EMISSIONS OF CH_4 AND N_2O FROM A WETLAND IN THE SANJIANG PLAIN

In order to understand more about mechanisms of and factors that influence CH_4 and N_2O production in wetlands, fluxes of CH_4 and N_2O 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 CH_4 and N_2O 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 CH_4 and N_2O emissions within and among plant zones.The wetland was a source of both CH_4 and N_2O during the growing season and emissions showed conspicuous temporal and spatial variations. Similar temporal variations of CH_4 and N_2O fluxes were observed in the C. pseudocuraica and C. lasiocarpa sites. Emission rates of CH_4 were higher in July and August while emissions of N_2O were higher in July and September. However, the highest emissions of CH_4 and N_2O in the C. angustifolia site occurred about one month earlier than in the C. pseudocuraica and C. lasiocarpa sites. The highest CH_4 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 N_2O emissions were in the D. angustifolia site on June 12 with a rate of 2.53 mg·m -2·d -1. The average CH_4 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 (p0.000 1). These results showed that average CH_4 fluxes in submerged wetlands were higher than in seasonal wetlands. N_2O 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 N_2O emissions.Air temperature, soil temperature, soil redox potential and standing water depth were important factors influencing emission rates of CH_4 and N_2O from the wetlands. Relationship analysis showed that CH_4 fluxes were correlated weakly with air temperature and soil temperature at 5 cm depth within a site (0.201r0.560) but not correlated with standing water depth ((0.100r0.176). Strong correlations were found between N_2O fluxes and standing water depth (r_1=-0.701; r_2=-0.528), but no correlation between N_2O fluxes and air temperature and soil temperature at 5 cm depth in the C. pseudocuraica and C. lasiocarpa sites (-0.089r0.211) was found. However, in the D. angustifolia site, there were no correlations between N_2O fluxes and the three factors (r0.344). These results indicated that temperature was more important in influencing CH_4 emissions in the seasonal and submerged wetlands whereas standing water depth was more important in influencing N_2O emissions in the submerged wetlands. Furthermore, standing water table was the main control of the difference in CH_4 emissions among plant zones. However, there appeared to be similar rates of N_2O emissions among plant zones in the wetlands with strongly anaerobic conditions.