What was done
The authors used the results from a five-year-long open-top chamber (OTC) experiment, which had been conducted in Colorado, USA (Mosier et al., 2002; Morgan et al., 2004), to parameterize CO2 and temperature impacts in the DAYCENT ecosystem model (Parton et al., 2001), after which they used the model to study the likely consequences of future increases in atmospheric CO2 concentration and air temperature on a number of semiarid grassland ecosystem processes.

What was learned
In the words of the four researchers:

1. "Comparison of field data and model results for the OTC experiment showed that the model represented the major seasonal and year-to-year changes of the control and elevated CO2 treatments, with the model correctly showing greater soil water content and above-ground plant production under elevated CO2."


2. "Combined warming and rising CO2 should result in substantive production increases in most years."


3. "Above-ground net primary production tends to be greatest in the combined elevated CO2 and warming treatment, especially in high-production years."


4. "The two most important direct impacts of elevated CO2 concentration on grassland ecosystems are the reduction of plant transpiration rates and the increase in the C to N ratio of live plant material."


5. "DAYCENT results show that the dominant impact of elevated CO2 is to increase soil water content (as a result of reduced transpiration) which then results in increased potential plant growth."


6. "The DAYCENT model predicts that net N release from soil organic matter in the CO2 treatment is a potential N source for increased N uptake by the plant."


7. "The DAYCENT simulated model results for the elevated CO2 experiment show deceased NOX gas fluxes comparable with the Mosier et al. (2003) results."


8. "The long-term impact (20-100 years) of lower N losses would be to increase soil N mineralization, organic soil N contents and plant production."

What it means
The results of this study, plus those of the prior studies that provide input for it, suggest that continued increases in the air's CO2 content and temperature imply nothing but good for the world's semiarid grassland ecosystems.

References
Morgan, J.A., Mosier, A.R., Milchunas, D.G., LeCain, D.R., Nelson, J.A. and Parton, W.J. 2004. CO2 enhances productivity of the shortgrass steppe, alters species composition, and reduces forage digestibility. Ecological Applications 14: 208-219.

Mosier, A.R., Morgan, J.A., King, J.Y., LeCain, D. and Milchunas, D.G. 2002. Soil-atmosphere exchange of CH4, CO2, NOX and N2O in the Colorado shortgrass steppe under elevated CO2. Plant and Soil 240: 201-211.

Mosier, A.R., Pendall, E. and Morgan, J.A. 2003. Effect of water addition and nitrogen fertilization on the fluxes of CH4, CO2, NOX, and N2O following five years of elevated CO2 in the Colorado Shortgrass Steppe. Atmospheric Chemistry and Physics 3: 1703-1708.

Parton, W.J., Holland, E.A., Del Grosso, S.J., Hartman, M.D., Martin, R.E., Mosier, A.R., Ojima, D.S. and Schimel, D.S. 2001. Generalized model for NOX and N2O emissions from soils. Journal of Geophysical Research 106: 17,403-17,420.