Arbuscular mycorrhizal mycelial respiration in a moist tropical forest.

*Arbuscular mycorrhizal fungi (AMF) are widespread in tropical forests and represent a major sink of photosynthate, yet their contribution to soil respiration in such ecosystems remains unknown. *Using in-growth mesocosms we measured AMF mycelial respiration in two separate experiments: (1) an experiment in a semi-evergreen moist tropical forest, and (2) an experiment with 6-m-tall Pseudobombax septenatum in 4.5-m(3) containers, for which we also determined the dependence of AMF mycelial respiration on the supply of carbon from the plant using girdling and root-cutting treatments. *In the forest, AMF mycelia respired carbon at a rate of 1.4 t ha(-1) yr(-1), which accounted for 14 +/- 6% of total soil respiration and 26 +/- 12% of root-derived respiration. For P. septenatum, 40 +/- 6% of root-derived respiration originated from AMF mycelia and carbon was respired < 4 h after its supply from roots. *We conclude that arbuscular mycorrhizal mycelial respiration can be substantial in lowland tropical forests. As it is highly dependent on the recent supply of carbon from roots, a function of aboveground fixation, AMF mycelial respiration is therefore an important pathway of carbon flux from tropical forest trees to the atmosphere.