Wildland fire ash enhances short-term CO2 flux from soil in a Southern African savannah

Abstract

Savannah fires are the largest contributor to global carbon (C) emissions from vegetation fires as a result of their high frequency and the large area burnt each year. Fires not only emit CO2 during the combustion process, they can also lead to enhanced CO2 fluxes from affected soils and vegetation, especially during the initial post-fire recovery period. Wildland fire ash is a ubiquitous product of vegetation fires known to enrich the nutrient content and pyrogenic C (PyC) of the underlying soil, and is expected to stimulate soil C fluxes in the post-fire period. However, the role of ash in post-fire soil C fluxes has rarely been examined. Thus, we investigated the effects of fire and wildland fire ash on CO2 fluxes from fire-affected savannah soils. Soil and ash material were sampled from three sites in an African savannah in the Kruger National Park (South Africa) before and immediately after experimental fires. CO2 fluxes from homogenised natural soils and ash were continuously monitored for 28 d in a laboratory incubation experiment under controlled conditions (20 °C, 60% water-holding capacity). Wildland fire ash enhanced CO2 emissions by up to 3 times compared with pre- and post-fire soils (without ash), likely as a result of the high content of readily available nutrients in the ash. Our results also show that as much as 40% of the total C released over 28 d occurred during a short-lived burst of CO2 (lasting between 20 and 61 h) following wetting of the post-fire soil with ash samples. This study, although based on homogenised soil and ash material, shows that not considering wildland fire ash during post-fire soil C observations will likely misrepresent natural conditions and might lead to underestimations of short-term post-fire C fluxes. The large differences observed between the post-fire soil with ash, and both the pre- and post-fire soil without ash, clearly suggest that ash is a key player in C fluxes and should be taken into account in post-fire C cycling studies. Our results also highlight the importance of high frequency observations during the short-term period following wetting of fire-affected soils, since bursts of CO2 upon wetting of burnt soil can comprise a substantial fraction of total C emissions from burnt savannah soil.