Soil organic carbon and related impacts of the Warrumbungles wildfire

Dr Mitchell Tulau1, Dr Xihua  Yang2, Ms Robin McAlpine2, Dr Mano Veeragathipillai3, Dr Senani Karunaratne2, Mr Mingxi Zhang2, Ms Sally McInnes-Clarke4

1NSW Office of Environment and Heritage, Port Macquarie , Australia, 2NSW Office of Environment and Heritage, Parramatta, Australia, 3NSW Office of Environment and Heritage, Yanco, Australia, 4NSW Office of Environment and Heritage, Gosford, Australia

A wildfire in the Warrumbungle range in January 2013 burnt 56,290 ha, 72% of it at high-extreme severity. We examined the effects of the fire on soil organic carbon (SOC), nitrogen and soil microbial activity, spatially extrapolated results to estimate the overall impacts of the fire and estimated the long-term post burn recovery trajectory. We measured SOC fractions at 64 sites across four fire severity classes and three main geology/soil types. Soils were sampled for LECO TOC, MIR soil C fractions and N (64 sites x 5 sub-sites x 4 depths) and soil microbial activity by MicroResp across the range of fire severities. Topsoil SOC in low severity sites was 14% lower than unburnt sites, and severely burnt sites were 54% lower. These results were also reflected in losses in N and microbial activity. Statistical models indicated that the key effects on SOC were fire severity and geology/soil type. The highest SOC values were from unburnt volcanic topsoils. Sandier and especially sandstone-derived soils have 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 resistant OC (including charcoal). Site data was extrapolated by 1m LiDAR DEM and a fire severity map based on RapidEye and ADS40 imagery. It was estimated that 2.5 Mt (44 t/ha) of SOC was lost over the fire ground to 10 cm, with ~74,000 t of N lost. Spatial fire history data were used to estimate rates of SOC accumulation and a timeline for long-term recovery of SOC in the absence of fire. Recovery of SOC to pre-burn levels is expected to take several decades.


Mitch has 30 years’ experience in soil survey and acid sulfate soils in south-eastern Australia, and 10 years in fire-related assessment and research. He is a founding member of the Burnt Area Assessment Team, which assesses post-fire risks in eastern Australia, and is the author of the most comprehensive review of the impacts of fire on soils in Australia. He designed and led the soils and geomorphology research projects following the 2013 Warrumbungles wildfire, and is currently scientific advisor to the Rural Fire Service on peat fires.

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