Methane emission from a Carex-dominated wetland in Poyang Lake

As the largest natural source of methane to the atmosphere,wetlands play an important role in global carbon cycles.Since methane flux data were not available in numerous wetland types over the world,there were still some uncertainties in global wetland methane budgets.In China,methane flux measurements in wetlands were mostly conducted in cold regions,such as the Sanjiang Plain and the Tibetan Plateau.However,few were conducted in southern wetlands for accurate methane budgets estimation in Chinese wetlands.Poyang Lake is the largest fresh lake in China,where the typical lake wetland covered approximately 3130 km2 area.In this study,the marshland dominated by Carex cinerascens was selected for methane flux measurements,which is located in the national nature reserve,Nanji wetlands of Poyang Lake.To monitor methane flux from this type of wetlands,two treatments were designed: maintaining aboveground parts(TC) and removing aboveground parts(TJ),with three replicates for each treatment.Methane flux was measured between 9:00 and 11:00 at local time using a closed chamber-gas chromatograph method from May 2009 to April 2010,twice or three times per month.We aimed to clarify the characteristics of methane flux from the Carex-dominated wetland and their control factors,and thus provide basic data for further carbon budgets in the Poyang Lake wetland.Methane fluxes of the two treatments showed a clear seasonal variation pattern.It varied from-0.094 to 17.75 mg CH4·m-2·h-1 for the TC treatment,and ranged from-0.122 to 19.16 mg CH4·m-2·h-1 for the TJ treatment,respectively.Methane emission occurred in most time,while its uptake only occurred in the winter.Methane emission rate increased dramatically when the soil surface was covered by substantial water,the maximum in the flooded period was nearly 100 times higher than in the non-flooded period.In contrast to wetlands in other regions,the mean value of methane flux(13.05 mg·m-2·h-1) from the one dominated by Carex cinerascens in the Poyang Lake under the standing water regime was higher than those in the Zoige Plateau,where they are dominated by Carex muliensis and Carex meyeriana,but lower than those in the Sanjiang Plain dominated by Carex lasiocarpa.Before flooding,methane flux from the TJ treatment was significantly higher than from the TC treatment(t=2.69,P0.05).However,methane flux from the TJ treatment was much lower than from the TC treatment when the standing water table reached 15 cm depth.When Carex cinerascens was completed submerged,methane emission rate decreased by 75% in contrast to the condition that Carex cinerascens was partly submerged,as methane transmission was suppressed by the deep standing water.This confirmed that the aboveground plants in wetlands act as an additional methane transmission pathway.Soil moisture and soil temperature at 5 cm depth were positively correlated to methane flux of both treatments before soil surface was submerged.The two factors mainly controlled the methane flux variation before flooding,which could explain 65% to 74% variability of methane flux.During the experiment,methane emission from the Carex cinerascens dominated wetland was 12.77gC/m2,which accounted for approximately 4% of soil microbial respiration,but less than 1% net primary productivity.Given the global warming potential of methane is 25 times higher than carbon dioxide,our result suggested that methane emission is as equally important as carbon dioxide despite its lower emission rates.