<i>In situ</i>¹³CO₂ pulse‐labelling of upland grassland demonstrates a rapid pathway of carbon flux from arbuscular mycorrhizal mycelia to the soil

Summary The flux of pulse‐derived 13 C 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 13 CO 2 at ambient CO 2 concentrations. Release of 13 CO 2 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.