Nitrogen inhibition of nodulation and N-2 fixation of a tropical N-2-fixing tree (Gliricidia sepium) grown in elevated atmospheric CO2

Interactive effects of elevated atmospheric CO2 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 eleva ted CO2 alters the inhibitory effects of soil N on N-2 fixation, and whethe r the response of Gliricidia to elevated CO2 was a function of N source ori ginating from either substrate N fertilizer or N-2 fixation. We hypothesize d that CO2 enrichment would ameliorate the inhibitory effects of N fertiliz ation on seedling nodulation and N-2 fixation through increased C partition ing to nodules. Seedlings were grown from seed for 100 d in growth chambers at either 350 or 700 mu mol mol(-1) CO2. Seedlings were inoculated with Rh izobium spp. and grown either with 0, 1 or 10 mM N fertilizer. The delta(15 )N isotope-dilution technique was used to determine N source partitioning b etween N-2 fixation and inorganic N fertilizer uptake. The addition of 10 m M N fertilizer significantly reduced nodule number and mass, specific nitro genase activity, the specific rate of N-2 fixation, and the proportion of p lant N derived from N-2 fixation. Elevated CO2, however, strongly ameliorat ed the inhibitory effects of N fertilization, indicating that increased C a vailability for nodule activity may partially offset the inhibition of N-2 fixation caused by substrate N, as nodule sugar concentrations were stimula ted with CO2 enrichment. This study clearly shows that elevated CO2 enhance d plant productivity and net N content of Gliricidia tree seedlings by stim ulating N-2 fixation. In addition,seedling biomass production was greatly e nhanced by elevated CO2, regardless of whether plant N was derived from the substrate or from the atmosphere. We conclude from this study that CO2 enr ichment mitigates the inhibitory effects of substrate N on nodule initiatio n and development and specific N-2 fixation, either through increased C all ocation 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 CO2 concent rations, C allocation to symbiotic N-2-fixing bacteria, and plant C and N a ccumulation that may occur when N-2-fixing plants are grown under condition s where substrate N may typically inhibit N-2 fixation.