Effects of fire on soil CO2 - Implications for karst processes

Abstract:

Fire reduces soil CO₂ concentration by destroying vegetation and soil dwelling microbial communities thus reducing both heterotrophic and autotrophic contributions to soil respiration. While reductions in microbial respiration are short-lived, root respiration can take longer to recover, depending on the dominant growth forms (e.g. trees vs herbs) and modes of post-fire recovery (e.g. resprouting vs seedling establishment). This study aimed to quantify whether soil CO₂ concentration was reduced ~5 years and ~10 years after a fire in a subalpine karst environment in south-eastern Australia and to consider the implications for karst dissolution processes and speleothem growth rate. Paired sites with burnt and unburnt soil were compared with regards to CO₂ concentrations, soil moisture and soil temperature. Samples were taken from a grassland site and woodland site burnt ~5 years ago and a woodland site burnt ~10 years ago. The results showed that soil respiration was depressed in burnt sites relative to the unburnt pair in both the grassland and woodland sites after ~5 years; however, after ~10 years there was no significant difference between the burnt and control woodland sites. This indicates that soil CO₂ concentration takes between 5-10 years to return to pre-fire levels in subalpine environments in south-eastern Australia. In a karst environment, this long-term reduction in soil CO₂ concentration caused by fire is likely to cause a decreased stalagmite growth that could be incorrectly attributed to multiannual climatic drying, such as the El Nino Southern Oscillation, when reconstructing past environmental change from stalagmite records.