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

摘要

Increasing carbon dioxide concentration (E: 680μlitre CO2 litre-1 vs ambient, A: 355μlitre CO2 litre-1) around late-successional Alpine sedge communities of the Swiss Central Alps (2450 m) for four growing seasons (1992-95) had no detectable effect on symbiotic N2 fixation in Trifolium alpinum, which is the sole N2-fixing plant species in these communities (74±30 mg N m-2 year-1, A and E plots pooled). 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. It is suggested 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.