Nitrogen (N) and phosphorus (P) affect the structure and function of grasslands by altering plant competitive interactions, shifting patterns of above and below ground biomass allocation, and increasing net primary production. However, the influence of N and P on net ecosystem CO₂ exchange (NEE) is poorly understood. In a field-based factorial N- and P-addition experiment, we measured shallow soil moisture, leaf area index, and component fluxes of midday ecosystem CO₂ exchange throughout the growing season in a restored temperate grassland near Buffalo, New York. Throughout the growing season, N-addition increased gross ecosystem CO₂ exchange (GEE) and correspondingly altered NEE to increase ecosystem CO₂ uptake. In contrast N-addition caused a seasonally dynamic decline in leaf area adjusted GEE, a pattern consistent with increased photosynthetic light limitation. P-addition did not significantly increase R_e, and N- and P-addition interacted to significantly weaken the ecosystem as a midday CO₂ sink. Moreover, water limitation and phenological constraints during the middle and late growing season appear to limit plant responses to nutrient addition. These results suggest influences of N- and P-addition on ecosystem processes are seasonally dynamic and by differentially influencing above and below ground components of ecosystems, the availability of N and P in soils may interact to influence ecosystem CO₂ exchange.