HIGH-DENSITIES OF ARBUSCULAR MYCORRHIZAL FUNGI MAINTAINED DURING LONGFALLOWS IN SOILS USED TO GROW COTTON EXCEPT WHEN SOIL IS WETTED PERIODICALLY

Sequential harvests of cotton seedlings grown in soil cores enabled th e quantification of the density of arbuscular mycorrhizal fungi to det ect the effects of time, cultivation and periodic wetting of the soil. Cotton seedlings grown in soil cores from three locations formed arbu scular mycorrhizas at similar rates when cores were stored dry for up to 18 months. Disturbance of dry cores followed by dry storage for 18 months did not reduce the rate of establishment of mycorrhizas. Period ic wetting and drying of the cores, especially if the cores had first been disturbed, significantly reduced the rate of establishment of myc orrhizas. We suggest that long fallow disorder is possibly caused by f alls of rain in clay soils of eastern Australia used to grow cotton. T he proportion of the root with mycorrhizas at 3 wk was strongly correl ated with the infection at 8 wk. We also suggest that it might be poss ible to predict maximum levels of infection and early uptake of phosph ate of seedlings by determining the proportion of roots that are mycor rhizal 3 wk after emergence of cotton seedlings.