Stimulation of symbiotic N-2 fixation in Trifolium repens L under elevated atmospheric pCO(2) in a grassland ecosystem

Symbiotic N-2 fixation is one of the main processes that introduces N into terrestrial ecosystems. As such, it may be crucial for the sequestration of the extra C available in a world of continuously increasing atmospheric CO2 partial pressure (pCO(2)). The effect of elevated pCO(2) (60 Pa) on symbiotic N-2 fixation (N-15-isotope dilution method) was investigated using Free-Air-CO2-Enrichment technology over a period of 3 years. Trifolium repens was cultivated either alone or together with Lolium perenne (a nonfixing reference crop) in mixed swards. Two different N fertilization levels and defoliation frequencies were applied. The total N yield increased consistently and the percentage of plant N derived from symbiotic N-2 fixation increased significantly in T. repens under elevated pCO(2). All additionally assimilated N was derived from symbiotic N-2 fixation, not from the soil. In the mixtures exposed to elevated pCO(2), an increased amount of symbiotically fixed N (+7.8, 8.2, and 6.2 g m(-2) a(-1) in 1993, 1994, and 1995, respectively) was introduced into the system. Increased N-2 fixation is a competitive advantage for T. repens in mixed swards with pasture grasses and may be a crucial factor in maintaining the C:N ratio in the ecosystem as a whole.