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

Abstract

Abstract To understand the mechanism of diel variation pattern of CH 4 emission from the freshwater marsh, we measured CH 4 emission as well as temperature, redox potential and CH 4 concentration in porewater in situ in Sanjiang plain, northeast China. There was an apparent diel variation in CH 4 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 CH 4 emission varied from 6.7 to 8.2 mg CH 4 m −2 h −1 since plants only mediated 29% of CH 4 emission and CH 4 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 CH 4 emission and CH 4 flux enhanced by 19% after plants were covered, the diel CH 4 emission ranged from 19.5 to 29.7 mg CH 4 m −2 h −1 . At this site CH 4 emission was significantly correlated to mean CH 4 concentration in porewater and the latter was related to redox potential. The diel pattern could be explained by change in CH 4 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. CH 4 emission and concentration in porewater increased with temperature when CH 4 oxidation was no larger than CH 4 production and attained the peak at 9:00 but then decreased when CH 4 oxidation was larger than CH 4 production. Although less fresh oxygen was transported into the rhizome or rhizosphere after sunset, CH 4 oxidation was still driven by oxygen inputted at daytime. When the decreasing CH 4 oxidation was again equal to CH 4 production, CH 4 emission and concentration reached the lowest at midnight and then increased.