Low concentrations of ammonium inhibit specific nodulation (nodule number g-1 root DW) in soybean (Glycine max [L.] Merr.)

Experiments on peas (Gulden and Vessey, 1997) have indicated that NH ₄ ⁺ stimulates both whole plant (nodules plant^-1) and specific nodulation (nodules g^-1 root DW). The effect of low concentrations of NH ₄ ⁺ on the soybean/Bradyrhizobium symbiosis is unknown. The objectives of the current study were to determine the immediate and residual effects of NH ₄ ⁺ on nodulation and N₂ fixation in soybean (Glycine max L.] Merr.) in sand culture. Soybean (cv. Maple Ridge) were exposed to 0.0, 0.5, 1.0 and 2.0 mM of ¹⁵N-labelled (NH₄)₂SO₄ for 28 days after inoculation (DAI). From 29 to 56 DAI the plants were grown on NH ₄ ⁺ -free nutrient solution. Plants were harvested at 7, 14, 21, 28 and 56 DAI for root, shoot and nodule dry weight (DW), total N content, nodule counts and ¹⁵N enrichment of plant composites. Nitrogenase activity was measured by gas exchange at 28 DAI. The plants in the control (0.0 mM NH ₄ ⁺ ) treatment had consistently lower relative growth rates than the plants in the NH ₄ ⁺ treatments during the first 28 DAI. Plant growth was also less at 2.0 mM NH ₄ ⁺ compared to growth at 0.5 and 1.0 mM NH ₄ ⁺ . At 28 DAI, plants exposed to 0.5 and 1.0 mM NH ₄ ⁺ had significantly more nodules per plant and larger individual nodules than either the NH ₄ ⁺ -free controls or the 2.0 mM NH ₄ ⁺ -supplied plants. However, specific nodulation (nodule number g^-1 root DW) and specific nitrogenase activity (nitrogenase activity g^-1 nodule DW) were on average approximately 286% and 60% higher in the control plants, respectively, than for plants in the NH ₄ ⁺ treatments at 28 DAI. Also at 28 DAI, specific nodule DW (nodule DW g^-1 root DW) were 17, 44 and 53% higher in control plants than plants that had been exposed to 0.5, 1.0 and 2.0 mM NH ₄ ⁺ . At 56 DAI, after an additional 4 weeks of NH ₄ ⁺ -free nutrition, the plants which had previously received 0.5 and 1.0 mM NH ₄ ⁺ still maintained the highest plant DW and N contents, however, specific nodule DW had become similar at 600 mg nodule DW g^-1 root DW among all treatments. It is concluded that NH ₄ ⁺ has a negative effect on the nodulation process in the soybean/Bradyrhizobium symbiosis (as best indicated by the negative effect of NH ₄ ⁺ on specific nodulation). Despite this negative effect on specific nodulation, 0.5 and 1.0 mM NH ₄ ⁺ resulted in higher whole plant nodulation and N₂ fixation due to a compensating, positive effect on overall plant growth (i.e. fewer nodules g^-1 root DW, but much larger roots). Once NH ₄ ⁺ was removed from all treatments, the soybean plants appeared to move toward a consistent level of nodule DW relative to root DW.