It has widely been demonstrated that nitrogen (N) addition enhances plant growth and net primary productivity of terrestrial ecosystems. Moreover, N enrichment could have a profound impact on ecosystem carbon ﬂuxes, especially in the regions where N is deﬁcient. However, there is still debate on how N affects net ecosystem CO2 exchange (NEE). A ﬁeld experiment manipulating N has been conducted in Calamagrostis anagustifolia community of Sanjiang Plain marsh of northeastern China from 2009 to 2010. N was added at a rate of 5 and 10gN m -2 yr -1 with NH4NO3. The results for the 2 yr showed that gross ecosystem productivity (GEP) was higher than ecosystem respiration, leading to net carbon(C)sink (measured by NEE) over the growing season in the study site. Furthermore, low nitrogen (LN) and high nitrogen (HN) addition all signiﬁcantly stimulated growing-season NEE, on average, by 22 and 36%, respectively. It’s indicated that air temperature plays a major role in regulating ecosystem net C exchange and their responses to climatic change in Sanjiang Plain of northern China. we need long-term ﬁeld studies to predict the long-term effects of N deposition on ecosystem processes.