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 × ambient concentration; +O3) 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 +O3 decreased root growth in Trifolium alpinum and F. rubra. Colonization with AMF was stimulated by N in Leontodon helveticus and was strongly reduced by O3 in F. rubra and L. helveticus, probably due to lower carbon allocation belowground. Conversely, AMF did not protect plants from O3 neither did they increase the species’ responsiveness to N. Our results indicate that irrespective of AMF colonization, juvenile plants are highly sensitive to O3 stress, probably since their growth is primarily limited by carbon assimilation.