Journal of Vegetation Science 23 (2012) 236–248 Fairy rings caused by a killer fungus foster plant diversity in species-rich grassland Giuliano Bonanomi, Antonio Mingo, Guido Incerti, Stefano Mazzoleni & Marina Allegrezza Keywords Agaricus campestris; Microbial physiological profiles; Species diversity; Soil-borne pathogens; Species co-existence Vegetation pattern Nomenclature Pignatti (1982) and Tutin et al. (1964-1980). Received 3 April 2011 Accepted 16 September 2011 Co-ordinating Editor: Martin Zobel Bonanomi, G. (corresponding author, giuliano.bonanomi@unina.it), Mingo, A. (antonio.mingo@unina.it), Incerti, G. (incerti@units.it) & Mazzoleni, S. (mazzolen@unina.it): Dipartimento di Arboricoltura, Botanica e Patologia Vegetale, Universita ` di Napoli Federico II, via Universita ` 100, Portici, 80055 (NA), Italy Allegrezza, M. (m.allegrezza@univpm.it): Dipartimento di Scienze Ambientali e delle Produzioni Vegetali, Facolta ` di Agraria, Universita ` Politecnica delle Marche, Via Brecce Bianche, I-60131 (AN), Italy Abstract Questions: Does the fairy ring fungus Agaricus campestris affects spatial distribu- tion of co-existing plant species? Is ring development related to changes of soil physical, chemical, enzymatic and microbiological properties? What are the causes of weakening and subsequent luxuriance of vegetation during fairy ring dynamics in the soil? Location: Species-rich, mowed calcareous grassland, central Italy (43° 17′26″ N, 12° 51′29″ E). Methods: Fairy rings were monitored for total plant biomass, species richness and composition in four zones: external grassland (OUT), fungal front (FF), area with flourishing vegetation (BELT) and internal grassland (IN). In each zone, 17 soil parameter were analysed: physical and chemical properties (water-holding capacity, pH, electrical conductivity, organic C, Olsen P, total N, NH 4 + , NO 3 À , hydrophobicity and cyanide concentration), total enzyme activity (FDA) and microbiological features (community-level physiological profile using BIOLOG EcoPlates ™ , microbial biomass, fungal mycelium, culturable actinomycetes, bacteria and fungi). A bioassay was performed to evaluate responses of three co-existing species (Bromus erectus, Cynosurus echinatus and Centaurea ambigua) growing on soil collected from different fairy ring zones. Results: Plant species composition dramatically changed in response to fairy ring development, with disappearance of most grassland species in the ring- affected area. Plant biomass and species richness were higher in OUT and IN areas, and lowest in FF. Profound changes in soil properties occurred after fungal passage, with consistent reductions of C and N content and increases of FDA and microbial physiological profiles. Soil from the FF zone had remarkable increases of mineral N forms, electrical conductivity and hydrophobicity, with no trace of cyanide. The bioassay showed species-specific responses to different soil types. Conclusions: This study provides evidence that the spread of fairy ring fungi, coupled with a reduction of perennial plant cover, creates empty niches for many short-lived species. Introduction Calcareous grasslands are among the most species-rich and dynamic ecosystems worldwide (Kull & Zobel 1991; van der Maarel & Sykes 1993). These plant communities have evolved as a result of a long history of human activity, gen- erating and maintaining open and semi-open habitats through the extensive use of fire, grazing and/or mowing. Many different mechanisms have been proposed to explain species co-existence in such highly diverse plant communities (Palmer 1994). Niche partitioning between species at equilibrium (Tilman 1994), non-equilibrium co- existence dynamics (Huston 1994) in relation to physical disturbance (Connell 1978) and biological predation (Jan- zen 1970), soil spatial heterogeneity or temporal fluctua- tions of environmental conditions (Chesson 2000), and a balance between immigration/speciation and extinction (McArthur & Wilson 1967) are some of the most well stud- ied mechanisms. In addition, in recent decades plant–soil feedback related to the build-up of soil-borne pathogen Journal of Vegetation Science 236 Doi: 10.1111/j.1654-1103.2011.01353.x © 2011 International Association for Vegetation Science