In situ 13CO2 pulse-labelling of upland grassland demonstrates a rapid pathway of carbon flux from arbuscular mycorrhizal mycelia to the soil

• The flux of pulse-derived 13C from upland pasture plants to the external mycelium of their arbuscular mycorrhizal (AM) symbionts was traced and quantified over a 7-d post-labelling period. • Mesh cores, which allowed in-growth of native AM mycelium but were impenetrable to roots, were inserted into unlimed and limed plots and the surrounding vegetation was exposed to 13CO2 at ambient CO2 concentrations. • Release of 13CO2 from cores colonized by AM mycelium peaked 9–14 h after labelling and declined within 24 h after severance of mycelial connections to roots. Between 5 and 8% of carbon lost by plants was respired by AM mycelium over the first 21 h after labelling. Liming increased the amount of carbon fixed by plants and subsequently allocated to fine roots and AM mycelium. • The results demonstrate for the first time under field conditions that AM mycelia provide a rapid and important pathway of carbon flux from plants to the soil and atmosphere