Activity and diversity of mycorrhizal symbionts associated with urban trees as affected by pavement treatments

Arbuscular mycorrhizal fungi (AMF) establish beneficial symbioses with the roots of most land plants, including food crops and shade trees. They promote plant growth and nutrition and improve plant tolerance to pathogens and abiotic stresses, such as drought, salinity and adverse growing conditions. In this work we studied the activity and diversity of AMF communities colonizing the roots of the shade trees Celtis australis and Fraxinus ornus, frequently grown in urbanized sites as affected by four types of pavements: impermeable monolithic asphalt, permeable modular inter-locking pavers, permeable concrete and unpaved soil. To this aim, we assessed the mycorrhizal status and colonization of the root systems of the two tree species, investigated the composition of native root AMF communities by using PCR-denaturating gradient gel electrophoresis (PCR-DGGE) of partial 18S rRNA gene, and identified native AMF by amplicon sequencing. In C. australis mycorrhizal colonization showed significant differences among the treatments, with decreases of 58 and 50% in plants grown in impermeable pavement and permeable pavers, respectively, compared with unpaved control. PCR-DGGE and cluster analyses differentiated AMF symbionts of plants growing in impermeable pavement from all the others, while permeable pavements and unpaved soil showed a similar AMF community. A total of 45 AMF sequence types were detected, with Sclerocystis and Septoglomus as the most abundant genera, accounting for 84% of the sequences. The predominance of Sclerocystis species in the roots of both trees under impermeable pavements indicated their high and unforeseen tolerance towards harsh environmental conditions. Our results detected the fungal symbionts differentially boosted or depressed in the different types of soil sealing, that will be isolated in order to produce the most resilient specific AMF inocula for trees growing in harsh environments, such as sealed soils in urbanized sites.