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

Abstract

Short-term studies of tree growth at elevated CO 2 suggest that forest productivity may increase as atmospheric CO 2 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 CO 2 and the N inputs these plants could provide to forest ecosystems in the future. We investigated the effect of twice ambient CO 2 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 CO 2 . Whole-plant biomass and N accretion increased 54% and 40%, respectively. Delta- 15 N analysis of leaf tissue indicated that plants from both treatments derived similar proportions of their total N from symbiotic fixation suggesting that elevated CO 2 grown plants fixed approximately 40% more N than did ambient CO 2 grown plants. Leaves from both CO 2 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 CO 2 grown plants. These results suggest that with rising atmospheric CO 2 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.