Mycorrhizal Infection and Growth of Trifolium Subterraneum: Use of Sterilized Soil as a Control Treatment

SUMMARY Effects of mycorrhizal infection on the growth of Trifolium subterraneum have been studied by comparing plants grown in sterilized soil/sand mixtures with plants grown in untreated soil/sand mixtures. The original soil contained a high level of endomycorrhizal inoculum. The aim was to consider whether this approach is valid, bearing in mind the possible effects of sterilization (γ‐irradiation or autoclaving) on soil properties. Plants in the untreated soil/sand mixtures became significantly larger than plants in the sterilized soil/sand mixtures about 4 weeks after planting. This response was associated with rapid infection by vesicular‐arbuscular mycorrhizal fungi; it still occurred when nitrate replaced N2 as the nitrogen source for the plants. Addition of soluble phosphate enhanced the growth of the non‐mycorrhizal plants to a level similar to that of the mycorrhizal plants. Mixing sterilized and untreated soil did not depress mycorrhizal infection or growth. These results indicate that the growth difference was a classic mycorrhizal response, and did not reflect toxic effects of soil sterilization. The growth of Brassica oleracea (broccoli), which does not form mycorrhizas, was better on sterilized soil.