Effects of forest fires on mercury biogeochemical cycling in terrestrial and aquatic ecosystems

"Mercury (Hg), a persistent and toxic element, is largely stored in forests including forest canopy and surface soils. Therefore, forest disturbances such as wildfires (natural) and prescribed fires (anthropogenic, forest management practice) would interfere with Hg storage in forests and its export to downstream aquatic environment where is a hotspot for methylmercury (MeHg) production by the anaerobic bacteria. Although the two types of forest fires are different in intensity, frequency and duration, earlier studies have shown both forest fires increased Hg emissions from the vegetation and surface soils, shifting forests from “Hg sinks” to “Hg sources”. However, little is known about the Hg reactivity and bioavailability in the burned materials on the forest floor, and its transport pattern to the downstream watersheds. Part I of this dissertation work (chapter II) examined Hg content, origin, reactivity, bioavailability in wildfire ash and potential effects of ash on Hg fate in the downstream aquatic ecosystems. It has been demonstrated that ash samples generated from two recent northern California wildfires contained measurable but highly variable Hg, most of which were shown by stable Hg isotopic compositions to be derived from vegetation, not from the atmospheric Hg deposition. Ash samples had a highly variable fraction of Hg in recalcitrant forms (0-75 %), and this recalcitrant Hg pool appeared to be associated with the black carbon fraction in ash. Importantly, ash could strongly sequester aqueous inorganic Hg and result in low methylation potential in the aquatic environment. Part II of this dissertation work (chapter III) showed Hg dynamic export in three controlled-field-observation studies, including wildfire-burned watersheds and prescribed-fire-burned watersheds. During the two-year monitoring study in the wildfire-burned watersheds in northern California, highly elevated total Hg (THg) input, mainly driven by total suspended solids (TSS), was observed in the burned watersheds compared to the unburned watershed, especially in the first year following the wildfire, with rapid recovery in the second year. In contrast, much less Hg and TSS exports were observed in stream water in both the prescribed-fire-burned (pile burning) watersheds in northern California (Sagehen Experimental Forest) and the prescribed-fire-burned (broadcast burning) watersheds (Santee Experimental Forest) in South Carolina following the prescribed fires. Particulate Hg (PHg) dominated in the wildfire-burned watersheds Hg export while dissolved Hg (DHg) dominated in the prescribed-burned watersheds. Dissolved organic carbon (DOC), other than TSS, was the main driver carrying DHg contributing to the THg export in the prescribed-fire-burned watersheds in the present study. This dissertation work showed alterations in the biogeochemical pool of Hg in wildfire ash materials in post-burn landscapes, as well as the post-burn hydrological responses of Hg transport by wildfires and prescribed fires. Regarding the Hg export to the downstream environment, prescribed fires result in lower input than the wildfires. This work increases our understanding of Hg biogeochemical cycling by natural and anthropogenic forest fires and provided evidence and suggestions to resources managers in forest management agencies and fishery consumption advisories."--Abstract from author supplied metadata.