N2O emission from tropical forest soils of Australia

Three different tropical rain forest sites (Kauri Creek, Lake Eacham, and Massey Creek) on the Atherton Tablelands, Queensland, Australia, were investigated for the magnitude of N2O emission from soils during different seasons, that is, wet season, dry season, and transition periods. Highest mean N2O emission rates were observed for soils derived from granite at the Kauri Creek site with 74.5 +/- 25.2 mug N2O-N m(-2) h(-1), whereas for soils derived from Metamorphics (Lake Eacham site) mean N2O emission rates were much lower (13.1 +/- 1.1 mug N2O-N m(-2) h(-1)). For the Massey Creek site, with soils derived from Rhyolite, a mean annual N2O emission rate of 46.2 +/- 1.1 mug N2O-N m(-2) h(-1) was calculated. The mean annual N2O emission rate calculated for all three sites over the entire observation period was 39.0 mug N2O-N m(-2) h(-1) and thus at the high end of reports from tropical rain forest soils. N2O emission rates showed at all sites pronounced temporal as well as spatial variability. The magnitude of N2O emissions was strongly linked to rainfall events; that is, N2O emissions strongly increased approximately 6-8 hours after precipitation. Correlation analysis confirmed the strong dependency of N2O emissions on changes in soil moisture, whereas changes in soil temperature did not mediate considerable changes in N2O fluxes. Spatial variability of N2O fluxes on a site scale could be explained best by differences in water-filled pore space, CO2 emission, and C/N ratio of the soil. On the basis of all published N2O flux rates from tropical rain forest soils we recalculated the contribution of such forests to the global atmospheric N2O budget and come up with a figure of 3.55 Tg N2O-N yr(-1), which is approximately 50% higher than reported by others.