Extraradical mycelium of arbuscular mycorrhizal fungi radiating from large plants depresses the growth of nearby seedlings in a nutrient deficient substrate

The effect of arbuscular mycorrhiza (AM) on the interaction of large plants and seedlings in an early succession situation was investigated in a greenhouse experiment using compartmented rhizoboxes. Tripleurospermum inodorum, a highly mycorrhiza-responsive early coloniser of spoil banks, was cultivated either non-mycorrhizal or inoculated with AM fungi in the central compartment of the rhizoboxes. After two months, seedlings of T. inodorum or Sisymbrium loeselii, a non-host species colonising spoil banks simultaneously with T. inodorum, were planted in lateral compartments, which were colonised by the extraradical mycelium (ERM) of the pre-cultivated T. inodorum in the inoculated treatments. The experiment comprised the comparison of two AM fungal isolates and two substrates: spoil bank soil and a mixture of this soil with sand. As expected based on the low nutrient levels in the substrates, the pre-cultivated T. inodorum plants responded positively to mycorrhiza, the response being more pronounced in phosphorus uptake than in nitrogen uptake and growth. In contrast, the growth of the seedlings, both the host and the non-host species, was inhibited in the mycorrhizal treatments. Based on the phosphorus and nitrogen concentrations in the biomass of the experimental plants, this growth inhibition was attributed to nitrogen depletion in the lateral compartments by the ERM radiating from the central compartment. The results point to an important aspect of mycorrhizal effects on the coexistence of large plants and seedlings in nutrient deficient substrates.