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Elevated CO2 Stimulates Both the Quantity and Quality of C3 and C4 Grasses

Paper Reviewed
McGranahan, D.A. and Yurkonis, K.A. 2018. Variability in grass forage quality and quantity in response to elevated CO2 and water limitation. Grass and Forage Science 73: 517-521.

Writing as background for their work, McGranahan and Yurkonis (2018) state that "temperate regions such as the northern U.S. Great Plains depend on both C3 and C4 grasses, and thus, the relative responses of important grass species under both elevated CO2 and water limitation merit investigation," as model scenarios predict periods of drought in this region in the future. Thus, it became their objective to examine the "forage quality and quantity responses to water deficit and elevated CO2" of five C3 (Agropyron cristatum, Bromus inermis, Poa pratensis, Schedonorus arundinaceus and Pascopyrum smithii) and three C4 (Bouteloua gracilis, Buchloë dactyloides and Sirghastrum nutans) grass species.

This work was conducted in a controlled-environment chamber, where for three weeks the grasses were exposed to atmospheric CO2 concentrations of either 450 (normal) or 700 (enriched) ppm and water treatments corresponding to well-watered or water-limited conditions to simulate drought. Prior to the experimental treatment, the aboveground tissue of the grasses was clipped; and at the end of the three-week experimental period the authors measured the biomass recovery as a percentage of pre-experimental biomass that was regrown over the treatment period. In addition, they measured the crude protein content (expressed as a percentage of biomass) and the total crude protein content (in grams) of the regrown biomass of the grasses.

Results of the experiment revealed, in the words of the authors, that "both C3 and C4 grasses increased forage quality and quantity under elevated CO2" (emphasis added). Indeed, analysis of the data presented in their Supplementary Table 1 indicate that elevated CO2 enhanced crude protein content percentages by an average of ~25% under both well-watered and water-limited conditions, albeit the percentage enhancements were higher for the C3 grasses than C4 grasses. Similarly, elevated CO2 stimulated biomass recovery. Surprisingly, this parameter was higher for the C4 grasses than the C3 grasses (238% vs 86%, respectively, under well-watered conditions and 205% vs 51%, respectively, under water-limited conditions). Lastly, significant enhancements in total crude protein content (g) were also noted under elevated CO2, where the mean enhancement for C3 grasses under both well-watered and water-limiting conditions was around 100%, whereas it was a whopping 226% and 248% for C4 grasses in the respective treatments.

The findings reported by McGranahan and Yurkonis are very important, for both the quality and quantity of forage are important characteristics of grazing ecosystems. And, as revealed by the data presented in this study, rising atmospheric CO2 concentrations stimulate both of these parameters. Thus, in the future, it can logically be inferred that grassland agricultural systems -- and the animals that rely upon them for food and nutrition -- will benefit as the air's CO2 content rises. And that is great news worth sharing!

Posted 5 September 2018