Laboratory experiments imply the conditionality of mycorrhizal benefits for Salix repens:: role of pH and nitrogen to phosphorus ratios

Responses of one arbuscular mycorrhizal fungus (Glomus mosseae) and two ectomycorrhizal fungi (Hebeloma leucosarx, Paxillus involutus) to a range of substrate conditions were investigated in the laboratory. Non-mycorrhizal controls were also included. Substrate conditions included three levels of nitrogen - phosphorus ratios, ranging from N limitation to P limitation (N/P ratio 5.4, 16.2, 48.6), and three pH's, ranging from acidic to alkaline (pH 4, 5.5, 7), in a full factorial experiment. Plant parameters (carbon gain, N and P-content, root length) were significantly affected by fungus, soil pH and soil N/P, and their interactions. Mycorrhizal benefits by ectomycorrhizal fungi (EcMF) were generally larger than by the arbuscular mycorrhizal fungi (AMF), when assessed by above-ground parameters. Glomus mosseae, despite low colonization, had a much larger positive effect on root length than those EcMF. Hebeloma leucosarx and P. involutus were equally effective, despite differences in proportional colonization. Hebeloma leucosarx was able to expand niche width of S. repens towards alkaline conditions. Results are discussed in the framework of a dune successional gradient from young, calcareous, humus-poor towards old, acidic, humus-rich soils.