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 NH4+ stimulates both whole plant (nodules plant(-1)) and specific nodulation (nodules g(-1) root DW). The effect of low concentrations of NH4+ on the soybean/Bradyrhizobium symbiosis is unknown. The objectives of the current study were to determine the immediate and residual effects of NH4+ on nodulation and N-2 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 N-15-labelled (NH4)(2)SO4 for 28 days after inoculation (DAI). From 29 to 56 DAI the plants were grown on NH4+-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 N-15 enrichment of plant composites. Nitrogenase activity was measured by gas exchange at 28 DAI. The plants in the control (0.0 mM NH4+) treatment had consistently lower relative growth rates than the plants in the NH, treatments during the first 28 DAI. Plant growth was also less at 2.0 mM NH4+ compared to growth at 0.5 and 1.0 mM NH4+. At 28 DAI, plants exposed to 0.5 and 1.0 mM NH4+ had significantly more nodules per plant and larger individual nodules than either the NH4+-free controls or the 2.0 mM NH4+-supplied plants. However, specific nodulation (nodule number g(-1) root DW) and specific nitrogenase activity (nitrogenase activity g-l nodule DW) were on average approximately 286% and 60% higher in the control plants, respectively, than for plants in the NH4+ 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 NH4+. At 56 DAI, after an additional 4 weeks of NH4+-free nutrition, the plants which had previously received 0.5 and 1.0 mM NH4+ 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 NH4+ has a negative effect on the nodulation process in the soybean/Bradyrhizobium symbiosis (as best indicated by the negative effect of NH4+ on specific nodulation). Despite this negative effect on specific nodulation, 0.5 and 1.0 mM NH4+ resulted in higher whole plant nodulation and N-2 fixation due to a compensating, positive effect on overall plant growth (i.e. fewer nodules g(-1) root DW, but much larger roots). Once NH4+ was removed from all treatments, the soybean plants appeared to move toward a consistent level of nodule DW relative to root DW.