Symbiotic N2 fixation in a high Alpine grassland:: effects of four growing seasons of elevated CO2

1. Increasing carbon dioxide concentration (E: 680 mu l CO2 litre(-1) vs ambient, A: 355 mu l CO2 litre(-1)) around late-successional Alpine sedge communities of the Swiss Central Alps (2450 m) for four growing seasons (1992-1995) had no detectable effect on symbiotic N-2 fixation in Trifolium alpinum-the sole N-2-fixing plant species in these communities (74 +/- 30 mg N m(-2) year(-1), A and E plots pooled). 2. This result is based on data collected in the fourth growing season showing that elevated CO2 had no effect on Trifolium above-ground biomass (4.4 +/- 1.7 g m(-2), A and E plots pooled, n = 24) or N content per unit land area (124 +/- 51 mg N m(-2), A and E pooled), or on the percentage of N Trifolium derived from the atmosphere through symbiotic N-2 fixation (%Ndfa: 61.0 +/- 4.1 across A and E plots) estimated using the N-15 dilution method. 3. Thus, it appears that N inputs to this ecosystem via symbiotic N-2 fixation will not be dramatically affected in the foreseeable future even as atmospheric CO2 continues to rise.