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

Abstract

1. Increasing carbon dioxide concentration (E: 680 μl CO 2 litre –1 vs ambient, A: 355 μl CO 2 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 CO 2 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 15 N 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 CO 2 continues to rise.