Ozone and Nitrogen Effects on Juvenile Subalpine Plants: Complex Interactions with Species and Colonization by Arbuscular Mycorrhizal Fungi (AMF)

Increasing concentrations of tropospheric ozone and reactive nitrogen threaten the composition and function of semi-natural plant communities. Using a free-air fumigation system, we investigated the effects of elevated ozone (1.73 x ambient concentration; +O-3)and nitrogen (+50 k g N ha year(-1); +N) deposition on growth of juveniles of three subalpine grassland species and their colonization by arbuscular mycorrhizal fungi (AMF) in situ. In a subsample, the extra-radical mycelium was regularly disrupted to determine the effect of AMF on the plants' pollutant sensitivity. The plants reacted sensitively to the pollutants: +N increased shoot growth in Festuca rubra, while +O-3 decreased root growth in Trifolium alpinum and F. rubra. Colonization with AMF was stimulated by N in Leontodon helveticus and was strongly reduced by O-3 in F. rubra and L. helveticus, probably due to lower carbon allocation belowground. Conversely, AMF did not protect plants from O-3 neither did they increase the species' responsiveness to N. Our results indicate that irrespective of AMF colonization, juvenile plants are highly sensitive to O-3 stress, probably since their growth is primarily limited by carbon assimilation.