Precipitation, Plant Communities and Methane Fluxes in the Ka‘au Crater Wetland, O‘ahu, Hawai‘i

Abstract

Methane emissions from the Ka`au crater tropical wetland on the island of O`ahu were monitored from August to November 2003. Net methane emission was measured using static chambers and the methane production and oxidation potentials of soil samples were assayed. Environmental parameters (water table level, precipitation, temperature and Photosynthetically Active Radiation (PAR)) were also measured. In addition, the organic chemical composition in different soils was measured. Average net methane emissions varied considerably among different vegetation patterns in the wetland, ranging between 17 and 160 mg m-2 day-1 . Considering the area covered by each pattern, the average net methane flux from the crater is 84 ± 4 mg m-2 day-1 . Methane production potentials ranged between 6*10-6 and 1*10-3 mg g(dry soil)-1 day-1 . Methane oxidation potentials were approximated using first order reaction kinetics and ranged between 0.08 and 0.026 hr-1 g(dry soil)-1 . Water table level was found to be the main environmental parameter influencing net methane emissions, affecting primarily the methane generation potential of soil samples. Methane oxidation potentials were mostly affected by ambient methane concentrations. The variation in methanogenic activity due to the ± 0.05°C temperature variation recorded in the soil throughout the sampling period was calculated to be 0.07%, much less than the observed variation in net emission and methane generation potentials. No correlation was observed between net methane emissions and PAR. Moreover, the soil organic carbon content was similar among three of the five vegetation patterns studied. Past changes in vegetation suggest that methane emissions may have decreased since the early 1900s. This trend is expected to prevail in the future if the vegetation patterns contributing the least to the overall methane flux spread. Additionally, anthropogenic greenhouse warming is likely to reduce precipitation in Hawaii during this century which may in turn reduce the net methane emissions from the Ka`au crater wetland even further.