| Wetlands in Xiaoxing'an Mountains, which one of the main mountainous wetlands distribution areas, play an important role of the wetlands study in China, because of the diversity wetland types including marshes, thicket swamps and forested swamps. We observed the methane fluxes from seven types of wetlands in Xiaoxing'an Mountains including a marsh (M), a thicket swamp (TS), a Alnus sibirica swamp (FS-1), a Betula platyphylla swamp (FS-2), a Larix gmelinii - Carex schmidtii swamp (FS-3), a Larix gmelinii - moss swamp (FF) and a Larix gmelinii - Sphagnum spp. swamp (FB), their spatial and temporal variations of methane fluxes and the affecting factors, in the growing seasons of 2007 and 2008 using the static opaque chamber and gas chromatography technique. Also the methane fluxes under the anthropogenic disturbances of draining for forestation, and cutting were measured. Results showed that:(1) No common diurnal variations were found in all the wetlands, and the diurnal patterns were weakly correlated to temperatures and water tables. FS-1 and FS-2 have the same pattern of diurnal fluxes with unique peaks at the same time in difference seasons. M, TS and FB have the patterns of diurnal fluxes with unique peaks at different time in difference seasons. No apparent diurnal variations were observed in FS-3 and FF because of the low flux rates.(2) Most of methane fluxes from wetlands in Xiaoxing'an Mountains in the growing season peaked in summer or autumn, but the rates kept at low levels in spring or late autumn. Seasonal variations of methane fluxes were correlated with temperatures, but the relationships were restricted by water wables. Seasonal variations of methane fluxes were correlated well with temperatures when wetlands water tables were high but the relationships were weak when water tables were low.(3) Interannual variations of methane fluxes were large in M, TS, FS-1 and FS-2 in the two growing seasons, mean methane fluxes in the growing season of 2008(19.88-820.22mg·m-2·d-1) were 5-31 times larger than the mean values of 2007(0.64-44.56mg·m-2·d-1). FB was an atmospheric methane source in the growing of 2007(56.08mg·m-2·d-1) but a sink in the same periods of 2008(-0.19mg·m-2·d-1). No significant differences of methane fluxes in the growing season of 2007 and 2008 in FS-3 and FB. Interannual variations of methane fluxes were controlled by the mean annual water table and the variations of water table fluctuations.(4) Apparent spatial variations of methane fluxes from wetlands in Xiaoxing'an Mountains were observed. M and FS-1 were big sources of atmospheric methane, FB, FS-2 and TS were small sources, but FS-3 and FF were weak sinks, and the average methane emission rates during the two growing seasons were 456.63mg·m-2·d-1, 239.39mg·m-2·d-1, 26.19mg·m-2·d-1, 10.86mg·m-2·d-1, 6.49mg·m-2·d-1,-0.69mg·m-2·d-1 and -1.42mg·m-2·d-1, respectively. Water table and aboveground herb biomass together controlled the spatial variations of methane fluxes. Seasonal average methane fluxes were high with higher water table and herb biomass but low with lower water table and herb biomass. Air temperature may contribute less to the spatial variations of methane fluxes from wetlands than water table and herb biomass. Episodic fluxes may contribute large to the spatial variations of methane fluxes from wetlands.Herbs are better indicators of methen flux rates from a region scale than shrubs or trees.(5) Episodic fluxes were detected in FB when water table falling, which greatly influenced the diurnal, seasonal, interannual and spatial variations of methane fluxes, also the correlations of methane fluxes and the affecting factors. It was important to understanding and evaluating the regional methane fluxes, so should not be overlooked.(6) Based on the in situ measurement, we preliminarily estimated the budget of methane emissions from wetlands in mountainous regions in Northeast China was 1.49Tg in the growing season and 1.56Tg in the year. Wetlands in mountainous regions of northeast China were likely to be the biggest sources of atmospheric methane from natural wetlands in China.(7) Methane fluxes decreased greatly after natural wetlands were drained for forestation. Natural M and TS were atmospheric methane sources, also were 10- years-old plantations, but the flux rates were only 1/3 and 1/50 as those from M and TS, respectively. 20- years-old plantations change to the atmospheric methane sink. Draining ditches in the plantations were atmospheric methane sources, however, flux rates decreased greatly. But planting ridges were weak sources or changed to the sinks. Water table lowering and vascular plants decreasing mainly contributed to the decrease of methane emission rates after wetlands draining for forestation. The decrease of draining ditches area may also cause methane emission rates decreasing in the plantations.(8) Three forested swamps after selective and clear cutting uptakes less methane than the natural ones, or changed to the atmospheric methane sinks. Changes in temperature and water table together contributed to the changes of methane fluxes after cutting. If the utilization of trees in forested swamps were necessary, we should choose selective cutting rather than clear cutting to reduce methane emission. |
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