Nitrogen and CO2 Affect Regrowth and Biomass Partitioning Differently in Forages of Three Functional Groups

Little work has been done to assess the impact of elevated CO2 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 μmol mol−1, and high CO2 (HC), 700 μmol mol−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 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 (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 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.