Fire in Organic-Rich Wetland Sediments: Inorganic Responses in Porewater

Declining rainfall and extraction of groundwater increase the vulnerability of wetland sediments to ignition and combustion. This study investigated the existence of a unique hydrochemical porewater signal associated with organic-rich sulfidic sediments that have been overheated, dried, cracked, and burnt, by the passage of fire. Porewater was collected from wetland sediments with recent fire histories, as well as a wetland that had not suffered any type of burn in recent times (i.e. 5+ years). The results show that fire brought about elevated base cation concentrations in addition to substantial increase in oxidation of sulfidic wetland sediments, the generation of acidic porewaters, and the concomitant mobilisation of metal species. These changes were episodic in nature, varying with seasonal fluctuations of groundwater and sediment hydration and saturation, and persistent for at least several years. The seasonally episodic nature of acid generation following fire leads to the depletion of the acid-neutralising capacity of the sediments (potentially faster than would otherwise have occurred as a result of drought-induced acidification events alone) and ultimately exhausts the buffering capacity of the sediments.