Effects of wildfire on long-term soil CO2 concentration: implications for karst processes

Abstract

Wildfires reduce soil CO2 concentration by destroying vegetation and soil-dwelling microbes, thus reducing soil respiration. Post-fire vegetation recovery is primarily determined by vegetation growth forms and modes of regeneration, whereas long-term recovery of soil microbes is largely dependent on vegetation rehabilitation. With previous research focussing on post-fire respiration recovery in the context of CO2 flux between the soil and atmosphere, there is a lack of studies measuring the long-term response of soil CO2 concentration in karst environments. Hence, this study aimed to quantify whether soil CO2 concentration was reduced 5 and 10 years after fires in a karst environment 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 soil CO2 concentration, soil temperature and soil moisture. Samples were taken from a grassland community and woodland community burnt 5 years prior and a forest community burnt 10 years prior. The results showed that soil respiration was depressed in the burnt site relative to the unburnt control in the woodland 5 years post-fire. A vegetation survey indicated that there substantially less biomass in the burnt site relative to the unburnt site. In the forest site 10 years post-fire there was no significant difference in soil CO2 concentration or vegetation between the burnt and control. This demonstrates that soil CO2 concentration takes >5 years to recover to pre-fire levels in woodlands and <10 years in subalpine forests and is determined by vegetation recovery. This long-term reduction in soil CO2 concentration caused by fire has the potential to affect karst subsurface processes governed by soil CO2 which lead to incorrect interpretation of speleothem proxy climate records.