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

Author
item BACHMAN, SARAH - University Of Wyoming
item HEISLER-WHITE, JANA - University Of Wyoming
item PENDALL, ELISE - University Of Wyoming
item WILLIAMS, DAVID - University Of Wyoming
item Morgan, Jack
item NEWCOMB, JOANNE - University Of Wyoming

Submitted to: Oecologia
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/29/2009
Publication Date: 11/27/2009
Citation: Bachman, S., Heisler-White, J.L., Pendall, E., Williams, D.G., Morgan, J.A., Newcomb, J. 2009. Elevated carbon dioxide alters impacts of precipitation pulses on ecosystem photosynthesis and respiration in a semi-arid grassland. Oecologia. 162:791-802.

Interpretive Summary: A central question in research investigating the responses of various terrestrial ecosystems to climate change and increased atmospheric CO2 is how will the exchange of ecosystem carbon (C) between the land and atmosphere be affected by future environmental change. This is important because the loss or gain of C by terrestrial ecosystems indirectly effects climate change depending on whether those systems release more or less C to the atmosphere in response to climate change or increased atmospheric CO2. There are two major mechanisms by which this land-atmosphere exchange of C occurs. C uptake is achieved by the assimilation of carbon dioxide (CO2) by plants in photosynthesis, and C release to the atmosphere occurs through respiration of plants, animals and microorganisms, all of which metabolize complex compounds and release CO2 in the process. This manuscript reports on research in which gas exchange chambers were used to quantify the effects of CO2 and water pulses on the exchanges of CO2 between a semi-arid grassland in Wyoming and the atmosphere. The results indicate that the balance between processes which assimilate and release CO2 from the grassland change seasonally and that both increased CO2 and altered precipitation patterns can affect those balances. These findings illustrate how terrestrial ecosystems can not only be affected by changes in atmospheric conditions associated with climate change, but can also feed-back and influence the atmosphere through changes in the land-atmosphere exchanges of CO2.

Technical Abstract: Predicting 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 water availability. We measured net ecosystem exchange of CO2 (NEE), GPP and Re prior to and following early (May 31st) and late (August 4th) growing season simulated precipitation ‘pulse’ events under ambient and elevated atmospheric [CO2] in a northern mixed-grass prairie of Wyoming, USA. Elevated atmospheric [CO2] differentially stimulated GPP and Re both within and between pulse events. Greater stimulation of Re compared to GPP under elevated atmospheric [CO2] caused a net efflux (positive NEE) one day following the May pulse not seen under ambient atmospheric [CO2]. Re and GPP were stimulated by elevated atmospheric [CO2] across a range of soil moisture conditions (approximately 3 to 30% volumetric soil moisture), and were nonlinearly related to one another during both pulses. Elevated atmospheric [CO2] extended the magnitude and duration of the response of Re and GPP to moisture inputs, and these effects varied seasonally. Overall, elevated atmospheric [CO2] had the greatest effect on Re following both pulses, and treatment differences were greater in May than in August. Estimates of cumulative NEE over the seven days following the pulses showed net losses of C from all treatments in May and net gains in C in August under both ambient and elevated atmospheric [CO2] treatments with simulated precipitation. This paper highlights the interaction between atmospheric [CO2] and ecosystem CO2 exchange responses to precipitation and demonstrates that this interaction varies seasonally.