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Title: Interactions between elevated atmospheric CO2 and defoliation on North American rangeland plant species at low and high N availability

Author
item Lecain, Daniel
item Morgan, Jack
item HUTCHINSON, GORDON - Retired ARS Employee
item REEDER, JEAN - Retired ARS Employee
item DIJKSTRA, FEIKE - University Of Sydney

Submitted to: Grass and Forage Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/19/2011
Publication Date: 8/15/2012
Citation: Lecain, D.R., Morgan, J.A., Hutchinson, G.L., Reeder, J.D., Dijkstra, F.A. 2012. Interactions between elevated atmospheric CO2 and defoliation on North American rangeland plant species at low and high N availability. Grass and Forage Science. 67:350-360.

Interpretive Summary: In addition to its well-established role in climate change, rising carbon dioxide in Earth’s atmosphere has a direct effect on rangelands through the stimulation of photosynthesis and secondary plant metabolic responses. This research investigated how increasing ambient carbon dioxide affects productivity and species responses in a northern mixed-grass prairie subjected to simulated grazing (defoliation) and variable soil N. Enrichment of the air with carbon dioxide increased production of mostly cool-season grasses, but lowered forage quality for all vegetation. Defoliation had little effect on the results. These findings, which are similar to previous research findings on rangelands that had not considered grazing, suggest that while rising carbon dioxide enhances productivity in rangelands managed for livestock grazing, forage quality will likely decline, leaving vegetation less useful for livestock.

Technical Abstract: Semi-arid rangelands are expected to be highly sensitive to global change, but few studies have explicitly investigated interactions between increased atmospheric CO2 and plant defoliation (such as occurs with animal grazing). This experiment subjected intact plant-soil cylinders from the Wyoming prairie to a 3-way factorial of CO2 (370 vs. 720 µl l-1), defoliation (non-clipped vs. clipped) and soil nitrogen (control vs. 10g m-2 added N) under nearly natural climate conditions. The functional groups C3-grasses, C4-grasses and forbs were investigated. Two- and three-way interactions on productivity and nitrogen resources were rare. Only the C3-grass had a positive (+50%) growth response to elevated CO2. Above-ground productivity was equal in defoliated and non-defoliated plants. Elevated CO2 consistently reduced “quality” of C3 and C4-grasses through reduced (-16%) tissue N concentration. Nitrogen-Use-Efficiency (NUE) was increased (+25%) under elevated CO2, but plant uptake of 15N from the soil was unaffected. In contrast, defoliation improved (+29%) tissue quality (%N) in all functional groups; however, NUE was reduced (-23%) by defoliation. Without additional N availability, forage quality with defoliation may not be maintained under increasing atmospheric CO2. Increased dominance of C3-grasses plus reduced forage quality will necessitate changes in grazing management practices in the northern mixed-grass prairie.