Emission of methane and nitrous oxide by Australian mangrove ecosystems

The fluxes of the greenhouse gases methane (CH4) and nitrous oxide (N2O) were measured in mangrove wetlands in Queensland, Australia, using the closed chamber technique. Large differences in the fluxes of both gases from different study sites were observed, which presumably depended on differences in substrate availability. CH4 emission rates were in the range of 20 to 350 mug m(-2) h(-1), whereas N2O fluxes were lower, amounting to -2 to 14 mug m(-2) h(-1). in general, the field sites with high substrate availability showed higher emissions than sites with poor nutrient supply. This assumption is supported by the observation of dramatically increased N2O emissions (150-400 mug m(-2) h(-1)) if study sites were artificially fertilised with additional N. As expected, N fertilisation did not alter CH4 fluxes during the period of investigation. In the present study, it was confirmed that the mangrove vegetation may play a role as a transport path for CH4 and N2O by facilitating diffusion out of the soil. Prop roots from Rhizophora stylosa emitted CH4 and N2O at rates of 2.6 and 3.3 mug m(-2) root surface h(-1), respectively, whereas the soil of this stand acted as a sink for CH4. As a consequence, the ecosystem as a whole could constitute a CH4 source despite CH4 uptake by the soil. In contrast to prop roots, the presence of pneumatophores in Avicennia marina led to a significant increase in CH4 emissions from mangrove soils, but did not enhance N2O emissions. These findings indicate that mangrove ecosystems may be considered a significant source of N2O and that anthropogenic nutrient input into these ecosystems will lead to enhanced source strengths. For an up-scaling of greenhouse gas emissions from mangrove forests to a global scale, more information is needed, particularly on the significance of vegetation.