CO₂, CH₄ and N₂O fluxes from soil of a burned grassland in Central Africa

Abstract. The impact of fire on soil fluxes of CO₂, CH₄ and N₂O was investigated in a tropical grassland in Congo Brazzaville during two field campaigns in 2007–2008. The first campaign was conducted in the middle of the dry season and the second at the end of the growing season, respectively one and eight months after burning. Gas fluxes and several soil parameters were measured in each campaign from burned plots and from a close-by control area preserved from fire. Rain events were simulated at each campaign to evaluate the magnitude and duration of the generated gas flux pulses. In laboratory experiments, soil samples from field plots were analysed for microbial biomass, net N mineralization, net nitrification, N₂O, NO and CO₂ emissions under different water and temperature soil regimes. One month after burning, field CO₂ emissions were significantly lower in burned plots than in the control plots, the average daily CH₄ flux shifted from net emission in the unburned area to net consumption in burned plots, no significant effect of fire was observed on soil N₂O fluxes. Eight months after burning, the average daily fluxes of CO₂, CH₄ and N₂O measured in control and burned plots were not significantly different. In laboratory, N₂O fluxes from soil of burned plots were significantly higher than fluxes from soil of unburned plots only above 70% of maximum soil water holding capacity; this was never attained in the field even after rain simulation. Higher NO emissions were measured in the lab in soil from burned plots at both 10% and 50% of maximum soil water holding capacity. Increasing the incubation temperature from 25 °C to 37 °C negatively affected microbial growth, mineralization and nitrification activities but enhanced N₂O and CO₂ production. Results indicate that fire did not increase post-burning soil GHG emissions in this tropical grasslands characterized by acidic, well drained and nutrient-poor soil.