The Sanjiang Plain is the largest low-altitudinal swampy plain in China where has been strongly affected by human activities particularly the conversion of marsh to cropland. To evaluate the effect of marsh reclamation on methane and nitrous oxide emissions, field observations were made at the Sanjiang Mire Wetland Experimental Station（47°35′N, 133°31′E）, Chinese Academy of Sciences, located in the eastern part of Heilongjiang Province. The mean annual precipitation and temperature in this region is 550-600 mm and 1.9 ℃, respectively. Dominant vegetations are marshes and wet meadows. Soil types are typically marsh soil, meadow soil and lessive soil. CH₄ and N₂O fluxes were measured by using the static opaque chamber and gas chromatography system. Four plots of Carex lasiocarpa marsh, Deyeuxia angustifolia marsh, rice field and upland were selected. During the experiment periods, measurements were made twice a week in growing season and once a month in the winter. Three replicates were conducted for each plot.
Results showed that there existed significant spatial difference in CH₄ and N₂O emissions from the two marsh plots. The Carex lasiocarpa marsh emitted more CH₄ while less N₂O than the Deyeuxia angustifolia marsh. Water regime and soil character regulated CH₄ and N₂O fluxes significantly. Gas exchange between the land and atmosphere in the winter season contributed greatly to the annual budget. CH₄ emission in the winter season contributed 2.3%-4.6% to the annual total, while N₂O uptake accounts for approximately 14% of the total N₂O emission in the growing-season.
Field observations also indicated that the conversion of marsh to cropland reduced CH₄ emission significantly but increased N₂O emission slightly. CH₄ emissions from marsh, rice field and upland were 329.56, 94.82 kg•hm^-2•a^-1and -1.37 kg•hm^-2•a^-1, respectively. N₂O emissions from corresponding plots were 1.93, 2.09 kg•hm^-2•a^-1and 4.90 kg•hm^-2•a^-1, respectively. An integrated global warming potential (GWP) of the CH₄ and N₂O emissions were assessed for the four plots. The results showed that the integrated GWP for rice field and upland were 30.8%-37.9% and 6.0%^-28.7% lower than those for marsh, respectively.The conversion of marsh to cropland resulted in a transformation of carbon sink to carbon source, namely marsh acted as a carbon sink and cropland acted as a carbon source. In 2004, carbon emissions were -3.08, 1.79 t•hm^-2 and 3.35 t•hm^-2 in Deyeuxia angustifolia marsh, rice field and upland, respectively. The carbon source of upland was stronger than that of rice field.