Effects of Wildfire Intensity on the Clay Mineralogy and Chemistry of California Chaparral Soils: An Experimental and Field Comparison

Wildfires have recently plagued the western United States due to higher temperatures and longer-lasting drought conditions driven by climate change, a problem exacerbated by U.S. Forest Service fire suppression policies that have allowed accumulation of tinder-like woody debris on the forest floor. Five soils were collected along a north-south transect of California chaparral ecosystems and underwent chemical and mineralogical analysis. Unburned and field burned soil samples were collected from five different bedrock parent materials: granite, clastic sedimentary rocks, extrusive volcanic rocks, metavolcanic rocks, and ultramafic rocks. Unburned soils from sites adjacent to burned sites were heated in a muffle furnace at controlled 200°C temperature increments ranging from 200°C to 800°C to assess mineralogical and chemical alterations as a function of burn temperature and compared with corresponding field burn equivalents. X-ray diffraction (XRD) indicated that with increased burn temperature, expandable 2:1 clays (smectite) collapse into nonexpandable illite (i.e. illitization takes place), while kaolinite is destroyed at temperatures between 400-600⁰C, in conjunction with the transformation of hydrous iron oxides (goethite and/or ferrihydrite) to hematite. Soil cation exchange capacity (CEC) showed variable response to fire intensity, with low-to-moderate burns (200-400°C) temporarily increasing CEC, while temperatures >400°C caused significant reductions to CEC, likely due to smectite interlayer collapse (which begins to occur as low as 200°C) and loss of organic matter by combustion (~300°C). Soil pH initially decreases by ~0.5 pH units (up to 200°C), followed by a large increase in pH (1-2 units of pH) with burn temperature >400°C. (Serpentine soil pH increased progressively and did not exhibit a slight pH drop at 200°C). Additionally, soils derived from serpentinite released nickel and chromium at elevated temperatures, highlighting potential environmental risks at certain burn temperatures in these particular soils. The capacity of XRD to assess topsoil temperature affected by wildfire was explored, using experimental burn data as a baseline for assessing field burn temperature. This approach indicates that the chaparral field burn topsoils tended to reach temperatures of ~200°C, and with closer-spacing of temperature intervals in experimental burns (e.g. 100, 150, 200, 250, 300 °C), more precision could be achieved. It is hoped that this study provides useful insights into the thermal alteration of soils, thus contributing to a better understanding of post-fire soil recovery and management.