Land use change and the impact on greenhouse gas exchange in north Australian savanna soils

Savanna ecosystems are subjected to accelerating land use change as human demand for food and forest products increases. Land use change has been shown to both increase and decrease greenhouse gas fluxes from savannas and considerable uncertainty exists about the non-CO₂ fluxes from the soil. We measured methane (CH₄), nitrous oxide (N₂O) and carbon dioxide (CO₂) over a complete wet-dry seasonal cycle at three replicate sites of each of three land uses: savanna, young pasture and old pasture (converted from savanna 5-7 and 25-30 yr ago, respectively) in the Douglas Daly region of Northern Australia. The effect of break of season rains at the end of the dry season was investigated with two irrigation experiments. Land use change from savanna to pasture increased net greenhouse gas fluxes from the soil. Pasture sites were a weaker sink for CH₄ than savanna sites and, under wet conditions, old pastures turned from being sinks to a significant source of CH₄. Nitrous oxide emissions were generally very low, in the range of 0 to 5 μg N₂O-N m^-2 h^-1, and under dry conditions soil uptake of N₂O was apparent. Break of season rains produced a small, short lived pulse of N₂O up to 20 μg N₂O-N m^-2 h^-1, most evident in pasture soil. Annual cumulative soil CO₂ fluxes increased after clearing, with savanna (14.6 t CO₂-C ha^-1 yr^-1) having the lowest fluxes compared to old pasture (18.5 t CO₂-C ha^-1 yr^-1) and young pasture (20.0 t CO₂-C ha^-1 yr^-1). Clearing savanna increased soil-based greenhouse gas emissions from 53 to ∼ 70 t CO₂-equivalents, a 30% increase dominated by an increase in soil CO₂ emissions and shift from soil CH₄ sink to source. Seasonal variation was clearly driven by soil water content, supporting the emerging view that soil water content is a more important driver of soil gas fluxes than soil temperature in tropical ecosystems where temperature varies little among seasons.