Arbuscular mycorrhizae (AM) are ancient fungal habitants going back more than 400 million years [ Parniske 2008 ]. They are mainly involved in the transfer of water and mineral nutrients, in particular phosphate (Pi), to their host plants. Roots of over 90 % terrestrial plant species can associate with AM fungi, establishing endosymbiotic relationships [Van Der Heijden and Horton 2009] . AM exchanges are estimated to consume up to 20 % of the photosynthetic production of terrestrial plants (approximately 5 billion tonnes of carbon per year) [Parniske 2008] . Therefore, AM symbiosis contributes significantly to global phosphate and carbon cycling and influences primary productivity in terrestrial ecosystems [Parniske 2008] .
AM fungi are coenocytic, that is their hyphae have a continuous cytoplasm (without cross walls) in which thousands of nuclei coexist, forming a syncytium. The nuclei encode surprisingly diverse genomes and several lines of
evidence indicate that the nuclei themselves are diverse, that is AM fungi are heterokaryotic [Parniske 2008] . This suggests that AM fungi could have a wide range of metabolic and symbiotic capabilities. Its long evolutionary history, its highly conserved primitive cellular form, combined with its ubiquitous symbiosis with terrestrial autotrophs leads us to believe that AM probably occupy an important ecological niche in the subsoil ecosystem comprising plants, bacteria and geochemical cycles [Johnson et al. 2005] .
Subsoil networks of AM fungi are known to interconnect multiple species of autotrophic plants [ Van Der Heijden and Horton 2009 ; Heijden et al. 2015 ]. They modulate [ Van der Heijden et al. 1998] and influence plant community structure [Montesinos-Navarro et al. 2012] and alter plant-plant interactions by supplying and recycling nutrients [ Van Der Heijden and Horton 2009 ].
AM fungi symbiotically provide phosphorous (and Nitrogen) from the soil in exchange for carbohydrates from plants. The extent of symbiosis in the plant root system is dependent on the plants Phosphorous and Nitrogen status and its photosynthetic capacity (carbohydrate production) [ Van Der Heijden and Horton 2009 ]. Since AM exchange subsoil Phosphorous for Carbohydrates from plants, they are able to simultaneously sense the status of subsoil geochemical cycles, and the photosynthetic productivity of the associated plant species. Through their modulator effects AM fungi are known to have both positive and negative effects on plant growth [ Van Der Heijden and Horton 2009 ], hence modulate flows in interspecies networks comprised of autotrophs, bacteria, and biogeochemical cycles [ Bonfante and Anca 2009 ].
While Phosphorous is predicted to be the major limiting element in terrestrial ecosystems high-energyCarbon is the limiting element for decomposers [Cherif and Loreau 2009] . Hence by exchanging Phosphorous for high-energy Carbon (carbohydrate) from plants, AM could be able to allocate growth limiting Phosphorous to modulate overall production of carbohydrate across networks of competing species of autotrophs and their subsoil bacterial ecosystems [Walder et al. 2012] .
