Abstract
Recently, soybean yields in Niigata Prefecture have been subjected to wide annual fluctuation, and both the seed yield and quality have decreased in the last decade. This trend was confirmed based on the soybean yield statistics in Niigata Prefecture. We investigated the relationship between soybean seed yield and cropping system, and the effects of nitrogen fertility in soil and climatic factors on the yield. The yield decreased dramatically in the fields where soybean cropping continued for over four years. The recent decline in soybean yield was attributed to the decrease in pod number and seed weight. Compared with a continuous paddy field, a long-term paddy-upland rotation field had less available soil nitrogen during the soybean-growing period, especially from the beginning of the bloom (R1) stage onward. The low level of available nitrogen in the soil of long-term rotated fields was suggested to reduce pod number and seed growth. It was also suggested that on increasing amount of solar irradiation increased the pod number in early August, from the beginning-bloom to beginning-pod (R1-3) stages, and that, the heavy rains in mid-August, during the full-pod to beginning-seed filling (R4-5) stages, caused a short-term wet damage, which triggered a decline in nodule activity along with premature pod fall.
