Ecosystem CO2 Exchange in Response to Nitrogen and Phosphorus Addition in a Restored, Temperate Grassland

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 Re, 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.