Fire interacts with season to influence soil respiration in tropical savannas

Soil respiration (Rs) is the second-largest source of CO2 to the atmosphere in terrestrial systems. In tropical savannas seasonal moisture availability and frequent fires drive ecosystem dynamics and may have a considerable impact on soil carbon (C) cycling, including Rs. In order to test the effect of fire on soil C cycling we measured Rs in annually burnt and unburnt plots in wet and dry seasons at a long-term fire experiment established in savanna woodlands of northern Australia. There was a significant interaction between season and fire, with highest rates of daily Rs (722 mmol CO2 m−2 d−1) observed in the wet season on unburnt, leaf litter patches. The three fold higher Rs rate on unburnt plots in the wet season was due to greater root-derived respiration (Rroot: 356 mmol CO2 m−2 d−1), while smaller changes to soil-derived respiration (Rsoil: 51 mmol CO2 m−2 d−1) were simply the result of C moving through decomposition rather than combustion pathways. Relationships between instantaneous Rs and soil temperature showed hysteresis with variable direction, suggesting that season and fire treatment also influence the soil depth at which CO2 is produced. We suggest that (1) changes to fire regimes, through active management or climate change, in tropical savannas could have an impact on Rs, and (2) the direct effect of fire on soil C cycling is limited to the removal of aboveground litter inputs.