Vesicular-arbuscular mycorrhizal associations with barley on sewage-amended plots

Barley plants grown on field plots amended with sewage from two sources were examined for vesicular-arbuscular (VAM) infection. Three treatments (control. 8.2 tonnes sludge ha⁻¹ and 32.9 ha⁻¹) were applied annually for 8 yr with no additions during years 9 and 10. A barley crop was grown on the site during each of the 10 yr of the study. Percent VAM infection on barley roots showed no effect of nonindustrial sludge application. The urban-industrial sludge treatments, however, produced a 6-fold drop in infection rate. Little correlation was found between percent VAM infection and available soil P content. Zn concentration in the soil were found to be negatively correlated with VAM infection. This study suggests that there are factors within sludge, in addition to P, that affect mycorrhizal occurrence 2 yr after the last application of sludge to soil.