Soil-atmosphere exchange of CH₄, CO, N₂O and NOx and the effects of land-use change in the semiarid Mallee system in Southeastern Australia

The semiarid and arid zones cover a quarter of the global land area and support one-fifth of the world's human population. A significant fraction of the global soil-atmosphere exchange for climatically active gases occurs in semiarid and arid zones yet little is known about these exchanges. A study was made of the soil-atmosphere exchange of CH₄, CO, N₂O and NOx in the semiarid Mallee system, in north-western Victoria, Australia, at two sites: one pristine mallee and the other cleared for approximately 65 years for farming (currently wheat). The mean (± standard error) rates of CH₄ exchange were uptakes of -3.0 ± 0.5 ng(C) m⁻² s⁻¹ for the Mallee and -6.0 ± 0.3 ng(C) m⁻² s⁻¹ for the Wheat. Converting mallee forest to wheat crop increases CH₄ uptake significantly. CH₄ emissions were observed in the Mallee in summer and were hypothesized to arise from termite activity. We find no evidence that in situ growing wheat plants emit CH₄, contrary to a recent report. The average CO emissions of 10.1 ± 1.8 ng(C) m⁻² s⁻¹ in the Mallee and 12.6 ± 2.0 ng(C) m⁻² s⁻¹ in the Wheat. The average N₂O emissions were 0.5 ± 0.1 ng(N) m⁻² s⁻¹ from the pristine Mallee and 1.4 ± 0.3 ng(N) m⁻² s⁻¹ from the Wheat. The experimental results show that the processes controlling these exchanges are different to those in temperate systems and are poorly understood.