reclamation efforts following catastrophic fires. This research project will provide a laboratory assessment of weathering characteristics that will be linked to associated field characteristics; this information will be able to be applied by forest and watershed managers. For example, where soil weathering has produced particle disintegration due to enhanced weathering, forest managers might anticipate soils of finer texture, with less water infiltration and higher surface runoff and erosion rates. However, finer soils tend to resist water repellency often observed after fires, and may promote an increase in nutrient availability. These soils would be most suitable for revegetation if surface runoff can be controlled. If fires promote the weathering-related release of cementing agents like iron, aluminum and silica, soil particles could combine into larger aggregates, resulting in a coarser soil. Coarse soils would have higher infiltration rates (with less surface runoff), but higher water repellency, and a greater susceptibility for nutrient leaching. Recovery of such soils from wildfire may be harder. Recognizing how these disparate processes occur is not well understood at present. This project will address these unknowns by applying advanced instrumentation and detailed field survey with modern knowledge sense of how soil, vegetation, and atmospheric systems interact.