Harmful soil organisms in coastal foredunes involved in degeneration of Ammophila arenaria and Calammophila baltica

The presence of harmful soil organisms in the root zone of Ammophila arenaria (marram grass) was examined by biotesting. For this investigation three locations along the sandy shoreline of The Netherlands were chosen: Voorne, Texel, and Schouwen. At all three locations harmful organisms were detected in sand from stable dunes, as well as in sand from mobile dunes (degenerated and vigorous A. arenaria, respectively). In beach sand, however, no harmful organisms occurred. Since A. arenaria shows vigorous growth only when it is buried regularly by windblown sand from the beach, it is concluded that this sand deposition enables the plants to escape from harmful soil organisms. Ammophila arenaria and Calammophila baltica (purple or hybrid marram grass) from the Voorne location were grown outdoors in containers filled with sand from the beach, the mobile dunes, and the stable dunes, and sterilized sand from the stable dunes. Biomass production of both species was highest in sterilized sand from the stable dune, followed by (in descending order) beach sand, sand from the mobile dune, and unsterilized sand from the stable dune. As compared with A. arenaria, however, growth of C. baltica was reduced less and without mortality of cuttings. Degree of growth reduction by harmful soil organisms could not be related to numbers of plant parasitic nematodes. Key words: Ammophila arenaria, Calammophila baltica, coastal sand dunes, succession, harmful soil organisms