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Wildfires are expected to become more frequent and intensive at the global scale due to climate change. Many studies have focused on the loss of mercury (Hg) from burned forests; however, little is known about the origins, concentration, reactivity, and bioavailability of Hg in residual ash materials in postfire landscapes. We examine Hg levels and reactivity in black ash (BA, low burn intensity) and white ash (WA, high burn intensity) generated from two recent northern California wildfires and document that all ash samples contained measurable, but highly variable, Hg levels ranging from 4 to 125 ng/g dry wt. (n = 28). Stable Hg isotopic compositions measured in select ash samples suggest that most Hg in wildfire ash is derived from vegetation. Ash samples had a highly variable fraction of Hg in recalcitrant forms (0–75%), and this recalcitrant Hg pool appears to be associated with the black carbon fraction in ash …