The direction of carbon (C) allocation in mycorrhizal mycelia is of fundamental importance to coexistence of individual plants. We therefore investigated the transfer of C from established plants to plant seedlings through fungal mycelia. C allocation by the arbuscular mycorrhizal (AM) fungus Glomus intraradices, from 'donor' plants to mycelia in soil and two different species of introduced 'receiver' seedlings, was investigated in a pot experiment using C-13 labelling and fatty acid analysis. After (CO2)-C-13 application to the shoots of Trifolium subterraneum or Plantago lanceolata, used as donor plants, T subterraneum and P. lanceolata receiver seedlings were introduced. Samples were collected 4-20 days after (CO2)-C-13 application and analysed regarding C-13 and the fatty acid 16:1 omega 5, the signature of AM fungi. C-13 transfer from T subterraneum donor plants was demonstrated by C-13 enrichment of the roots of the receiver seedlings, but not from the P. lanceolata donor plants. C-13 allocation to the neutral lipid fatty acid 16:1 omega 5 was only 1 ng in each receiver seedling, but 2 mu g of the fatty acid in whole soil. The results indicate that C allocation through mycelial networks is influenced by the donor plant species, but is not directed towards receiver seedlings to any higher degree than towards other directions. The importanice of the extraradical AM fungal mycelium as a C sink was demonstrated. (c) 2007 Elsevier Ltd. All rights reserved.