10:45 〜 12:15
[BBC02-P06] Daily patterns of methane emissions from trees and sediment in a subtropical mangrove forest
キーワード：greenhouse gas, mangrove forest, methane, CH4 flux, tidal cycle
Mangrove forests are among the most carbon-rich tropical forests, storing carbon in the biomass of the trees, both above and belowground, and in the sediments. They are considered effective carbon sinks and the carbon buried in these systems is often refers to as blue carbon. However, it has recently been recognized that methane (CH4) emissions from sediments can partially offset the burial of blue carbon in sediments of mangrove forests. Moreover, only very few studies have considered the role of trees in the CH4 budget of mangrove forests. How CH4 emissions from trees and sediment (FCH4) vary with the day/night alternation and the tidal cycle, have not yet been elucidated.
The study site is a mangrove forest extending about 1500 m along the intertidal banks of the Miyara River estuary on Ishigaki Island (Japan, 24°21'N 124°12'E), which are inundated twice daily (mixed semidiurnal tides). We measured FCH4 from sediments, and from buttress roots and stems on two individuals of Bruguiera gymnorrhiza over a 24-hour cycle, using closed chambers connected to a cavity-enhanced absorption spectroscopy gas analyser.
Higher FCH4 was measured on the buttress roots than on the stems, with a decreasing trend with height along the stems. FCH4 exhibited large variations over a 24-hour cycle. FCH4 from sediment was lower just before or after the flood period than around low tide. FCH4 from stems at position along the trunks that were not submerged at high tide was lower during the night than during the day. There was no evidence of any tidal influence on stem CH4 emissions measured above 1 m. On buttress roots and on stem portion which were submerged, high FCH4 were measured at the beginning of the ebb tide even at night.
Measurements conducted at low tide during day time cannot be considered as representative of the 24-h period owing to the large diurnal variation of FCH4 and the tidal influence. A better understanding of FCH4 variability from mangrove trees and sediments, and its controls at different time scales is needed for accurate estimation of CH4 budgets of mangrove forests.
The study site is a mangrove forest extending about 1500 m along the intertidal banks of the Miyara River estuary on Ishigaki Island (Japan, 24°21'N 124°12'E), which are inundated twice daily (mixed semidiurnal tides). We measured FCH4 from sediments, and from buttress roots and stems on two individuals of Bruguiera gymnorrhiza over a 24-hour cycle, using closed chambers connected to a cavity-enhanced absorption spectroscopy gas analyser.
Higher FCH4 was measured on the buttress roots than on the stems, with a decreasing trend with height along the stems. FCH4 exhibited large variations over a 24-hour cycle. FCH4 from sediment was lower just before or after the flood period than around low tide. FCH4 from stems at position along the trunks that were not submerged at high tide was lower during the night than during the day. There was no evidence of any tidal influence on stem CH4 emissions measured above 1 m. On buttress roots and on stem portion which were submerged, high FCH4 were measured at the beginning of the ebb tide even at night.
Measurements conducted at low tide during day time cannot be considered as representative of the 24-h period owing to the large diurnal variation of FCH4 and the tidal influence. A better understanding of FCH4 variability from mangrove trees and sediments, and its controls at different time scales is needed for accurate estimation of CH4 budgets of mangrove forests.