Impacts of a wildfire on soil organic carbon in Warrumbungle National Park, Australia

Abstract

A wildfire in the Warrumbungle Range in January 2013 burnt 56,290 ha of forest land, 72% of it at high-extreme severity. We investigated the effects of fire on soil organic carbon (SOC), soil carbon fractions (Particulate Organic Carbon (POC), Humus Organic Carbon (HOC) and Resistant Organic Carbon (ROC)) at 64 sites stratified according to geology and fire severity across Warrumbungle National Park. Statistical models were used to identify the main factors controlling the soil chemical parameters and we spatially extrapolated results based on these main factors to estimate the overall impacts of the fire. Statistical models indicated that the key effects on SOC were fire severity and geology/soil type. SOC declined with increasing fire severity − topsoil SOC in low severity sites was 14% lower than unburnt sites, and severely burnt sites were 54% lower than unburnt. There were also significant differences in SOC fractions between the different geology/soil types. These results were also reflected in N and pH changes. The highest SOC values were from unburnt volcanic topsoils. Sandier and especially sandstone-derived soils had less SOC irrespective of the fire severity class. The lowest SOC values were from severely burnt sandstone ridges, where most of the remaining SOC occurs as ROC (including charcoal). Site data was classified according to a fire severity map and geological mapping, and class averages spatially extrapolated to obtain an estimate of the amounts of SOC lost due to the fire. An estimated 1.52 Mt (26.99 t/ha) of SOC was lost over the fire ground to 10 cm. SOC levels in unburnt control sites are much higher than averages in the generally cleared central west of NSW, thus underlining the importance of forested ecosystems in carbon sequestration in soils, and of Warrumbungle National Park with its high proportion of trachytic clayey soils in particular.