Carbon flow into soil and external hyphae from roots of mycorrhizal cucumber plants

summary Cucumber ( Cucumis sativus L.) plants grown in PVC tubes with a partially sterilized soil‐sand mixture were inoculated with the vesicular‐arbuscular mycorrhizal fungus Glomus fasciculatum (Thaxter) Gerdemann & Trappe emend. Walker & Koske or left uninoculated. The soil column of each PVC tube was divided into a root and a hyphal compartment by a mesh bag (60 μm), which retained the roots but allowed external hyphae to pass. Inoculated plants rapidly became infected and an extensive mycelium developed. Three weeks after seedling emergence plants were labelled with 14 CO 2 for 16 h. The distribution of 14 C within the plants and the 14 C flow into external hyphae and soil were measured during an 80 h chase period. Below‐ground respiration in mycorrhizal plants accounted for 27% of the photo assimilated 14 C. Organic 14 C in the soil represented 3˙1 % of the fixed 14 C, and 26 % of this was located in external hyphae. Based on conservative assumptions concerning dry weight of internal mycorrhizal infection and growth yield of the fungus, it was estimated that mycorrhizal events consumed 20 % of photoassimilated 14 C. The specific incorporation of C by the external mycelium in the hyphal compartment was 41 μg C mg −1 dry wt. d −1 . The importance of external VA mycorrhizal hyphae for the distribution of plant‐derived C in the soil volume and as a substrate source for the soil biota is discussed.