Mobility of nitrogen in ashes and soils impacted by wildfires in northern California and Nevada

Wildfires drastically alter biogeochemical cycling and transport of nutrient elements, including nitrogen (N), from terrestrial to aquatic ecosystems, with the potential to degrade water quality. Understanding the impact of characteristics of wildfire-derived ashes and burned soils on the mobilization of N is essential for effectively managing wildfires and mitigating adverse effects on watershed functions. This study quantified the mobility of N in soils and ashes influenced by wildfires in the northern California/Nevada region in the western United States (Dixie, Beckwourth, and Caldor fires) and the impact of soil/ash characteristics. The mobile fraction of N ranged from 0.025-0.070 for the ashes, and the mobile fraction of N was composed of 13.1-39.6% as NO₃^-, up to 0.011% as NO₂^-, 0.004-86.9% as NH₃/NH₄⁺, and up to 49.4% as dissolved organic N. The speciation indicates possible nitrification occurring during the wildfires, but suggests no substantial denitrification. The mobile fraction of N was 11.3 ± 7.4 times that of organic carbon (OC), due to the high mobility of inorganic N (mainly NO₃^- and NH₃/NH₄⁺) and nitrogenous organic compounds. The mobile N fraction was associated with redox reactions of iron during wildfires, and was regulated by the redox reactivity of OC. N mobility in the ashes was lower than in control soils, potentially due to the transformation in the speciation of N. However, the total amount of mobile N was increased by wildfire, with the amount of increase being closely related to the severity of wildfires. Overall, wildfires lead to more mobile N, including both organic and inorganic N regulated by redox reactions and severity of wildfires, with subsequent concerns for water quality and water/wastewater treatment processes.