The variation of methane emission from freshwater marshes and response to the exogenous N in Sanjiang Plain Northeast China

The methane emitted from freshwater marsh is the difference between the production and oxidation of CH4 which reaches the atmosphere via diffusion, ebullition, or by passage through aerenchymous tissues of higher plants. To evaluate the variation of CH4 emission and the effects of increased exogenous N on CH4 emission, we have raised in situ the exogenous N with the rates of (control), (low nitrogen level), (medial nitrogen level) and (high nitrogen level), respectively, in Sanjiang Plain marsh ecosystem Northeast China. CH4 emissions were measured using the static chamber and gas chromatogram techniques. The daily average CH4 emission during the different growing stages from the marsh varies from -0.06 to with the daily observation. The exogenous N significantly increased the mean seasonal CH4 emission rate by 181% (low nitrogen level, P<0.01), 254% (medial nitrogen level, P<0.01) and 155% (high nitrogen level, P<0.01), respectively. The effects of exogenous N strongly interacted with the plant aboveground biomass, which was a variable explaining most variation in CH4 emission. We also find the net effect of exogenous N on CH4 emission from freshwater marshes most likely depends on the counterbalance between the N-induced increases in CH4 production and oxidation, as a N excess may result in the inhibition of CH4 emission, and in the stimulation of CH4 emission.