Modification of Soil Hydroscopic and Chemical Properties Caused by Four Recent California, USA Megafires

Abstract

While it is well known that wildfires can greatly contribute to soil water repellency by changing soil chemical composition, the mechanisms of these changes are still poorly understood. In the past decade, the number, size, and intensity of wildfires have greatly increased in the western USA. Recent megafires in California (i.e., the Dixie, Beckwourth Complex, Caldor, and Mosquito fires) provided us with an opportunity to characterize pre- and post-fire soils and to study the effects of fires on soil water repellency, soil organic constituents, and connections between the two. Water drop penetration time (WDPT) tests performed in the field showed a significant increase (from <1 s up to >600 s) in WDPT from pre- to post-fire soils. This increase in soil water repellency after fires was confirmed by increases in apparent contact angle (ACA) between 1.1 and 9 times from unburned to burned soils. The chemical characterization of burned soils with high resolution mass spectrometry showed the increased abundance of hydrophobic organics (e.g., PAH-like compounds and organic molecules with a low number of oxygen atoms) as well as the correlation of the average H/C ratio and aromaticity index (AI) with ACA. Most likely, these compounds contribute to post-fire soil water repellency that triggers hydrological effects such as landslides, flooding, and debris flows.

Hyperspectral Reflectance and Chemical Composition of Pre- and Post-Fire Soils from Three 2021 Western USA Megafires

“…Based on visual observation of the areas, medium-and high-severity burned areas were chosen for ash and soil sampling. Approximately 10 to 100 g of each of the samples, including ash, and burned and unburned soil, were collected into aluminum foil envelopes and were kept in a sample cooler during transport to the laboratory [3]. The ash samples were collected from the ash layer above the soil.…”

“…Over the past two decades, wildfire activity has increased in terms of frequency, intensity, and size in the western USA, especially in California and Nevada [1][2][3][4]. This has been attributed to climate change causing rising temperatures and altered precipitation patterns [1,3], but also to population growth, especially in the Wildland Urban Interface (WUI) [5][6][7][8]; accumulation of fuels due to historical fire suppression [5,6]; and extended drought conditions and heat waves [5,6]. As part of the increase in fire size, western USA megafires (>160 km 2 in size [7]) [2,[9][10][11] have increased in frequency by up to 1000% over the past 40 years [3] despite increased suppression efforts.…”

“…This has been attributed to climate change causing rising temperatures and altered precipitation patterns [1,3], but also to population growth, especially in the Wildland Urban Interface (WUI) [5][6][7][8]; accumulation of fuels due to historical fire suppression [5,6]; and extended drought conditions and heat waves [5,6]. As part of the increase in fire size, western USA megafires (>160 km 2 in size [7]) [2,[9][10][11] have increased in frequency by up to 1000% over the past 40 years [3] despite increased suppression efforts. Roughly 5224 km 2 of land were burned by three 2021 California megafires: the Dixie, Beckwourth Complex, and Caldor fires [3].…”

“…As part of the increase in fire size, western USA megafires (>160 km 2 in size [7]) [2,[9][10][11] have increased in frequency by up to 1000% over the past 40 years [3] despite increased suppression efforts. Roughly 5224 km 2 of land were burned by three 2021 California megafires: the Dixie, Beckwourth Complex, and Caldor fires [3]. For example, the 2021 Dixie fire was the largest single wildfire in the history of California and burned 3898 km 2 of land, mostly conifer forest [7,12].…”

“…For example, the 2021 Dixie fire was the largest single wildfire in the history of California and burned 3898 km 2 of land, mostly conifer forest [7,12]. Accelerated runoff and soil erosion were two major consequences of this and other recent California wildfires [3], which were largely caused by the formation of water-repellent soils during these fires [8,[12][13][14][15][16]. Previous studies discussed changes in soil chemical and physical properties caused by wildfires [8,13,[17][18][19][20].…”