1 Graduate School of Agriculture, Department of International Environmental and Agricultural Science, Tokyo University of Agriculture and Technology, Japan.
Hairy vetch (Vicia villosa Roth) is a leguminous plant that is widely used as a green manure and a cover crop. Recent observations have shown that previous hairy vetch cultivation on a plot promotes the subsequent growth of velvet bean (Mucuna pruriens). Here we investigated soil nutrients and microorganisms in an attempt to identify the factors responsible for the growth-promotion effect of hairy vetch on velvet bean. Field experiments showed that the dry weight of velvet bean plants was six times heavier in plots that had previously been sown with hairy vetch than in previously unplanted control plots. Soil analysis revealed that the concentrations of nitrate and ammonium nitrogen (N) in hairy vetch plots were up to 20% and 10% higher, respectively, than those in control plots. The root-nodule bacteria (rhizobia) isolated from the hairy vetch nodules were all closely related to Rhizobium leguminosarum, whereas those isolated from velvet bean nodules were Bradyrhizobium species in both types of plot. There was no significant difference in the infection rate of arbuscular mycorrhizal fungi in velvet bean roots between the two types of plot. These findings suggest that the growth-promoting effect of hairy vetch on velvet bean is not caused by the propagation of symbiotic microorganisms, but rather is due to other factors including an increase in soil nutrients.