Plant and soil response to single and mixed species of arbuscular mycorrhizal fungi under fungicide stress

Defining sustainable agricultural practices requires an understanding of both aboveand below-ground consequences to management strategies. While alternatives to biocides are sought for the control of weeds, insects and pathogens, biocide use will continue with the goal of reducing quantities used in an integrated approach. The impact of three fungicides on plant growth, seed yield, seed nutrient composition, and on soil aggregation as mediated by arbuscular mycorrhizal (AM) fungi was studied in a silty-clay loam soil with a high extractable P concentration. Shoot dry mass, seed yield and seed nutrient (N, P, K) contents of pea (Pisum sativum L.) plants were enhanced by three AM fungi and a mixture of three fungi compared with nonAM plants. Each AM fungus produced a distinct pattern of shoot responses of plants, but the mixed inoculum treatment was as good or better than each single species for all of the above-ground measures of plant performance. Soil aggregation was improved by two of the three AM fungi as individual inocula, and was further increased in the mixed inoculum treatment. Two of the three fungicides reduced shoot dry mass and seed yield, but none of the fungicides affected soil aggregation. Fungicides inhibited mycorrhiza formation least in the mixed inoculum treatment which gave the best overall plant and soil responses. Since the three fungi together were more tolerant of fungicides than each fungus alone, it appeared that as a community, AM fungi modified and alleviated fungicide stress, resulting in high levels of plant performance and soil aggregation. © 1997 Elsevier Science B.V.