Nitrogen inhibition of nodulation and N 2 fixation of a tropical N 2 ‐fixing tree ( Gliricidia sepium ) grown in elevated atmospheric CO 2

Interactive effects of elevated atmospheric CO 2 and soil N availability on N 2 fixation and biomass production were examined using Gliricidia sepium , a tropical leguminous tree species. Our objective was to determine if elevated CO 2 alters the inhibitory effects of soil N on N 2 fixation, and whether the response of Gliricidia to elevated CO 2 was a function of N source originating from either substrate N fertilizer or N 2 fixation. We hypothesized that CO 2 enrichment would ameliorate the inhibitory effects of N fertilization on seedling nodulation and N 2 fixation through increased C partitioning to nodules. Seedlings were grown from seed for 100 d in growth chambers at either 350 or 700 μmol mol −1 CO 2 . Seedlings were inoculated with Rhizobium spp. and grown either with 0, 1 or 10 mM N fertilizer. The δ 15 N isotope‐dilution technique was used to determine N source partitioning between N 2 fixation and inorganic N fertilizer uptake. The addition of 10 mM N fertilizer significantly reduced nodule number and mass, specific nitrogenase activity, the specific rate of N 2 fixation, and the proportion of plant N derived from N 2 fixation. Elevated CO 2 , however, strongly ameliorated the inhibitory effects of N fertilization, indicating that increased C availability for nodule activity may partially offset the inhibition of N 2 fixation caused by substrate N, as nodule sugar concentrations were stimulated with CO 2 enrichment. This study clearly shows that elevated CO 2 enhanced plant productivity and net N content of Gliricidia tree seedlings by stimulating N 2 fixation. In addition, seedling biomass production was greatly enhanced by elevated CO 2 , regardless of whether plant N was derived from the substrate or from the atmosphere. We conclude from this study that CO 2 enrichment mitigates the inhibitory effects of substrate N on nodule initiation and development and specific N 2 fixation, either through increased C allocation to nodule production and activity, or through increased N demand by the plant for biomass production. This experiment with Gliricidia provides evidence for a positive feedback between increased atmospheric CO 2 concentrations, C allocation to symbiotic N 2 ‐fixing bacteria, and plant C and N accumulation that may occur when N 2 ‐fixing plants are grown under conditions where substrate N may typically inhibit N 2 fixation.