Native grass facilitates mycorrhizal colonisation and P uptake of tree seedlings in two anthropogenic substrates

Two microcosm experiments were conducted to study the role of extraradical mycelium (ERM) of arbuscular mycorrhizal fungi (AMF) in establishment and growth of tree species used for revegetation in anthropogenic substrates. Inoculated or non-inoculated Acer pseudoplatanus, Alnus glutinosa or Salix purpurea seedlings were grown with Calamagrostis epigejos (a grass spontaneously colonising degraded ecosystems) in two substrates (fly ash and coal mine spoil) either in direct root contact or in rhizoboxes with interaction only via ERM network. In both experiments, inoculation with AMF mostly had a positive effect on the growth of trees and increased the aggregation of fly ash. When plants grew in direct root-to-root contact, grass presence negatively affected tree growth, but it significantly improved mycorrhizal development (colonization of tree seedlings, spore number and ERM length). When grass and tree roots interacted via the ERM network, tree seedlings were successfully colonised by the ERM spreading from the C. epigejos roots. Mechanical disturbance of the ERM links between plants reduced AMF development and tree height in both substrates, but tree shoot biomass was not affected. In fly ash, inoculated, non-disturbed treatments showed significant transfer of P-32 from the grass to the tree seedlings. It can be concluded that roots of A. pseudoplatanus, A. glutinosa or S. purpurea seedlings can be colonised from the ERM network radiating from quickly growing grasses, which can act as important agents for AMF distribution and facilitate mycorrhization of planted trees. In particular for willow, grass seems to be an essential nurse plant to achieve successful root colonisation.