Growth and nitrogen accretion of dinitrogen-fixing Alnus glutinosa (L) Gaertn under elevated carbon dioxide

Short-term studies of tree growth at elevated CO2 suggest that forest productivity may increase as atmospheric CO2 concentrations rise, although low soil N availability may limit the magnitude of this response. There have been few studies of growth and N-2 fixation by symbiotic N-2-fixing woody species under elevated CO2 and the N inputs these plants could provide to forest ecosystems in the future. We investigated the effect of twice ambient CO2 on growth, tissue N accretion, and N-2 fixation of nodulated Alnus glutinosa (L.) Gaertn. grown under low soil N conditions for 160 d. Root, nodule, stem, and leaf dry weight (DW) and N accretion increased significantly in response to elevated CO2. Whole-plant biomass and N accretion increased 54% and 40%, respectively. Delta-N-15 analysis of leaf tissue indicated that plants from both treatments derived similar proportions of their total N from symbiotic fixation suggesting that elevated CO2 grown plants fixed approximately 40% more N than did ambient CO2 grown plants. Leaves from both CO2 treatments showed similar relative declines in leaf N content prior to autumnal leaf abscission, but total N in leaf litter increased 24% in elevated compared to ambient CO2 grown plants, These results suggest that with rising atmospheric CO2 N-2-fixing woody species will accumulate greater amounts of biomass N through N-2 fixation and may enhance soil N levels by increased litter N inputs.