Nitrogen and [CO.sub.2] Affect Regrowth and Biomass Partitioning Differently in Forages of Three Functional Groups

Little work has been done to assess the impact of elevated [CO.sub.2] on responses of forages to defoliation. This study examines regrowth, biomass partitioning, and labile C and N metabolites in three functional plant-types: a [C.sub.3] grass [Pascopyrum smithii (Rydb.) A. Love], a [C.sub.4] 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 [CO.sub.2] [low [CO.sub.2] (LC), 355 µmol [mol.sup.-1], and high [CO.sub.2] (HC), 700 µmol [mol.sup.-1]] and two N nutrition regimes [low N (LN), watered twice weekly with half-strength Hoagland's containing 0 N, and high N (HN), half-strength Hoagland's containing 14 mM N]. High N enhanced regrowth in all three species, while high [CO.sub.2] enhanced regrowth only in the two [C.sub.3] species. In M. sativa, [CO.sub.2] 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 [CO.sub.2] so that k was greatest for plants grown at HN/ HC (0.95) and HN/LC (0.89), intermediate at LN/LC (0.58), 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 [CO.sub.2]. Significant correlations were established between k and several measures of plant N status, suggesting that the effects of [CO.sub.2] on plant biomass partitioning involve N status.
DESPITE THE IMPORTANCE of the effect of grazing pressure on the production and ecology of rangelands and pastures (Harris, 1978; Milchunas and Lauenroth, 1993), only a limited amount of attention [...]