Effects of pavements on diversity and activity of mycorrhizal symbionts associated with urban trees

This study investigated the impact of different types of soil sealing on the communities of a group of beneficial plant symbionts, arbuscular mycorrhizal fungi (AMF), colonizing the roots of two shade trees, Celtis australis and Fraxinus ornus, frequently grown in urbanized sites. Such plants were grown in an experimental site, in northern Italy, established in November 2011 and subjected to four different pavement treatments: impermeable monolithic asphalt, permeable pavers, permeable concrete and unpaved soil. The diversity and composition of root AMF communities were assessed by PCR denaturating gradient gel electrophoresis of partial 18S rRNA gene, AMF taxa were identified by amplicon sequencing and mycorrhizal colonization was evaluated after root clearing and staining. For the first time, our molecular work revealed that impermeable pavements induced shifts in the composition of AMF communities associated to the roots of C. australis and F. ornus and impacted on the percentage of mycorrhizal root length. When the root-zone was covered with permeable pavements, a similar AMF community as that observed in the unpaved soil was detected, providing novel information to be utilised for reducing the disturbance caused by specific types of soil sealing on AMF symbionts, which play a key role in plant nutrition and health. A total of 45 AMF sequence types were detected, with Sclerocystis and Septoglomus as the most abundant phylotypes, accounting for 84% of the sequences. The predominance of Sclerocystis species in the roots of both tree species under impermeable pavements indicated their high and unforeseen tolerance towards harsh environmental conditions. Such species could be utilized as AMF inocula specifically selected for their proven resilience in paved sites, in order to exploit their ability to boost biogeochemical processes fundamental for energy fluxes and plant nutrition and health.