Submitted to: Plant and Soil
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 20, 1997
Publication Date: N/A
Interpretive Summary: The roots and soils of crop plants are colonized by mycorrhizal fungi. The resulting plant-fungus association (mycorrhiza) fundamentally alters host-plant development and affects soil stability. Because mycorrhizal fungi colonize the bulk soil that surrounds the roots, their hyphae in the soil are in intimate contact with soil organisms. We grew nodulated or nitrogen-fertilized soybean plants in pot cultures to determine how the microscopic members or the soil food web (bacteria, springtails, nematodes bacteria-feeders, mycorrhizal fungi) affect plant growth and soil aggregation. We found that aggregation was better in the soils of nodulated and ammonia-fertilized plants than in nitrate-fertilized ones. However, mycorrhiza development had the greatest effect on the formation of stable soil aggregates, and some of the microscopic soil animals counteracted this effect, apparently by feeding on the hyphae. Plant and soil responses were not related to each other directly, but to the mycorrhizal fungi, which appeared to mediate interactions between plant and soil. We conclude that the complex web of soil organisms all influence each other but that their effects on plant and soil are to some extent mediated by the hyphae of mycorrhizal fungi. These effects are measurable in terms of plant growth and soil stability.
Technical Abstract: The effects of the form of N nutrition on soil stability is an important consideration for the management of sustainable agricultural systems. We grew soybean plants in unsterilized soil. Treatments were: inoculation with Rhizobium, fertilization with nitrate or urea, or no N for the controls. The soils were sampled (3-wk intervals) for water-stable soil aggregation (WSA), soil pH, root growth, arbuscular mycorrhizal (AM) soil and root colonization, and rhizobacteria. The soil fauna was assayed at the end of the experiment (9 weeks).We found: (1) positive correlations between AM soil mycelia and roots and WSA, (2) negative correlations between total bacterial counts and WSA, and (3) negative correlations between AM soil hyphal length and Collembola numbers. Soils of nodulated and NH4NO3-fertilized plants had the highest WSA and the lowest pH at week 9. Sparse root development in the soils of the N-deficient control plants indicated that WSA formation was primarily influenced by AM hyphae. The ratio of bacterial counts in the water-stable vs. unstable soil fractions increased for the first 6 wk and then declined, while anaerobic bacteria increased with increasing WSA. Nematode and protozoa numbers did not correlate with bacterial counts or AM hypha lengths. Soil pH did not affect mycorrhiza development, but actinomycete counts declined with decreasing soil pH. The data indicate that AM fungi and roots interacted as the factors that affect soil aggregation, regardless of N nutrition.