Elevated carbon dioxide alters impacts of precipitation pulses on ecosystem photosynthesis and respiration in a semi-arid grassland

Predicting net C balance under future global change scenarios requires a comprehensive understanding of how ecosystem photosynthesis (gross primary production; GPP) and respiration (Re) respond to elevated atmospheric [CO2] and altered water availability. We measured net ecosystem exchange of CO2 (NEE), GPP and Re under ambient and elevated [CO2] in a northern mixed-grass prairie (Wyoming, USA) during dry intervals and in response to simulated precipitation pulse events. Elevated [CO2] resulted in higher rates of both GPP and Re across the 2006 growing season, and the balance of these two fluxes (NEE) accounted for cumulative growing season C uptake (-14.4 ± 8.3 g C m-2). Despite lower GPP and Re, experimental plots under ambient [CO2] had greater cumulative uptake (-36.2 ± 8.2 g C m-2) than plots under elevated [CO2]. Non-irrigated control plots received 50% of average precipitation during the drought of 2006, and had near-zero NEE (1.9 ± 6.4 g C m-2) for the growing season. Elevated [CO2] extended the magnitude and duration of pulse-related increases in GPP, resulting in a significant [CO2] treatment by pulse day interaction, demonstrating the potential for elevated [CO2] to increase the capacity of this ecosystem to respond to late-season precipitation. However, stimulation of Re throughout the growing season under elevated [CO2] reduced net C uptake compared to plots under ambient [CO2]. These results indicate that although elevated [CO2] stimulates gross rates of ecosystem C fluxes, it does not necessarily enhance net C uptake, and that C cycle responses in semi-arid grasslands are likely to be more sensitive to changes in precipitation than atmospheric [CO2].