Characteristics of methane emission from different vegetations on a wetland

Abstract

Methane flux was observed on a floating mat of temperate sphagnum bog, Mizorogaike, Japan, during the period April to October in 1993, to investigate the factors controlling CH₄ emission, especially differences in vegetation and variation of water level. Comparing the CH₄ flux among reed dominant site, marsh trefoil (a broad-leaved perennial aquatic herb) dominant site and sphagnum dominant site, the largest CH₄ flux was observed at the marsh trefoil site and the smallest flux was at the sphagnum site, reflecting the difference in decomposability of organic matter. Namely, the decomposition rate of marsh trefoil was high, while that of sphagnum was extremely low. At the marsh trefoil site, lowering of the water table below the surface during summer caused an increase of surface soil temperature due to direct exposure to solar radiation. Consequently, it caused a low Eh and enhanced methane production which was supported by the supply of organic matters with high decomposition rate derived from marsh trefoil. At the reed site, CH₄ flux increased in late summer. Such a delayed CH₄ emission at the reed site was possibly caused by decomposition of roots growing during the summer and delay of soil temperature increase due to the waterlogging. Average CH₄ fluxes for seven months were 450, 290, and 70 mgC/m2/day for the marsh trefoil, reed and sphagnum sites, respectively. The δ¹³C of dissolved CH₄, higher than that of bubbles, suggested CH₄ oxidation in the soil water. However, oxidation was not a deterministic process controlling CH₄ flux at the observed temperate wetland where CH₄ production rate was high because of high soil temperature. Sphagnum can only grow in oligotrophic (limited amount of nutrients) water, and its domination suppresses growth of other plants because of low concentration or low availability of nutrients. However, sphagnum can be easily replaced by other wetland plants, if the water in a sphagnum bog becomes rich in nutrients due to water pollution or change in the water circulation in the area. Such a vegetation change from a sphagnum dominant bog to a fen dominated by other wetland plants can accelerate CH₄ emission to the atmosphere.