What is the Significance of Symbiotic N2 Fixation for Grassland Ecosystems in a CO2-Rich Word?

Abstract

The significance of symbiotic N2 fixation (measured as 15N-isotope dilution) for grassland ecosystems under elevated atmospheric pCO2 (60 Pa) was investigated under field conditions using the free air carbon-dioxide enrichment (FACE) technology. There was a fundamental difference in the CO2 response of plant biomass production in ecosystems depending on the presence of Trifolium repens: Under elevated pCO2, Lolium perenne grown in monoculture showed symptoms of N limitation (1, 3, 6) in such a way that the above-ground N-yield decreased under elevated pCO2 (6). This was in contrast to L. perenne growing with T. repens or to T. repens growing in monoculture where N nutrition appeared to be adequate (1, 3, 6). An evaluation of the N-sources clearly showed that under elevated pCO2 all nitrogen that was additionally assimilated in T. repens came from symbiotic N2 fixation (4, 5). This was a consequence of a consistent increase in the relative contribution of symbiotically fixed N to the total N yield (4, 5); the amount of symbiotically fixed N increased by 60% in the grass/legume mixtures through both increased clover proportion and increased N2 fixation in each individual clover plant (3, 4, 5). This led to a simultaneously enhanced apparent transfer of N from clover to grasses (6). These data suggest that increased photosynthetic CO2 fixation under elevated pCO2, although not entirely reflected in biomass production, was counterbalanced by an appropriately increased symbiotic N2 fixation, thus maintaining the C: N ratio at the whole ecosystem level. Since inadequate N supply would restrict an increase in extra C-sequestration into the ecosystem under elevated pCO2, symbiotic N2 fixation is considered to be a crucial driving force for increased carbon sequestration in a CO2-rich world (2).