Background and Aima:Climate warming and increased atmospheric nitrogn(N) deposition both have the potential to increase plant productivity over the next century, yet they can also increase decomposition and respiration. Our aim was to examine the extent to which warming and N addition can, on balance, alter nrt ecosystem CO2 exchange (NEE) in a grass-dominated system. Methods: We measured NEE responses to warming and N addition over two growing seasons in a temperate old field using stedy-state flow-through chambers, which allowed for the integrated measurement of respiration and photoassimilation effects on net CO2 flux over diel periods. We also assessed the relationship between NEE and plant biomass response to the warming and N treatements.Results:In both years, our studysystem was a net source of carbon (C) during the snow-free season. N addition did not significantly affect diel NEE or dark respiration in either year, despite a doubling in aboveground plant biomass in response to N addition in the second year, and a corresponding increase in peak daily net CO2 photoassimilation in N addition plots. The warming treatement also had no significant effect on NEE, althouogh the flow-through chambers required warming to be temporarily halted during NEE measurements. Conclusions: Overall, our results both highlight ythe potential divergence of plant and soil responses to N addition and demonstrate the capacity for a grass dominated system to function as a net source of C in consecutive years.