Effect of Japanese Knotweed (Fallopia japonica) Mulching on Continuous Potato Cropping: Modern Evaluation of Traditional Japanese Knotweed-Mulch Farming in Nishi-Awa Steep Slope Land Agriculture System, Japan

Poaceae species such as silver grass or reed are commonly used in traditional mulch farming in Japan, where the Nishi-Awa area is a Globally Important Agricultural Heritage Systems (GIAHS) site. Farmers here have traditionally used silver grass for mulch farming; furthermore, local farmers have learned from long-standing experience that Japanese knotweed (Fallopia japonica, Polygonaceae) is better for cultivation of solanaceous crops in this area. However, it is unclear why Japanese knotweed mulching is beneficial for cultivation of solanaceous crops. Thus, in this study, we hypothesized that Japanese knotweed mulching may be effective in avoiding hazards associated with continuous potato cropping, as native potato used to be cultivated twice a year in the past. Therefore, we investigated the effects of Japanese knotweed mulching on continuous potato cropping and after tomato cropping, which is another solanaceous crop species. Field experiments were conducted in 2018. First, we compared Japanese knotweed mulching, silver grass mulching and no grass mulching (control) in a soil under continuous potato cultivation and in an uncultivated soil. As a result, the extent of the potato yield decrease was reduced by Japanese knotweed mulching. Secondly, we compared Japanese knotweed-mulching and no grass mulching in a soil after a tomato crop and in an uncultivated soil. The extent of decrease in potato growth and yield was also reduced by Japanese knotweed mulching. These findings indicate that mulching with Japanese knotweed helped to avoid the risks associated with continuous potato cropping.