Underground resource allocation between individual networks of mycorrhizal fungi.

Fusions between individual mycelia of arbuscular mycorrhizal (AM) fungi have been observed in two-dimensional systems but never in soil systems. Here, phosphorus ((32)P) labelling was used to demonstrate nutrient transfer between individual mycelia and to investigate the possible role of anastomosis. Trifolium subterraneum colonized by Glomus mosseae were grown in root-retaining mesh bags, which were placed 20 cm apart. The mycelium of one plant, the donor, had access to (32)P-labelled soil placed adjacent to the mesh bag. Transfer of (32)P from the donor mycelium to the receiver plant was measured at three harvests. In a second-harvest control treatment the receiver was colonized by Glomus caledonium in order to determine whether transfer occurred by other means than hyphal fusions. Significant amounts of P were transferred to the receiver plant at the last harvests when the two mycelia of G. mosseae overlapped. The transfer probably occurred via anastomoses between the mycelia as no transfer of (32)P was detected between the mycelia of different fungi at the second harvest. The indicated ability of AM fungal mycelia to anastomose in soil has implications for the formation of large plant-interlinking functional networks, long-distance nutrient transport and retention of nutrients in readily plant-available pools.