Effects of exogenous nitrogen input on the CH4 andN2Ofluxes in freshwater marshes

CH4 and N2O are the two main greenhouse gases with great contribution to global warming. Natural wetland in the north hemisphere is recognized as one of the most important global sources of atmospheric CH4. But its role in contribution toN2Ois not clear. Nutrient enrichment may impact the greenhouse gas emission from freshwater marshes. As the largest freshwater marsh distribution area in China, Saijiang Plain has experienced the extensive reclamation over the past 50 years. Large area of the marshes was drained for agricultural production. The remained marshes are often subject to high levels of nitrogen enrichment from atmospheric deposition, surface runoff loading and agricultural drainage, which affected the CH4 andN2Oemission from marsh ecosystem. In this work, we discuss the effects of exogenous Nitrogen on the CH4 andN2Ofluxes of marshes using an in situ field fertilization experiment with the static chamber technique. Ammonia Nitrate solution were applied at a rate of 0 (control), 60, 120kgN·hm-2 and 240kgN·hm-2. Monitoring results showed the maximum CH4 flux of these four levels is 3.67mg CH4 m-2 h-1, 11.01 mg CH4 m-2 h-1, 11.28 mg CH4 m-2 h-1 and 5.82 mg CH4 m-2 h-1, While the maximumN2Oflux is 0.27 mg·m-2·h-1, 0.65 mg·m-2·h-1, 1.0 mg·m-2·h-1, 2.26 mg·m-2·h-1. The results indicated exogenous nitrogen input increases the CH4 andN2Oemission. Comparing with the control level, the CH4 flux of other three fertilization levels increases by 181%, 254% and 155%, and theN2Ofluxes increased 21%, 100% and 533%, respectively. During the growing season, nitrogen input does not have obvious influence upon the seasonal dynamic pattern of CH4 emission flux. But every Nitrogen input, especially of the highest level, brings about the obvious fluctuation of theN2Oemission flux. With the increasing of nitrogen input, there is an exponential increase trend ofN2Oflux (R2 =0.97, p <0.01). CH4 flux under the high nitrogen input was lower than those under the low and medial nitrogen input, which was related to the plant growing stages and the microbial activity of CH4-producing. The exogenous nitrogen input changed the CH4 andN2Ofluxes not only via the direct effect but also by affecting the wetland microbial process.