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 CO2 litre–1vs ambient, A: 355 μl 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 N2 fixation in Trifolium alpinum—the sole N2‐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 N2 fixation (%Ndfa: 61·0 ± 4·1 across A and E plots) estimated using the 15N dilution method. 3. Thus, it appears that N inputs to this ecosystem via symbiotic N2 fixation will not be dramatically affected in the foreseeable future even as atmospheric CO2 continues to rise.