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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. [ABSTRACT FROM AUTHOR]

Summary: 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. See also the Commentary on this article by Hempel, 220: 952–953. [ABSTRACT FROM AUTHOR]

Mowing is a crucial management strategy for conservation and restoration of semi‐natural grasslands. In the past decades, dominant mowing methods have changed from manual to mechanical, using heavy machines causing soil compaction. Whereas the negative effects of compaction are well understood in agriculture and forestry, they have received little attention so far in nature management. This study aims to investigate the effects of compaction on soil quality, arbuscular mycorrhizal fungal communities, and survival of a target plant species. We subjected seven grasslands to three contrasting mowing types: manual mowing with a brush cutter, softtrack mowing—designed to minimize soil compaction—and tractor mowing. We compared compaction, soil properties, and arbuscular mycorrhizal communities among the types. Additionally, we experimentally subjected intact grassland sods containing Devil's bit scabious (Succisa pratensis Moench), a target species of grassland conservation, to compaction levels corresponding to the three mowing types. Subsequently, we assessed plant survival. We found that soil compaction, soil nutrient availability, and toxic soil elements were significantly higher in the softtrack‐ and tractor‐mown parts of grassland sites compared to the manually mown parts. Also, the arbuscular mycorrhizal community composition differed significantly among mowing types, with the largest differences between “manual” and “tractor.” Succisa pratensis survival significantly decreased by 43 and 71% on the short term and by 71 and 86% on the longer term with increasing compaction. Consequently, mowing‐associated soil compaction compromises nature management targets via at least four mechanisms: nutrient enrichment, soil toxicity, changes in arbuscular mycorrhizal communities, and decreased survival of the target plant species. [ABSTRACT FROM AUTHOR]