Post-wildfire mobilization of nutritional elements
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Authors
Numan, Travis Ray
Issue Date
2025
Type
Dissertation
Language
en_US
Keywords
nitrogen , organic carbon , soil properties , wildfire
Alternative Title
Abstract
Wildfires significantly alter watershed dynamics by mobilizing organic carbon (OC) and nitrogen (N) from terrestrial to aquatic ecosystems, impacting watershed functions and water quality. This Ph.D. project investigated both immediate (0-year) and extended (2-year) effects of wildfires on OC and N mobilization in soils and ashes from three major wildfire sites in northern California and Nevada (Dixie, Beckworth, and Caldor fires), emphasizing the role of physicochemical properties of ashes and soils as well as aging process in controlling nutrient mobilization over time.Immediately after the fires, ashes had elevated bulk OC (9.2 – 57.3 mg/g) and N (2.81 – 5.15 mg N/g) contents compared to control soils. Mobile fraction of OC ranged from 0.0093 to 0.029 in ashes and 0.010 to 0.065 in soils. The total mobile OC was substantially increased by wildfires, when the redox reactions played an important role. Wildfire increased the total amount of mobile OC substantially by 5.2 – 574% compared to control soils.
The mobile fraction of N ranged from 0.025 to 0.070 in ashes, with NO3-, NO2-, NH3/NH4+, and organic N representing distinct contributions. Ashes exhibited lower N mobility than soils, potentially due to transformations in N speciation. Notably, the mobile fraction of N was approximately 11 times the values for OC reflecting the greater solubility and high mobility of inorganic and nitrogenous organic compounds post wildfire. The mobile fraction of N was associated with redox reactions of iron (Fe) during wildfires, regulated by the redox reactivity of OC. Similarly as OC, total amount of mobile N was increased by wildfires, for which the relative degree was closely related to the severity of wildfires.
Two years after wildfires, OC mobility ranged from 0.009 to 0.114 in ashes and 0.002 to 0.582 in soils, with a higher aromatic fraction in mobile OC from ashes (40.0 – 65.3%) than control soils (23.3 – 47.0%). The mobility of OC exhibited a significant negative correlation with the aromatic fraction in mobile OC for 2-year samples, not present for 0-year samples, indicating the more important role of aromatic OC in its mobility after aging.
The mobile fraction of N in ashes ranged from 0.052 to 0.118, with significantly higher NO2- concentrations than 0-year samples, indicating substantial denitrification over time. Dissolved organic nitrogen (DON) mobilized from aged ashes was also markedly higher than in fresh samples. Redox reactions played an important role for the mobile fraction of OC and N, when X-ray absorption near edge spectra showed reduction of iron (Fe) through the aging process. The impact of aging on the mobility of OC was closely dependent on the aromatic carbon in ashes and soils impacted by wildfire.
Overall, these findings demonstrate both immediate and prolonged effects of wildfires on OC and N mobilization, with implications for downstream water quality. The sustained export of OC and N, influenced by physicochemical properties and aging, posed risks for eutrophication and disinfection byproduct formation. This study underscores the need for watershed management strategies that address both short- and long-term impacts of wildfires on nutrient dynamics in fire-prone regions.