Continental drying and carbon sequestration along a subambient to elevated CO2 gradient

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The concentration of CO2 in the atmosphere has increased dramatically since the last glacial maximum. A major focus of ongoing research is the current and future role of terrestrial ecosystems in sequestering CO2. In grasslands, which are North America 's largest potential vegetation type, the interactions of water availability with increased CO2 can be as important as the direct effects of CO2 in shaping primary production, decomposition, and C storage. The proposed research and modeling synthesis is a collaboration among Duke (Jackson, Maherali), Colorado State (Parton), and Kansas State (Owensby) Universities and the USDA-ARS (Polley, Johnson) to predict the consequences of increased atmospheric CO2 concentrations and climate change (increased summer drought) for the Southern Great Plains region of the U.S.The field experiment in an intact C3/C4 grassland in central Texas is unique in providing a continuous gradient of Ca from 200 to 550 umol mol -1, allowing the examination of critical threshold and nonlinear responses to past, present, and future atmospheric CO2. Nested within the CO2 gradient is a replicated factorial water manipulation of ambient and 60% ambient precipitation, the latter consistent with the summer droughts predicted for the central U.S. by general circulation models (GCMs) in the coming century and similar to an extended historical drought from the 1950s in the region. The experiments described in this proposal will permit rigorous accounting of CO2 effects on carbon sequestration and the soil water balance, a factor that has emerged from previous studies as critical in explaining the integrated effects of CO2 enrichment on grasslands.Key hypotheses to be addressed include:H-1 Ecosystems respond nonlinearly to atmospheric CO2, with less carbon sequestration at future CO2 concentrations than observed in the recent past.H-2 Although the stimulation of plant production with increasing CO2 will be greater under the summer droughts predicted by GCMs for the central U.S., carbon storage will ultimately depend on the balance of soil water availability and decomposition.H-3 Water availability plays a key role in grassland carbon sequestration with CO2 through interactions with soil N availability.The DAYCENT ecosystem model will be used to evaluate the potential long-term impact of increasing atmospheric CO2 and climate change for grasslands in the Southern Great Plains. A substantial amount of data is already available to describe the potential impact of increasing atmospheric CO2 on grassland ecosystems in the Southern Great Plains from long-term CO2 experiments in Texas, Colorado, and Kansas. The impact of different environmental conditions and climate change will be evaluated for above-and belowground plant production, soil C and N levels, and net ecosystem production. Therefore, the proposed research and modeling integration directly addresses the ability of ecosystems to continue as carbon sinks in the coming century.The proposed project is relevant to three of the five goals outlined in the U.S. Carbon Cycle Science Plan (CCSP): 1) Improve projections of future atmospheric concentrations of CO2 through a combination of manipulative experiments and model development, 2) Establish accurate estimates of the impact of historical and current land use patterns and trends on the evolving C budget, and 3) Establish accurate estimates of the potential Northern Hemisphere terrestrial C sink and the underlying mechanisms that regulate it. The proposed research also contributes to the Global Change and Terrestrial Ecosystems (GCTE) and Biospheric Aspects of the Hydrological Cycle (BAHC) core projects of the International Geosphere Biosphere Programme (IGBP).