Ericoid mycorrhizal fungi recover from nitrogen additions in a dry heathland experiment in southern England

Elena Arrigoni, Ruairi Hafferty, Thomas D. Brekke, Gerhard Gebauer, Kerry Bradshaw, Jeffrey G. Duckett, Silvia Pressel, Carrie Andrew, Martin I. Bidartondo,, Laura M. Suz, Jill Kowal

1. Ericoid mycorrhizal fungi are fundamental for heathlands’ establishment and functioning and contribute to the essential ecosystem services these endangered habitats provide, yet they are overlooked by environmental monitoring policies. As nitrogen pollution declines across Europe, policymakers need measurable indicators of habitat recovery encompassing above- and below-ground parts of the ecosystem. 2. We conducted a manipulation experiment simulating atmospheric nitrogen pollution on pioneer and mature heather (Calluna vulgaris) stands in a dry heathland to determine its impact on the functionality and composition of ericoid mycorrhizal communities, and to assess their viability as short-term recovery indicators in three substrates: heather roots, a common leafy liverwort (Cephaloziella divaricata), and soil. Additionally, we measured aboveground liverwort biomass, rhizoid colonisation and density, soil and heather leaf chemistry. 3. After an initial negative impact of nitrogen fertilisation, ericoid mycorrhizal colonisation in heather roots, ericoid mycorrhizal fungi of the genus Hyaloscypha in roots and soil, and symbiotic Pezoloma in liverworts, all recovered within seven months. Liverwort biomass, density and rhizoid colonisation were negatively affected, and did not recover. 4. The effect of nitrogen fertilisation and recovery trajectories depended on substrate, age of heather stands, and ericoid mycorrhizal morphotype and taxon. Pioneer plants were more sensitive to treatments than mature plants. 5. Policy implications: Our findings open new perspectives on the potential use of mycorrhizal fungi as indicators of recovery from nitrogen pollution in heathlands. The rapid recovery observed in pioneer heather communities suggests that ericoid mycorrhizal fungi can provide short-term measures of belowground recovery, but plant successional stage and fungal identity must be considered. Our results support the development of targeted and stage-specific belowground indicators to complement existing aboveground-focussed indicators, to improve assessment of recovery from nitrogen pollution in heathland conservation and restoration policy.