Nitrogen inhibition of nodulation and N₂ fixation of a tropical N₂-fixing tree (Gliricidia sepium) grown in elevated atmospheric CO₂

Abstract

Interactive effects of elevated atmospheric CO₂ and soil N availability on N₂ fixation and biomass production were examined using Gliricidia sepium, a tropical leguminous tree species. Our objective was to determine if elevated CO₂ alters the inhibitory effects of soil N on N₂ fixation, and whether the response of Gliricidia to elevated CO₂ was a function of N source originating from either substrate N fertilizer or N₂ fixation. We hypothesized that CO₂ enrichment would ameliorate the inhibitory effects of N fertilization on seedling nodulation and N₂ 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 CO₂. Seedlings were inoculated with Rhizobium spp. and grown either with 0, 1 or 10 mM N fertilizer. The δ¹⁵N isotope-dilution technique was used to determine N source partitioning between N₂ 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 N₂ fixation, and the proportion of plant N derived from N₂ fixation. Elevated CO₂, however, strongly ameliorated the inhibitory effects of N fertilization, indicating that increased C availability for nodule activity may partially offset the inhibition of N₂ fixation caused by substrate N, as nodule sugar concentrations were stimulated with CO₂ enrichment. This study clearly shows that elevated CO₂ enhanced plant productivity and net N content of Gliricidia tree seedlings by stimulating N₂ fixation. In addition, seedling biomass production was greatly enhanced by elevated CO₂, regardless of whether plant N was derived from the substrate or from the atmosphere. We conclude from this study that CO₂ enrichment mitigates the inhibitory effects of substrate N on nodule initiation and development and specific N₂ 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 CO₂ concentrations, C allocation to symbiotic N₂-fixing bacteria, and plant C and N accumulation that may occur when N₂-fixing plants are grown under conditions where substrate N may typically inhibit N₂ fixation.