Soybean Symbiotic Nitrogen Fixation and Translocation of Nitrate and Potassium Ions within Plants under Various Rhizobium Inoculation and Early-season Nitrogen Use, Poster Board #163

In-season patterns of soybean symbiotic N fixation and translocation of NO₃ and K ions within soybean leaves, stems, petioles, pods and beans were examined in a marsh soil in Nova Scotia in 2010. The objectives were to determine the effects of Rhizobium inoculants and early-season N inputs on soybean NO₃ and K ion repartitioning within plants and NO₃ and their influence on grain yield and quality. The field study treatments consisted of four rates of Bradyrhizobium japonicum inoculants at 0, 1.5, 3, and 4.5 g·kg^-1seeds (IR₀, IR_1.5, IR₃, and IR_4.5) and four rates of starter N fertilizers at 0, 10, 20, and 30 kg·ha^-1 (N₀, N₁₀, N₂₀, and N₃₀), arranged in a split-plot design with three replications. The soybean cv. ‘Lynx RR’ used in the study was genetically modified, Round-up ready. Nodulation rates, determined using active nodules (presence of leghaemoglobin red pigments), were significantly higher (46 nodules/plant) at the N₂₀ level (P < 0.05) than other treatments at flowering. The symbiotic N fixation ratios, determined using ureide-N method, increased significantly from 20% at flowering to 53% at podding stage for IR₃ and IR_4.5 treatments. At flowering the highest daily N fixation was estimated to 0.86 g/plant at the IR_4.5 rate, which was significantly higher than the control I₀ (0.49 g/plant). The ion-selective-determined leaf and stem sap NO₃ concentrations were significantly higher in the IR treatments and the NO₃ concentrations were in the order of pods (4100 ppm) > leaves (2062 ppm) > stems (1660 ppm) > petioles (1245 ppm). The leaf and stem sap K concentrations were mostly comparable with the seasonal as in NO₃ ions. The main effects and interactions of the treatments were significant on the temporal storages of sap NO₃ and K ions in leaves, stems, petioles and pods (P < 0.0502). Total N uptake increased from 0.3 g/plant at flowering to 1.2 g/plant at podding stage for all inoculant treatments. The repartition of total N was found in a portion of 80% in seeds, 11% in pods wall and 9% in stems (P < 0.05). Soybean grain yield was shown the similar trends as in symbiotic N fixation. It was suggested that high symbiotic N fixation could stimulate translocation of NO₃ and K ions from leaves and stems to seeds and the early-season N₂₀ input could promote soybean nodulation at flowering and IR_4.5treatment could enhance the symbiotic N fixation.