Abstract:

In order to understand more about mechanisms of and factors that influence CH₄ and N₂O production in wetlands, fluxes of CH₄ and N₂O were measured using static-chamber and gas-chromatography methods in a marsh wetland, located at the Honghe Farm in eastern part of Heilongjiang Province, China (47°35'17.8″ N, 133°37'48.4″ E), from June to September,2003. Three plant communities, Carex pseudocuraica, Carex lasiocarpa and Deyeuxia angustifolia, were selected to measure fluxes of CH₄ and N₂O to contrast the variance of the emission rates of both greenhouse gases in these different plant zones. Air temperature and soil temperature at 5 cm depth, soil redox potential (0-100 cm), and standing water depth at each site also were measured to determine the main factors that control CH₄ and N₂O emissions within and among plant zones.

The wetland was a source of both CH₄ and N₂O during the growing season and emissions showed conspicuous temporal and spatial variations. Similar temporal variations of CH₄ and N₂O fluxes were observed in the C. pseudocuraica and C. lasiocarpa sites. Emission rates of CH₄ were higher in July and August while emissions of N₂O were higher in July and September. However, the highest emissions of CH₄ and N₂O in the C. angustifolia site occurred about one month earlier than in the C. pseudocuraica and C. lasiocarpa sites. The highest CH₄ emissions observed in the wetland were in the C. pseudocuraica site on July 19 with a rate of 696.24 mg·m^-2·d^-1, and the highest N₂O emissions were in the D. angustifolia site on June 12 with a rate of 2.53 mg·m^-2·d^-1. The average CH₄ flux from the C. pseudocuraica site was 273.6 mg·m^-2·d^-1, the highest among the three sites over the growing season but was not significantly different from 259.2 mg·m^-2·d^-1 of the C. lasiocarpa site. However, both were significantly higher than the 38.16 mg·m^-2·d^-1 measured in the D. angustifolia site (p<0.000 1). These results showed that average CH₄ fluxes in submerged wetlands were higher than in seasonal wetlands. N₂O fluxes from the C. pseudocuraica, C. lasiocarpa and D. angustifolia sites were not significantly different (p=0.967) with an average flux of 0.969, 0.932 and 0.983 mg·m^-2·d^-1, respectively, suggesting that submerged and seasonal wetlands had similar rates of N₂O emissions.

Air temperature, soil temperature, soil redox potential and standing water depth were important factors influencing emission rates of CH₄ and N₂O from the wetlands. Relationship analysis showed that CH₄ fluxes were correlated weakly with air temperature and soil temperature at 5 cm depth within a site (0.201<r<0.560) but not correlated with standing water depth ((0.100<r<0.176). Strong correlations were found between N₂O fluxes and standing water depth (r₁=-0.701; r₂=-0.528), but no correlation between N₂O fluxes and air temperature and soil temperature at 5 cm depth in the C. pseudocuraica and C. lasiocarpa sites (-0.089<r<0.211) was found. However, in theD. angustifolia site, there were no correlations between N₂O fluxes and the three factors (r<0.344). These results indicated that temperature was more important in influencing CH₄ emissions in the seasonal and submerged wetlands whereas standing water depth was more important in influencing N₂O emissions in the submerged wetlands. Furthermore, standing water table was the main control of the difference in CH₄ emissions among plant zones. However, there appeared to be similar rates of N₂O emissions among plant zones in the wetlands with strongly anaerobic conditions.

Key words: CH₄, N₂O, Growing seasons, Plant zones, Wetland, Sanjiang Plain