Phosphorus uptake by a community of arbuscular mycorrhizal fungi in jarrah forest

Communities of indigenous arbusuclar mycorrhizal (AM) fungi are expected to alter phosphorus uptake and biomass productivity of plants according to characteristics of the life cycles of the fungi present and the way they interact with each other inside roots and with host plants. Differences in the relative abundance of AM fungi inside roots could influence P uptake if the fungi present differ in effectiveness at accessing P and transferring it to the plant. However, it is difficult to assess the contribution of AM fungi under field conditions. We investigated P uptake, from point sources of P placed 2, 4 and 6 cm from roots, by plants colonised by a community of AM fungi in jarrah forest soil. Roots were retained within a mesh bag to prevent them from growing towards the point source of P. The relative abundance of morphotypes of fungi inside roots and the P status of plants were assessed after 12 and 16 weeks. First, a bioassay was carried out in undisturbed forest soil cores using two host plants, a forest understorey plant Phyllanthus calycinus Labill and the annual pasture species subterranean clover (Trifolium subterraneaum L.), to assess the infectivity of the indigenous community of AM fungi. Roots of both bioassay host plants were colonised in similar proportions by morphotypes of AM fungi resembling Glomus, Acaulospora, Scutellospora and fine endophytes. In this bioassay, there were positive correlations between the proportion of root length colonised and plant biomass and P uptake for P. calycinus, but not for subterranean clover. In the experiment assessing the capacity of P. calycinus to access P placed at increasing distances from the root, shoot P content and concentration in P. calycinus were greater when P was placed 2 cm compared with 4 and 6 cm from roots. The length of hyphae in the vicinity of the point source of P decreased with increasing distance from the plant. The extent to which the individual AM fungi were involved in P uptake is not known. The Glomus morphotype was dominant at both times of sampling.