Evaluation of field data from the KSU-DOE CO2 enrichment study has indicated that significant increases in WUE and NUE of the C4 grasses do occur (Owensby et al. 1993a). We implemented this effect in CENTURY and tested the implementation against field data using observed climate records. The simulations indicated that implementation of both the NUE and WUE effects was necessary to obtain the appropriate aboveground production and net annual N mineralization. The implementation of WUE allowed for a greater CO2 response to be expressed during drier years. The NUE effect allowed for plant production to remain elevated despite lower N availability. The implementation was used to simulate long-term impacts of climate change and CO2 increases. Overall the climate change impact on net primary production and net mineralization at the KSU-DOE site was positive. Soil organic matter declined slightly with climate change projections from the two GCMs. The climate change projections tended to increase decomposition and net N mineralization which enhanced N availability and stimulated greater plant growth. Doubling CO2 concentrations and maintaining climate at current levels resulted in a 15% increase in plant production, soil C increased by only 3%, and net N mineralization declined by about 10% (Table 1a). Lower litter quality under higher CO2 levels contributed to the lower net N mineralization. Changes in NUE allowing the C4 plants to assimilate more C per gram N taken up results in the higher production. Most of the positive NPP response to CO2 could be attributed to greater increases in WUE in drier years. Similarly, Owensby et al. (1993a) found that NPP of a C4 grass was observed by CO2 in a dry rainfall year, while it was not observed in a wet year. These are consistent with the simulations at the KSU-DOE site with elevated CO2.
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