Seasonal and diurnal CO2 efflux from red wood ant (Formica aquilonia) mounds in boreal coniferous forests

Organic mounds of the red wood ants (Formica rufa group; RWA) have been shown to be “hot spots” of carbon dioxide (CO2) efflux from the European forest soils. However, little information is available on the variability of CO2 effluxes from RWA mounds and on the factors regulating CO2 efflux. We assessed the seasonal and diurnal changes in CO2 effluxes, temperatures and volumetric water contents from mounds of Formica aquilona, the important RWA of the boreal forests in Finland. The daily average CO2 efflux from RWA mounds ranged 1.1-6.9 g CO2 m-2 h-1 during the active ant season (May-September), and from 0.2 to 1.1 g CO2 m-2 h-1 during their dormant period (October-April). Mound CO2 efflux from May to September was 3.6-6.0 times higher than from the surrounding forest floors, and most likely came from RWA respiration. Seasonal changes in mound CO2 effluxes were significantly correlated with mound temperature, but not with volumetric water content (7% on average). The high CO2 efflux associated with increased volumetric water content (up to 34%) after a RWA mound was abandoned indicated that these dry mound conditions restrict microbial decomposition of mound organic matter. CO2 effluxes were highest at night and lowest during the day, which is likely due to an increased ant activity or numbers in the mound at night. Diurnal changes in mound CO2 efflux were negatively correlated with air temperature, and positively correlated with the difference between the mound and air temperature. This suggests that thermal convection of warmer mound air to the colder outside air at night might be also a cause of the diurnal changes. We conclude that seasonal and diurnal variations in mound CO2 effluxes are dependent on RWA activities and fluctuation in RWA mound and outside temperatures.