Savanna ecosystems comprise 22% of the global terrestrial surface and 25% of Australia (almost 1.9 million km2) and provide significant ecosystem services through carbon and water cycles and the maintenance of biodiversity. The current structure, composition and distribution of Australian savannas have coevolved with fire, yet remain driven by the dynamic constraints of their bioclimatic niche. Fire in Australian savannas influences both the biophysical and biogeochemical processes at multiple scales from leaf to landscape. Here, we present the latest emission estimates from Australian savanna biomass burning and their contribution to global greenhouse gas budgets. We then review our understanding of the impacts of fire on ecosystem function and local surface water and heat balances, which in turn influence regional climate. We show how savanna fires are coupled to the global climate through the carbon cycle and fire regimes. We present new research that climate change is likely to alter the structure and function of savannas through shifts in moisture availability and increases in atmospheric carbon dioxide, in turn altering fire regimes with further feedbacks to climate. We explore opportunities to reduce net greenhouse gas emissions from savanna ecosystems through changes in savanna fire management.

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Abstract. 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-CO 2 fluxes from the soil. We measured methane (CH 4 ), nitrous oxide (N 2 O) and carbon dioxide (CO 2 ) over a complete wetdry 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 CH 4 than savanna sites and, under wet conditions, old pastures turned from being sinks to a significant source of CH 4 . Nitrous oxide emissions were generally very low, in the range of 0 to 5 µg N 2 O-N m −2 h −1 , and under dry conditions soil uptake of N 2 O was apparent. Break of season rains produced a small, short lived pulse of N 2 O up to 20 µg N 2 O-N m −2 h −1 , most evident in pasture soil. Annual cumulative soil CO 2 fluxes increased after clearing, with savanna (14.6 t CO 2 -C ha −1 yr −1 ) having the lowest fluxes compared to old pasture (18.5 t CO 2 -C ha −1 yr −1 ) and young pasture (20.0 t CO 2 -C ha −1 yr −1 ). Clearing savanna increased soil-based greenhouse gas emissions from 53 to ∼ 70 t CO 2 -equivalents, a 30 % increase dominated by an increase in soil CO 2 emissions and shift from soil CH 4 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.