Abstract: Ecosystem carbon exchange is a key component of terrestrial ecosystem carbon cycle. Nitrogen (N) deposition is one of the most common disturbances in grassland ecosystems. In this study, we assessed the effects of N addition on ecosystem carbon exchange and its responses to the variations of temperature and moisture. Results showed that the daily variations of NEE, ER and GEP under N addition showed the same trend, but different magnitudes of variation during different growing periods with a significant level at vegetation vigorous period (P<0.05). NEE had no significant correlation with sunshine duration and temperature (P>0.05), but was positively correlated with biomass and soil moisture (P<0.05). N addition enhanced the sensitivity of NEE to soil moisture, from 0.18 to 0.37. Both ER and GEP showed a significant exponential relationship with sunshine duration and temperature (P<0.01). N addition increased the sensitivity of ER and GEP to sunshine duration and temperature. Aboveground biomass had a good correlation with ER and GEP at the beginning of the growing season. Changes of biomass following Naddition may play an important role in mediating the effects of N addition on grassland ecosystem carbon exchange.

Key words: ammonia-oxidizing archaea, long-term fertilization, ammonia-oxidizing bacteria, diversity