Abstract
Objective. Because most Japanese soybeans are cultivated in paddy fields under rotational cropping, life cycle assessment (LCA) of soybean production needs to take into account a whole crop rotation including rice cultivation. In order to investigate the environmental impact associated with organic soybean production in Japan, this study carried out comparative LCA of organic and conventional soybean-rice rotation systems including 3-year organic and conventional rotations and a 2-year organic rotation. The inventories of arable products were built in the SimaPro software with the NARO LCI database, and were analyzed using the impact categories of global warming, acidification, eutrophication, non-renewable energy consumption, and pesticide application.
Results and Discussion. Organic soybean production reduced the environmental impact per 1 kg of soybeans in all the categories by more than 30% compared to conventional soybean production. The main contributors to the superiority of organic soybean production were decreases in direct field emissions and fertilizer consumption as well as the higher yield of organic soybeans. Sensitivity analysis demonstrated that organic soybean production remained more environmentally friendly than conventional soybean production, even if the yield of organic soybeans was assumed to decrease to 80% of the yield of conventional soybeans. However, the impact of organic rice production was significantly higher than that of conventional rice production in the categories of acidification and eutrophication due to overfertilization including rice bran and irregular soybeans for weed control. The impact of whole crop rotations was dominated in most cases by rice production owing to the higher yield of rice and the higher frequency of rice production. Consequently, the reduced impact of organic soybean production was canceled by the relatively large impact of organic rice production, depending on the impact categories; the impact of the organic rotation on acidification and eutrophication was higher than that of the conventional rotation. The 2-year organic rotation environmentally outperformed the 3-year organic rotation mainly because the effect of environmentally friendly soybean production had more influence on the former than on the latter.
Conclusions. This study showed that the environmental impact of organic soybean production was definitely lower than that of conventional soybean production. However, comparative LCA of whole crop rotations showed unfavorable results particularly with regard to acidification and eutrophication. This paper suggests two solutions to reduce the total impact associated with the organic paddy rotation including organic soybean production; one is reduction of organic fertilization in organic rice production and the other is introduction of a 2-year soybean-rice rotation.
