Interpretation of bioassays in the study of interactions between soil organisms and plants: involvement of nutrient factors

Abstract

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.