Soil–atmosphere exchange of CH₄, CO, N₂O and NO<i>_x</i> and the effects of land‐use change in the semiarid Mallee system in Southeastern Australia

Abstract

Abstract 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 CH 4 , CO, N 2 O and NO x 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 CH 4 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 CH 4 uptake significantly. CH 4 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 CH 4 , 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 N 2 O 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.