Colonization of the root zone of Ammophila arenaria by harmful soil organisms

The role of harmful soil organisms in the degeneration of Ammophila arenaria at coastal foredunes was examined by the growing of seedlings of A. arenaria in soil samples collected from its root zone. Three sites, each representing a successive stage in foredune succession were examined: (1) a highly mobile dune (sand accretion of 80 cm year⁻¹) with vigorous A. arenaria, colonizing only the upper 30-cm of the annually deposited layer of sand, (2) a mobile dune with vigorous A. arenaria (sand accretion of 22 cm year⁻¹) and a 1-metre soil profile completely colonized by roots and (3) a stable dune (no sand accretion) with degenerated A. arenaria and young roots mainly present in the upper 0-10cm. In the upper part of the highly mobile site, the presence of harmful soil organisms was confined to the root layers and at the mobile site for all depth layers a significant growth reduction of A. arenaria was observed due to the activity of harmful soil organisms. At the stable site, however, growth had only been reduced in some of the depth layers. At all sites newly formed roots of A. arenaria had been colonized by harmful soil organisms within one year. If present in sand prior to root growth harmful soil organisms reduced root length and root hair formation severely and they enhanced branching of the roots. It is concluded that harmful soil organisms initiate degeneration of A. arenaria in stable dunes by attack of the root system, which makes the plants suffer from abiotic stress.