Effects of nitrogen concentrations on nodulation and nitrogenase activity in dual root systems of soybean plants

Currently, the mechanisms underlying the effects of nitrogen on nodulation and nitrogen fixation of leguminous plants remain uncertain. To further investigate the relationship between nitrogen with nodulation and nitrogen fixation of soybean plants, a young seedling grafting technique was applied to generate soybean plants with dual root systems. The two root systems were separately cultured in sand medium, with one irrigated with nutrient solution containing NH4+ or NO3- as nitrogen sources (the N+ side) and another irrigated with nitrogen-free nutrient solution (the N- side). Various nitrogen concentrations, 0, 25, 50, 75, and 100 mg/L, were applied to the N+-side root system to determine the effects of nitrogen concentrations on nodulation and nitrogenase activity of soybean plants. The results showed that in the dual root systems constructed by seedling grafting, the number and weight of root nodules at the N+ side both exhibited a single-peak changing trend of first increasing and then decreasing with nitrogen concentrations. Root nodulation was accelerated by low concentrations of nitrogen (< 50 mg/L) and significantly suppressed by high concentrations of nitrogen (> 50 mg/L). In contrast, the number and weight of root nodules at the N- side showed a constant increasing trend with the increase in nitrogen concentrations. Taken together, these results indicated that high nitrogen concentration exerts a contact-dependent localized inhibitory effect on the formation (number of nodules) and growth of root nodules (weight of nodules). The nitrogenase activities in the root nodules at both the N+ and the N- sides displayed trends of first increasing and then decreasing with nitrogen concentrations, reaching a single peak when the nitrogen concentration was 50 mg/L. In addition, the promotive or inhibitory effects of nitrogen at different concentrations on nitrogenase activity in the root nodules were synchronous between the N+ and the N- sides, suggesting that the effect of nitrogen is systematically regulated via interactions between the aboveground and underground parts of the plants.