**Abstract**

Soybean plants require a large amount of nitrogen either from nitrogen fixation in nodules or nitrogen absorption from roots. It is known that nitrate, a major inorganic nitrogen compound in upland soils, represses nodule growth and nitrogen fixation. Rapid and reversible inhibition of nodule growth and nitrogen fixation activity was found in the hydroponically cultivated soybeans after changing the nutrient solution with or without nitrate. Isotope tracer analysis revealed that the major cause of this inhibition depended on the changes in the partitioning of photo-assimilate between nodules and roots and was not directly related to the transported N compounds. Transcriptome and metabolome analyses supported that nitrate strongly promotes nitrogen and carbon metabolism in the roots but represses them in the nodules. The application of ammonium, glutamine, or urea also inhibited the nodule growth and nitrogen fixation like nitrate, although the inhibition was lower than that of nitrate. The degree of inhibition was related to the decrease in carbon isotope partitioning into the nodules, rather than the import of nitrogen isotope to nodules. Urea was detected in xylem sap and all parts of soybean, and some urea might be originated from ureide degradation.

**Keywords:** soybean, nitrogen fixation, nodules, roots, nitrate, ammonium, urea, ureides
