Arbuscular mycorrhizal fungi (AMF; phylum Glomeromycota) are ubiquitous in terrestrial ecosystems. Despite their acknowledged importance in ecology, most research on AMF has focused on effects on individual plant hosts, with more recent efforts aimed at the level of the plant community. Research at the ecosystem level is less prominent, but potentially very promising. Numerous human-induced disturbances (including global change and agro-ecosystem management) impinge on AMF functioning; hence study of this symbiosis from the ecosystem perspective seems timely and crucial. In this paper, I discuss four (interacting) routes via which AMF can influence ecosystem processes. These include indirect pathways (through changes in plant and soil microbial community composition), and direct pathways (effects on host physiology and resource capture, and direct mycelium effects). I use the case study of carbon cycling to illustrate the potentially pervasive influence of AMF on ecosystem processes. A limited amount of published research on AMF ecology is suited for direct integration into ecosystem studies (because of scale mismatch or ill-adaptation to the ‘pools and flux’ paradigm of ecosystem ecology); I finish with an assessment of the tools (experimental designs, response variables) available for studying mycorrhizae at the ecosystem scale.