Methane soil-vegetation-atmosphere fluxes in tropical ecosystems

Recently, a surprising discovery indicated that, by an unknown process, vegetation emits methane to the atmosphere. This finding could have serious implications in atmospheric chemistry, climate, and mitigation of global change. In order to evaluate the magnitude of the tropical vegetation source, a re-evaluation of results obtained at various Venezuelan ecosystems is made. CH(4) fluxes from the soil-grass system in savanna ecosystems indicate that grasses produce CH(4) at similar to 10 ng center dot m(-2 center dot)s(-1). Furthermore, CH(4) accumulation within the nocturnal mixing layer at the Guri site, which is affected by savanna and forest emissions, was used to make a rough upper limit estimation of < 70 ng center dot m(-2).s(-1) for CH(4) emission from forest vegetation. These estimates are likely to be somewhat low as they do not take into account the light-induced production of CH(4) by the vegetation. Global extrapolation of these fluxes indicates that, ignoring the possible stimulating effects of solar radiation, savanna and forest vegetation result in CH(4) emissions of similar to 5Tg center dot yr(1) and < 22Tg center dot yr(1), respectively. These estimates are in agreement with the lower estimates based on laboratory CH(4) flux measurements, reported in the literature. On the other hand, the global extrapolation of the atmosphere-soil uptake fluxes results CH(4) sinks of similar to 1.3Tg center dot yr(1) in savannas and of 3.3Tg center dot yr(1) in forests. In conclusion, Venezuelan field measurements support the discovery that vegetation emits CH(4). However global extrapolation indicates that tropical vegetation would contribute modestly to global methane emission, which, additionally, is offset in part by savanna and forest CH(4) soil uptake. Most likely, carbon sequestration benefits from forestation should not be significantly affected by CH(4) emissions by trees.