Black and White Fire Ash Alters Greenhouse Gas Emissions and Temporarily Reverses Carbon Source-Sink Status in Aquatic Mesocosms

Wildfire ash is transported in large quantities to receiving water bodies, where it may exert strong chemical controls on ecosystem function. To assess the role of fire ash in altering CO₂ and CH₄ fluxes in aquatic sediments, we designed three mesocosm experiments that compared the changing fluxes of these gases and water quality parameters under different loads and types of ash. Black ash (char) caused substantial drops in pH and increased CO₂ and CH₄ emissions through abiotic and biotic mechanisms, while white ash dramatically increased pH and enhanced CH₄ emissions, possibly due to inhibition of methanotrophy. White ash-driven increases in pH also instigated CO₂ uptake. If this abiotically driven CO₂ uptake could interact with ash-driven nutrient fertilization to synergistically enhance biotic CO₂ uptake in surface waters after a fire, these initial increases in pH may represent an important priming effect. Our findings suggest that strong ash flows following fires may trigger substantial pulses of heterotrophic or abiotically driven greenhouse gas emissions or uptake in recipient lentic aquatic ecosystems, which─although they may be overshadowed by autotrophic responses─may nonetheless be central to altered lake or wetland carbon balance following a fire.