Emission of Methane and Nitrous Oxide by Australian Mangrove Ecosystems

Abstract

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 µg m^-2 h^-1, whereas N₂O fluxes were lower, amounting to - 2 to 14 µg 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 µg 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 µg 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.

Key words

Methane and nitrous oxide emissions - mangrove - pneumatophores - prop roots.

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