Soybean is an important crop for human food and feed for livestock. World soybean production is increasing especially in North and South America. Soybean seeds contain a high percentage of protein about 35-40%, and they require a large amount of nitrogen compared with other crops. Soybean plants make root nodules with rhizobia, and rhizobia can fix atmospheric N 2 and give the fixed N to the host soybean plants. Also, soybean can absorb nitrogen usually nitrate from soil or fertilizers. The amount of total assimilated nitrogen in shoot is proportional to the soybean seed yield either from nitrogen fixation or from nitrogen absorption, and the nitrogen availability is very important for soybean cultivation. Maintenance of a high and long-term nitrogen fixation activity is very important for a high production of soybean. However, application of chemical nitrogen fertilizers usually depresses nodule formation and nitrogen fixation. Nitrate in direct contact with a nodulated part of roots causes severe inhibition of nodule growth and nitrogen fixation, although a distant part of nodules from nitrate application gives no or little effect. Deep placement of slow-release nitrogen fertilizers, coated urea, or lime nitrogen promoted the growth and seed yield and quality of soybean without depressing nitrogen fixation.
‘Enrei’ is the second leading variety of soybean (Glycine max (L.) Merr.) in Japan. Its cultivation area is mainly restricted to the Hokuriku region. In order to expand the adaptability of ‘Enrei’, we developed two near-isogenic lines (NILs) of ‘Enrei’ for the dominant alleles controlling late flowering at the maturity loci, E2 and E3, by backcrossing with marker-assisted selection. The resultant NILs and the original variety were evaluated for flowering, maturity, seed productivity and other agronomic traits in five different locations. Expectedly, NILs with E2 or E3 alleles flowered later than the original variety in most locations. These NILs produced comparatively larger plants in all locations. Seed yields were improved by E2 and E3 in the southern location or in late-sowing conditions, whereas the NIL for E2 exhibited almost the same or lower productivity in the northern locations due to higher degrees of lodging. Seed quality-related traits, such as 100-seed weight and protein content, were not significantly different between the original variety and its NILs. These results suggest that the modification of genotypes at maturity loci provides new varieties that are adaptive to environments of different latitudes while retaining almost the same seed quality as that of the original.
The deep placement of urea fertilizer (DMU) containing 1% (W/W) of the nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) on soybean growth and seed yield was as effective as those of the coated urea (CU) and lime nitrogen (LN) in a field research. The average seed yields were high in LN (464 g·m−2) and DMU (461 g·m−2) and relatively low in CU (405 g·m−2), U (396 g·m−2), and Cont (373 g·m−2) treatments. The accumulations of dry matter and nitrogen in soybean shoots were higher in the plants with deep placement of CU, LN, and DMU than U and Cont. The daily nitrogen fixation activity and daily nitrogen absorption rate were calculated based on the relative ureide method. Both nitrogen fixation activity and nitrogen absorption rate were higher in DMU, CU, and LN compared with control treatment, suggesting that the deep placement of DMU did not repress nitrogen fixation. Soil incubation test was performed using the same field soil with DMU, U, LN, and urea with DMPP 1%, 2%, and 4%. DMU inhibits nitrification similar to the pattern of LN until 8 weeks. The increasing DMPP concentration did not markedly increase the nitrification inhibition. From these results, it was concluded that urea fertilizer with 1% DMPP is efficient for deep placement of N fertilizer for soybean cultivation due to its lower price compared with CU and LN.
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 soybeangrowing 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.
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