Although it is well known that arbuscular mycorrhizal fungi (AMF) play a key role in the functioning of natural ecosystems, the underlying drivers determining the composition of AMF communities remain unclear. In this study, we established 138 sampling plots at 46 grassland sites, consisting of 26 acidic grasslands and 20 calcareous grasslands spread across eight European countries, to assess the relative importance of abiotic and biotic filtering in driving AMF community composition and structure in both the grassland soils and in the roots of 13 grassland plant species. Soil AMF communities differed significantly between acidic and calcareous grasslands. In root AMF communities, most variance was attributable to soil variables while very little variation was explained by host plant identity. Root AMF communities in host plant species occurring in only one grassland type closely resembled the soil AMF communities of that grassland type and the root AMF communities of other host plant species occurring in the same grassland type. The observed AMF-host plants networks were not modular but nested. Our results indicate that abiotic conditions, rather than biotic filtering through host plant specificity, are the most important drivers in shaping AMF communities in European seminatural grasslands.

Species diversity is commonly hypothesized to result from trade-offs for different limiting resources, providing separate niches for coexisting species. As soil nutrients occur in multiple chemical forms, plant differences in acquisition of the same element derived from different compounds may represent unique niche dimensions. Because plant productivity of ecosystems is often limited by phosphorus, and because plants have evolved diverse adaptations to acquire soil phosphorus, a promising yet untested hypothesis is phosphorus resource partitioning. Here, we provided two different chemical forms of phosphorus to sown grassland mesocosms to investigate phosphorus acquisition of eight plant species that are common in European grasslands, and to identify subsequent patterns of plant abundance. For the first time, we show that the relative abundance of grassland plant species can be influenced by soil phosphorus forms, as higher abundance was linked to higher acquisition of a specific form of phosphorus. These results were supported by a subsequent isotope dilution experiment using intact grassland sods that were treated with different inorganic or organic phosphorus forms. Here, 5 out of 14 species showed greater phosphorus acquisition in the inorganic phosphorus treatment, and 4 in the organic phosphorus treatments. Furthermore, for the species used in both experiments we found similar acquisition patterns. Our results support the hypothesis of phosphorus resource partitioning and may provide a new mechanistic framework to explain high plant diversity in phosphorus-poor ecosystems. As world biodiversity hotspots are almost invariably related to phosphorus limitation, our results may thus also be key to understanding biodiversity loss in an era of ever-increasing nutrient enrichment.

Aim Our aim was to quantify the extent to which nutrient pollution explains arbuscular mycorrhizal fungal community richness and composition. Location Europe. Time period 2014–2016. Major taxa studied Arbuscular mycorrhizal fungi. Methods We sampled soils of calcareous and acidic grasslands and roots of 34 host plant species across a large geographical gradient of atmospheric nitrogen deposition and soil phosphorus availability. Furthermore, we performed an independent pairwise comparison between fertilized and unfertilized grasslands in Belgium and Iceland to compare results. Results We found that nitrogen deposition had a significant negative relationship to arbuscular mycorrhizal fungal richness, with a negative community threshold of 7.7 kg N/ha/year corresponding to the greatest reduction in operational taxonomic units. Additionally, we found that soil phosphorus had a significant negative relationship to mycorrhizal fungal richness. Main conclusions Our results highlight the necessity to revisit the critical loads of atmospheric nitrogen deposition used in European environmental policy, currently set at 10–15 kg N/ha/year. Importantly, our observed threshold of 7.7 kg N/ha/year does not correspond to a critical load below which there is no environmental harm, because the least negative changes in arbuscular mycorrhizal fungal communities were observed at < 5 kg N/ha/year. Therefore, to avoid compromising the policy tenet of no environmental harm with respect to grassland mycorrhizal fungi, areas of zero tolerance to nitrogen pollution should be delimited. Our results also indicate that environmental policy biased towards reducing nitrogen pollution alone will fail to preserve mycorrhizal biodiversity in European grasslands. We advocate increased policy attention to avoid phosphorus enrichment, particularly through agricultural fertilization. Here too, areas of zero phosphorus input, ideally set in the currently unpolluted (or least polluted) areas, seem key for effective environmental policy, because elevated levels of soil phosphorus after phosphorus fertilization are known to be extremely persistent.

Ectomycorrhizal communities in urban areas are significantly related to soil characteristics, while heavy metal pollution and biogeography had little or no effect.

Summary Despite the ecological significance of ericoid mycorrhizal fungi, little is known about the abiotic and biotic factors driving their diversity and community composition. To determine the relative importance of abiotic and biotic filtering in structuring ericoid mycorrhizal fungal communities, we established 156 sampling plots in two highly contrasting environments but dominated by the same Ericaceae plant species: waterlogged bogs and dry heathlands. Plots were located across 25 bogs and 27 dry heathlands in seven European countries covering a gradient in nitrogen deposition and phosphorus availability. Putatively ericoid mycorrhizal fungal communities in the roots of 10 different Ericaceae species were characterized using high‐throughput amplicon sequencing. Variation in ericoid mycorrhizal fungal communities was attributed to both habitat and soil variables on the one hand and host plant identity on the other. Communities differed significantly between bogs and heathlands and, in a given habitat, communities differed significantly among host plant species. Fungal richness was negatively related to nitrogen deposition in bogs and phosphorus availability in bogs and heathlands. Our results demonstrate that both abiotic and biotic filtering shapes ericoid mycorrhizal fungal communities and advocate an environmental policy minimizing excess nutrient input in these nutrient‐poor ecosystems to avoid loss of ericoid mycorrhizal fungal taxa.