Elevated atmospheric carbon dioxide and precipitation alter ecosystem carbon fluxes over northern mixed-grass prairie at the prairie heating and carbon dioxide enrichment (PHACE) experiment in Cheyenne, Wyoming, United States

Understanding net carbon (C) balance under future global change scenarios requires a comprehensive understanding of photosynthetic (GPP) and ecosystem respiration (Re) responses to atmospheric CO2 concentration and soil moisture availability. We measured midday net ecosystem exchange of CO2 (NEE), GPP and Re prior to and following early and late growing season simulated precipitation 'pulse' events (May 31st and August 4th) and diurnally on five additional dates over the growing season under ambient and elevated atmospheric [CO2] in a northern mixed-grass prairie of Wyoming, USA. We also measured soil CO2 concentration on 10 days over the growing season and modeled soil respiration using a flux gradient approach. Elevated atmospheric [CO2] differentially stimulated GPP and Re both within and between pulse events and across the growing season. Elevated [CO2] increased the magnitude and duration of GPP and Re responses to moisture inputs, supporting the hypothesis that atmospheric [CO2] interacts with individual precipitation events to alter CO2 exchange patterns, and demonstrates that this interaction varies seasonally. Over both the pulses and growing season, the greatest effects on CO2 exchange patterns were due to soil moisture, and elevated [CO2] had the greatest effect on respiration fluxes, Re an Rs. Estimates of cumulative C uptake and loss both following individual precipitation events and over the growing season suggest that elevated [CO2] did not enhance C uptake in the 2006-growing season.