The development and significance of vesicular-arbuscular mycorrhizas as influenced by agricultural practices

Abstract

The development and significance of vesicular- arbuscular mycorrhizas (VAM) in wheat and potatoes have been studied in relation to various farming systems and agricultural practices. The effects of farming systems on VAM have been observed on three neighbouring experimental farms in the vicinity of Nagele, in the North East Polder, The Netherlands. It appeared that varying amounts of organic matter used during a period of 26 years had little effect on the presence of VAM fungal spores. The results of a new project started on these farms in 1979 showed that nitrogen deficiency greatly stimulated the development of VAM in wheat. In potatoes, however, no such results were found. After winter wheat the number of spores of Glomus mosseae dominated the number of spores of the other species in the soil. After potatoes spores of Glomus macrocarpus were found more frequently.The influence of various agricultural practices on the development of VAM has been studied in long-term field experiments on the Lovink-farm, another experimental farm in the North East Polder. The relation between VAM and the amounts of nitrogen fertilizers applied has also been assessed in these experiments. From the results it can be concluded that the effect of various agricultural practices on VAM is based primarily on the effect of these practices on the growth of the plants and the inoculum potential of VAM in the soil. in addition, it appears that the effect of nitrogen on the development of VAM depends on the host plant species involved. This is in accordance with the results obtained on the farms at Nagele.Greenhouse experiments with wheat showed a strong interaction between the application of nitrogen and phosphorus fertilizers in relation to the development of VAM. When sufficient phosphorus was applied, the effect of nitrogen on VAM development depended on the way in which the VAM inoculum was applied.The significance of VAM fungi for the growth of plants depends largely on the environmental conditions in which the plants grow, particularly the nutrition of the plants, plant density and inoculum potential of VAM. These factors largely determine whether the symbiosis results in a positive or even a negative effect on the growth of plants.In spite of the fact that VAM fungi occur in the relatively rich arable soils in these polders, and that VAM fungi isolated from these soils greatly stimulate the growth of red clover in a poor soil, VAM does not appear as yet to be important for Dutch agriculture. Favourable conditions for the development of these mycorrhizas and conditions under which they have the greatest effect on the growth of plants are, in general, not present. Nevertheless, in circumstances in which little or no additional plant nutrients are used, or nutrients are not readily available for the plants, the importance of these fungi for the growth of plants increases.Protection given by VAM to plants against root pathogens seems to be limited. It is predominantly based on improved nutrition of poorly-fed plants and in a few special cases on competition for a site inside or outside the mycorrhizal root