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item Morgan, Jack
item Skinner, Robert
item Hanson, Jonathan

Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/5/2000
Publication Date: 1/2/2001
Citation: Morgan, J.A., Skinner, R.H., Hanson, J.D. 2001. Nitrogen and CO2 affect regrowth and biomass partitioning differently in forages of three functional groups. Crop Science 41(1):78-86.

Interpretive Summary: The amount of carbon dioxide in the earth's atmosphere is increasing, primarily from the burning of fossel fuels and secondarily from the loss of significant portions of important ecosystems, like forests, which have the capacity to assimilate and therefore remove some of the carbon dioxide from the atmosphere in photosynthesis. Higher carbon dioxide levels are known to effect plant physiology, but the details of those metabolic reactions are still largely unknown. This study was designed to investigate how increase atmospheric carbon dioxide concentrations will affect the recovery of forage plants from the stresses of defoliation due to livestock grazing. Higher carbon dioxide concentrations led to improved re-growth following defoliation in a cool-season forage grass, western wheatgrass, and also in the popular legume alfalfa, while having no effect on re-growth of a warm season grass, blue grama. Growth under elevated carbon dioxide also led to increased growth of roots in the grasses, but not alfalfa. The results suggest that increasing atmospheric carbon dioxide may differentially enhance re-growth of forages following defoliation. The outcome of this may be that as atmospheric carbon dioxide levels continue to increase, plant communities may eventually transition towards ecosystems primarily composed of those species which respond more strongly to carbon dioxide.

Technical Abstract: Little work has been done to assess the impact of elevated C02 on responses of forages to defoliation. This study examines regrowth, biomass partitioning, and labile C and N metabolites in three functional plant-types: a C3 grass [Pascopyrum smithii (Rydb.) A. Love], a C4 grass [Bouteloua gracilis (H.B.K.) Lag.], and a forage legume (Medicago sativa L.). Plants were grown from seed, defoliated twice, and grown in a controlled environment under a factorial arrangement of two CO2 [low CO2 (LC), 355 umol mol-1, and high CO2 (HC), 700 umol mol-1] and two N nutrition regimes [low N (LN), watered twice weekly with half strength Hoaglund's containing 0 N, and high N (HN), half-strength Hoaglund's containing 14 mM N]. High N enhanced regrowth in all three species, while high CO2 enhanced regrowth only in the two C3 species. In M. sativa, CO2 and N treatments had no significant effect on k, the allometric growth coefficient. In contrast, k was reduced in P. smithii plants grown under LN (0.63) compared with HN (0.99). In B. gracilis, low N also reduced k, but it interacted with CO2 so that k was greatest for plants grown at HN/HC (0.95) and HN/LC (0.89), intermediate at LN/LC (.58), and least at LN/HC (0.44). These results indicate greater partitioning to below-ground organs (reduced k) when N is limiting, particularly under elevated CO2. Significant correlations were established between k and several measures of plant N status, suggesting that the effects of CO2 on plant biomass partitioning involve N status.