Diel variation in methane emissions from the stands of Carex lasiocarpa and Deyeuxia angustifolia in a cool temperate freshwater marsh

To understand the mechanism of diel variation pattern of CH4 emission from the freshwater marsh, we measured CH4 emission as well as temperature, redox potential and CH4 concentration in porewater in situ in Sanjiang plain, northeast China. There was an apparent diel variation in CH4 emission with the peak at 9:00 and the lowest at 0:00. However, the difference between the highest and lowest emissions was much smaller in the Deyeuxia angustifolia site than in the Carex lasiocarpa site. In the D. angustifolia site with 3 cm standing water, the diel CH4 emission varied from 6.7 to 8.2 mg CH4 m(-2) h(-1) since plants only mediated 29% of CH4 emission and CH4 flux increased by merely 8% after plants were covered with a black cloth. In contrast, in the C lasiocarpa site with 16 cm standing water where plants released 81% of CH4 emission and CH4 flux enhanced by 19% after plants were covered, the diel CH4 emission ranged from 19.5 to 29.7 mg CH4 m(-2) h(-1). At this site CH4 emission was significantly correlated to mean CH4 concentration in porewater and the latter was related to redox potential. The diel pattern could be explained by change in CH4 oxidation driven by plant photosynthesis. Oxygen generated by plant photosynthesis in the rhizome or rhizosphere and temperature in the vertical profile gradually incremented and reached the peak with a lag time after sunrise. CH4 emission and concentration in porewater increased with temperature when CH4 oxidation was no larger than CH4 production and attained the peak at 9:00 but then decreased when CH4 oxidation was larger than CH4 production. Although less fresh oxygen was transported into the rhizome or rhizosphere after sunset, CH4 oxidation was still driven by oxygen inputted at daytime. When the decreasing CH4 oxidation was again equal to CH4 production, CH4 emission and concentration reached the lowest at midnight and then increased. (C) 2003 Elsevier Ltd. All rights reserved.