Organic nitrogen uptake and endophytic, mutualistic fungi in Dutch heathland ecosystems

Dutch heathlands were formerly dominated by the evergreen dwarf shrubs Calluna vulgaris and Erica tetralix, but during the 1970s and 1980s both species were increasingly displaced by the grasses Deschampsia flexuosa and Molinia caerulea . The causes of these changes include the direct and indirect effects of increased deposition of atmospheric nitrogen. This thesis focuses on the role of organic nitrogen uptake and endophytic, mutualistic fungi in Dutch heathland ecosystems. I tested the hypothesis that tannin-rich plant species are able to monopolize the nutrient cycle by increasing the amounts of organic nitrogen forms relative to inorganic nitrogen forms. In addition, those species will be favored which absorb organic nitrogen compounds through their associations with ericoid or ectomycorrhizal fungi. A field inventory and the results of related experiments under controlled conditions showed that nitrogen addition and shading both negatively affect the concentration of tannins in Calluna plants. Only in the greenhouse experiment was the presence of mycorrhizal structures in roots negatively affected by the addition of nitrogen. In the field experiment, shading reduced the amount of mycorrhizal structures in roots. It is concluded that when ericaceous plants are shaded by grasses that have become dominant due to increased nitrogen supplies, these effects will be intensified and displacement by competition will be accelerated. To test the effect of tannin-rich litter on nitrogen mineralization rates, an incubation experiment was performed with different types of shrub litter and grass litter. It was found that litters with C :N ratios above 30 and high tannin concentrations (as found in C. vulgaris and Vacciniumvitis-idaea ) decrease the amounts of inorganic nitrogen and concomitantly increase the amounts of dissolved organic nitrogen in soils. When searching for the root inhabitants of ericaceous plants there was a great diversity of endophytic fungal species present. Several new species of fungi were identified, one of which was published as Cryptosporiopsis rhizophila Verkley & Zijlstra. Synthesis trials and Bavendamm tests to elucidate the ecological role of this new fungal species revealed that C. rhizophila isolates are able to associate with roots of C. vulgaris and have the potential to fulfil the same ecological function to their ericoid host as well-known mycorrhiza formers. Surprisingly, it was discovered that fungi isolated from grass roots contained endophytic fungal species related to the Helotiales , the phylogenetic group to which most of the ericaceous fungal isolates isolated in this research belong. A competition experiment tested the effects of tannin-rich litter types on soil nitrogen and the outcome of the competition between grass and shrub species. In none of the treatments were C. vulgaris plants able to outcompete D. flexuosa . Grass plants were able to benefit more efficiently from the available soil nitrogen released from the types of litter added. Furthermore, in the treatments with low nutrient availability, there was hardly any competitive suppression of shrub plants by the grasses. This suggests that grasses from the heathland systems have adapted well to the high amounts of organic nitrogen - including adaptation to related ericaceous fungal symbionts. In conclusion, this thesis shows that being competitive for heathland plants by monopolizing the nutrient cycle with tannin-rich litter is not that simple if competitors have adopted the same strategy to use nutrients.