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 NH þ 4 or NO À 3 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.

Lodging can reduce grain yield and quality, especially in crops planted at high densities. Force analysis, which relates stem mechanical properties and morphological characteristics under high densities, was used to study lodging in soybean (Glycine max (L.) Merr.). We applied mechanical lodging and examined the relationships between stem bending moment of the breaking force and morphological characteristics in two soybean varieties of contrasting heights grown at four crop population densities (200000, 300000, 400000 and 500000 plants ha–1). The experiment was performed in a split-plot design during 2015–16. Measurements were made during the R1–R8 growth stages. Results showed that the full seed stage was the most sensitive period for lodging. Stem strength of the soybean plants increased with growth and development; however, during the R5–R6 stages, the weakened degree of stem strength was greater than the gravity moment. This was the primary reason for the soybeans tending to lodge during this period. In addition, increasing the planting density weakened the mechanical properties significantly. During breeding efforts to improve the soybean lodging resistance, more attention should be paid to strengthening soybean stems. This study also showed that the ratio of stem diameter to plant height could be a new quantitative index for evaluating the lodging resistance of soybeans.

The specific mechanism by which nitrogen application affects nodulation and nitrogen fixation in legume crops remains uncertain. To further study the effects of nitrogen application on soybean nodulation and nitrogen accumulation, three consecutive tests were performed during the VC-V4, V4-R1 (10 days), and R1-R2 (10 days) growth periods of soybean. In a dual-root soybean system, seedlings on one side were watered with a nutrient solution containing NH 4 + or NO 3 − as the N source (N+ side), and those on the other side were watered with a nitrogen-free nutrient solution (N-side). During the VC-R2 period, on the N+ side, high nitrogen treatment inhibited nodule growth and nitrogenase activity (EC 1.18.6.1), and the inhibition was significantly increased with increasing high nitrogen supply time (10 days, 20 days). When the high nitrogen treatment time reached 20 days, the specific nitrogenase activity (C 2 H 4 μmol −1 g −1 nodule dry mass h −1 ) was similar to that in the low nitrogen treatment, indicating that the nitrogen fixation capacity per gram of dry mass nodules was almost the same. Therefore, it is assumed that long-term high nitrogen treatment mainly reduces nitrogen fixation by reducing the nodule number. The effect of nitrogen concentration on the roots on the N+ side was greater than that on the N-side. Taken together, these results indicate that nitrogen application affects a contact-dependent local inhibition of root nodule growth, nitrogenase activity, and nitrogen accumulation. The whole plant systematically regulates specific nitrogenase activity, and high nitrogen inhibition is recoverable.