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item PEPPER, D
item Del Grosso, Stephen - Steve

Submitted to: Global Biogeochemical Cycles
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/1/2004
Publication Date: 1/2/2005
Citation: Pepper, D.A., Del Grosso, S.J., Mcmurtie, R.E., Parton, W. 2005. Simulated carbon sink response of shortgrass steppe, tallgrass prairie and forest ecosystems to rising [co2], temperature and nitrogen input. Global Biogeochemical Cycles. 19:GB1004, doi:10.1029/2004gb002226.

Interpretive Summary: Human activity has led to increased atmospheric carbon dioxide concentrations and increased nitrogen deposition. Increased atmospheric carbon dioxide contributes to the greenhouse effect and can also impact plant growth rates. Increased nitrogen deposition also impacts plant growth. We used two ecosystem models to project how future increases in carbon dioxide concentration, temperature, and N deposition will impact carbon dynamics. Model results suggest that increased CO2 alone will initially enhance carbon storage in the grassland and boreal forest systems studied but that this is a transient effect that is not sustained unless increased nitrogen deposition is also included. Warming alone decreased carbon levels in the grassland sites but increased carbon in the boreal forest. When increased carbon dioxide, enhanced nitrogen deposition, and warming were all included, all the sites remained carbon sinks 100 years into the future.

Technical Abstract: The response of plant ecosystems to environmental change will determine whether the terrestrial biosphere will remain a substantial carbon sink or become a source during the next century. We use two ecosystem models, the Generic Decomposition And Yield model (G’DAY) and the daily time step version of the Century model (DAYCENT), to simulate net ecosystem productivity (NEP) for three contrasting ecosystems (shortgrass steppe in Colorado, tallgrass prairie in Kansas, and Norway spruce in Sweden) with varying degrees of water, temperature, and nutrient limitation, to determine responses to gradual increases in atmospheric CO2 concentration ([CO2]), temperature, and nitrogen input over 100 years. Using G’DAY, under rising[CO2], there is evidence of C sink ‘‘saturation,’’ defined here as positive NEP reaching an upper limit and then declining toward zero, at all three sites (due largely to increased N immobilization in soil organic matter) but a positive C sink is sustained throughout the 100 years. DAYCENT also predicts a sustained C sink at all three sites under rising [CO2], with evidence of C sink saturation for the Colorado grassland and the C sink levels off after 80 years for the Kansas grassland. Warming reduces soil C and the C sink in both grassland ecosystems but increases the C sink in the forest. Warming increases decomposition and soil N mineralization, which stimulates net primary productivity (NPP) at all sites except when inducing water limitation. At the forest site some of the enhanced N release is allocated to a woody biomass pool with a low N:C ratio so that warming enhances NEP without increased N input at the forest site, but not at the grassland sites. Responses to combinations of treatments are generally additive for DAYCENT but more interactive for G’DAY, especially under combined rising [CO2] and warming at the strongly water-and N-limited shortgrass steppe. Increasing N input alleviates C sink saturation and enhances NEP, NPP, and soil C at all sites. At the water-limited grassland sites the effect of rising [CO2] on growth is greatest during the drier seasons. Key sensitivities in the simulations of NEP are identified and include NPP sensitivity to gradual increase in [CO2], N immobilization as a long-term feedback, and the presence or not of plant biomass pools with low N:C ratio.