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Abstract.A role for pathogens in plant competition has often been suggested, but examples are rare and, in the case of soil pathogens, virtually absent. In this paper we examine if and how soil-borne pathogens may play a role in plant competition. As a model, two successional plant species from coastal sand dunes were used: Ammophila arenaria (marram grass) and Festuca rubra ssp. arenaria (sand fescue). The root zone of A. arenaria contains pathogens that contribute to the degeneration of their host when dunes become stabilized. These pathogens (plant parasitic nematodes and pathogenic fungi) are relatively harmless to the immediate successor F. rubra. We tested the hypothesis that F. rubra, when grown in a mixed culture with A. arenaria, will be favored when A. arenaria is exposed to soil-borne pathogens from its own root zone.In a greenhouse, seedlings of both species were grown in replacement series in sterilized (pathogen-free) and unsterilized (pathogen-containing) soils originating from the root zone of natural A. arenaria. Nutrient additions, soil moisture, and the length of the experiment were based on two pilot studies. When exposed to its soil-borne pathogens, A. arenaria was outcompeted, especially when it constituted Ͻ50% of the planted mixture. Nutrient limitation enhanced the replacement of A. arenaria by F. rubra especially in unsterilized soil. This was due to reduced plasticity of A. arenaria in responding to nutrient limitation when exposed to its pathogens. The present results support previous suggestions that soil pathogen-driven competition may be an important mechanism in species replacement in coastal foredune vegetation.

Summary• Increased plant growth in sterilized soil is usually ascribed to the elimination of (often unidentified) soil-borne pathogens. Plant-soil bioassays are reported here for three dune soils and two plant species ( Ammophila arenaria and Carex arenaria ).• Dynamics of plant growth, availability and uptake of nutrients were compared in sterilized (25 kGy gamma-irradiation) vs control soils.• Plant growth, availability and acquisition of nutrients, for example P, even when provided in apparent excess, were significantly enhanced in gamma-irradiated calcareous dune sands. With A. arenaria , the positive sterilization effect occurred independently of initial plant dry mass. The addition of extracts of planted soils to A. arenaria growing in unsterilized sand caused an increase in root growth that could not be related to either nutrients or pathogens.• Increased availability and acquisition of nutrients in sterilized soil may contribute to nonsterile : sterile ratios of plant growth that are < 1. Any ecological speculation involving the role of soil-borne biological factors should be based on fully validated plant-soil bioassays, which account for nutritional or other nonpathogen-related side-effects induced by soil sterilization.

A role for pathogens in plant competition has often been suggested, but examples are rare and, in the case of soil pathogens, virtually absent. In this paper we examine if and how soil‐borne pathogens may play a role in plant competition. As a model, two successional plant species from coastal sand dunes were used: Ammophila arenaria (marram grass) and Festuca rubra ssp. arenaria (sand fescue). The root zone of A. arenaria contains pathogens that contribute to the degeneration of their host when dunes become stabilized. These pathogens (plant parasitic nematodes and pathogenic fungi) are relatively harmless to the immediate successor F. rubra. We tested the hypothesis that F. rubra, when grown in a mixed culture with A. arenaria, will be favored when A. arenaria is exposed to soil‐borne pathogens from its own root zone. In a greenhouse, seedlings of both species were grown in replacement series in sterilized (pathogen‐free) and unsterilized (pathogen‐containing) soils originating from the root zone of natural A. arenaria. Nutrient additions, soil moisture, and the length of the experiment were based on two pilot studies. When exposed to its soil‐borne pathogens, A. arenaria was outcompeted, especially when it constituted <50% of the planted mixture. Nutrient limitation enhanced the replacement of A. arenaria by F. rubra especially in unsterilized soil. This was due to reduced plasticity of A. arenaria in responding to nutrient limitation when exposed to its pathogens. The present results support previous suggestions that soil pathogen‐driven competition may be an important mechanism in species replacement in coastal foredune vegetation.

Arnrnophila arenaria is a dominant sand-fixing plant species of the European coastal foredunes. It remains vigorous under regular burial conditions on seaward slopes, but starts to degenerate when sand accumulation diminishes. Several hypotheses have been put forward to explain this degeneration. In this study, we test the hypothesis that upward growth of plants following sand burial enables them to escape harmful soil organisms. Plants in a degenerating field stand of A. arenaria and potted plants grown in sterilized sand (outdoor pot experiment) were buried with sterilized or nonsterilized sand. Burial in both sterilized and nonsterilized sand resulted in stem elongation, increased numbers of living shoots, and increased shoot and root biomass. However, when plants were grown in and buried with sterilized sand, the numbers of shoots were significanlty higher than those buried with nonsterilized sand. The new root zone of buried plants was colonized by pathogenic soil organisms (nematodes and fungi) during the same growing season. It is concluded that by upward growth through pathogen-free sand, the plants benefit, at least temporarily, from escaping its pathogens and parasites.Rdsumd : L'Arnrnophila arenaria est une espkce de plante dominante capable de fixer les sables des dunes c8tikres frontales en europe. Elle demeure vigoureuse sous des conditions rCgulikres d'ensablement sur les pentes opposCes a la mer, mais commence 21 dtgCnCrer lorsque l'accumulation de sable diminue. Plusieurs hypothkses ont Ct C mises de I'avant pour expliquer cette dCgCnCrescence. Dans cette ttude, les auteurs Cvaluent l'hypothtse selon laquelle la croissance dressCe des plantes, suivant le recouvrement par le sable, rendrait ces plantes capables d'Cchapper aux organismes nuisibles du sol. Des plantes situCes dans une station en dtgtnCrescence de I'A. arenaria et des plantes produites en pot dans du sable stCrilisC (expCrience en pots a I'extCrieur) ont Ct C enterrCes avec du sable stCrilisC et du sable non-stCrilist. Dans les deux cas, l'enterrement avec du sable stCrilisC ou non-stCrilisC conduit a 1'Clongation des tiges, a I'augmentation du nombre des tiges vivantes ainsi que l'augmentation des biomasses des tiges et des racines. Cependant, lorsque les plantes poussent dans le sable stCrilisC et sont enterrCes avec du sable stCrilisC, les nombres de tiges sont significativement plus grands que chez les plantes enterrCes avec du sable non-stCrile. La nouvelle zone racinaire des plantes enterrCes est colonisCe avec des organismes tdaphiques pathogknes (nimatodes et champignons) au cours de la m&me saison de croissance. Les auteurs concluent qu'au moyen de la croissance dressCe a travers le sable sans pathogkne, la plante ont profitC, du moins temporairement, de I'Cloignement de ses pathogknes et parasites.