Summary• Species of arbuscular mycorrhizal fungi (AMF) differ markedly in their improvement of plant nutrition and health. However, it is not yet possible to relate the diversity of an AMF community to its functional properties due to the lack of information on the functional diversity at each taxonomic level. This study investigates the inter-and intraspecific functional diversity of four Glomus species in relation to a phylogenetic analysis of large ribosomal subunit (LSU) sequences.• Growth and P nutrition of cucumber ( Cucumis sativus ) associated with 24 different isolates of AMF were measured in a two-compartment system with a 33 P-labelled root-free soil compartment.• Intraspecific differences were found in plant growth response and the extension of the fungal mycelium into the root-free soil patch whereas length-specific P uptake of the hyphae remained rather constant within each AMF species. Hence, the length-specific P uptake differed according to species, whereas lower phylogenetic levels were required to match functional characteristics such as fungal growth pattern and plant growth promotion.• The large intraspecific diversity observed for mycelium growth and improvement of P uptake means that AMF communities of low species diversity may still contain considerable functional heterogeneity.

Abstract. Large herbivores can influence plant and soil properties in grassland ecosystems, but especially for belowground biota and processes, the mechanisms that explain these effects are not fully understood. Here, we examine the capability of three grazing mechanisms-plant defoliation, dung and urine return, and physical presence of animals (causing trampling and excreta return in patches)-to explain grazing effects in Phleum pratense-Festuca pratensis dairy cow pasture in Finland. Comparison of control plots and plots grazed by cows showed that grazing maintained original plant-community structure, decreased shoot mass and root N and P concentrations, increased shoot N and P concentrations, and had an inconsistent effect on root mass. Among soil fauna, grazing increased the abundance of fungivorous nematodes and Aporrectodea earthworms and decreased the abundance of detritivorous enchytraeids and Lumbricus earthworms. Grazing also increased soil density and pH but did not affect average soil inorganic-N concentration. To reveal the mechanisms behind these effects, we analyzed results from mowed plots and plots that were both mowed and treated with a dung and urine mixture. This comparison revealed that grazing effects on plant attributes were almost entirely explained by defoliation, with only one partly explained by excreta return. Among belowground attributes, however, the mechanisms were more mixed, with effects explained by defoliation, patchy excreta return, and cow trampling. Average soil inorganic-N concentration was not affected by grazing because it was simultaneously decreased by defoliation and increased by cow presence. Presence of cows created great spatial heterogeneity in soil N availability and abundance of fungivorous nematodes. A greenhouse trial revealed a grazing-induced soil feedback on plant growth, which was explained by patchiness in N availability rather than changes in soil biota. Our results show that grazing effects on plant attributes can be satisfactorily predicted using the effects of defoliation, whereas those on soil fauna and soil N availability need understanding of other mechanisms as well. The results indicate that defoliation-induced changes in plant ecophysiology and the great spatial variation in N availability created by grazers are the two key mechanisms through which large herbivores can control grassland ecosystems.

Roots of 76 plant species collected in West Spitsbergen (Svalbard), in the middle-northern Arctic zone, were examined for mycorrhiza and root-associated fungi. Dryas octopetala, Pedicularis dasyantha and Salix polaris were ectomycorrhizal and Cassiope tetragona and Empetrum hermaphroditum ericoid mycorrhizal. Pedicularis dasyantha was only slightly infected. Structures resembling vesicular-arbuscular mycorrhizal (VAM) fungi were not found in the roots, and soil samples screened for VAM fungi contained only one spore. Root endophytic fungi commonly occurred in Spitsbergen, but only Olpidium brassicae, Pleospora herbarum, Papulaspora, Microdochium bolleyi and Rhizoctonia solani were identified with reasonable certainty. A sterile endophytic dark-septate fungus (DSF) was in 39.5~ of the flowering-plant species examined, especially in the Brassicaceae, Caryophyllaceae, Saxifragaceae and Poaceae. DSF were categorized into four slightly overlapping groups. Sterile endophytic hyaline septate fungi were rare. In the literature it is suggested that at least some of the DSF species or the hyaline septate fungi are functionally mutualistic rather than saprophytic or pathogenic. The literature on ectomycorrhizal fungi and plants in Spitsbergen is reviewed, including about 50 species, mainly of the genera Cortinarius, Hebeloma, Inocybe, Laccaria, Lactarius and Russula. These are symbiotic with the above-mentioned ectomycorrhizal plants. Four further ectomycorrhizal plants (Betula nana, Salix spp.) are very rare in the area.

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By acquiring symbiosis from the mycelium supported by neighboring plants, seedlings become connected to a common mycorrhizal network (CMN). Although the maintenance of a CMN may lead to mutual benefit among the neighboring plants, the benefits may be unequally distributed if the plants differ in their sink strength for the shared resources in the CMN. Hence, seedlings may not obtain any net benefit from a CMN maintained by competitively superior, mature plants. In a controlled greenhouse experiment, we showed that the presence of a mycorrhizal adult plant does not improve the growth of seedlings, although solitary seedlings benefit from mycorrhizae. Seedlings of the perennial, mycorrhizal, subarctic herbs Antennaria dioica, Campanula rotundifolia, Sibbaldia procumbens, and Solidago virgaurea were germinated alone or in the vicinity of an adult established Sibbaldia plant. The seedlings were either left nonmycorrhizal (NM) or were inoculated by spores or by a hyphal network connected to established Sibbaldia plants. Three arbuscular mycorrhizal fungi (AMF) were involved: Glomus claroideum, G. hoi, or an undescribed Glomus sp. isolate. In the absence of an adult Sibbaldia plant, the plant species responded differently to each AMF species, of which G. claroideum was most infective and most improved the growth of solitary seedlings. In NM seedlings grown alternatively with or without a NM adult plant, competition had no or only weakly adverse effects on seedling growth. Interestingly, the situation was the same in seedlings grown with a mycorrhizal adult plant: although heavily infected, seedlings did not show improved growth as compared to NM seedlings. A common mycorrhizal network may well imply some mutual aid for the connected plants, but competitive interactions within the CMN overwhelm any benefits. Corresponding Editor: S. H. Faeth.