Effect of N Fertilizer on Yield, Protein Content, and Symbiotic N Fixation in Fababeans

Fababeans (Vicia faba L. var. minor) are potentially an important source of crude protein in Western Canada. Thus the effect(s) of combined inorganic N on fababean growth performance are of considerable interest. This experiment was conducted to determine the effect(s) of fertilizer-N on fababean shoot and root yields, protein contents, N uptake, N fertilizer uptake, and symbiotic N fixation. Nitrogen fertilizer as NH₄NO₃, was applied to fababeans grown in an Udic Haploborall soil in rates up to 900 mg N/pot (300 mg N/kg soil), as split applications of 75 mg N/pot (25 mg N/kg soil), and as single large mid-season applications of 300 mg N/pot (100 mg N/kg soil. ¹⁵N-labelled fertilizers were used to measure N fertilizer uptake, and were also used with a reference crop to measure symbiotic N fixation. The split application treatment received labelled-¹⁵N in the manner described by Fried et al. (1975), enabling efficiency of N fertilizer uptake for each application time to be determined. Fababeans, when nodulated with effective strains of Rhizobia, obtain their N from soil and symbiotic fixation. Results obtained from this experiment indicate that these two sources of N were able to fully satisfy fababeans' N demand throughout their entire growth cycle. Aerial yield was not affected by N fertilizer up to 600 mg N/pot (200 mg N/kg soil) applied at seeding, by 300 mg N/pot applied in four 75.mg portions, nor by single midseason applications of 300 mg N/pot. Only the highest rate of N employed, 900 mg N/pot at seeding, significantly increased fababean yield, the increase being 13.2%. However, protein content and total N uptake into fababean shoots were unaffected by all N applications used. Fababeans, nodulated with an effective Rhizobia strain were efficient symbiotic fixers. Fababeans receiving no N fertilizer fixed 708 mg N/plant, or 87.10/, of their total N content. Fababeans were capable of fixing substantial amounts of N after pod-fill, amounting to at least 28% of the total seasonal symbiotically fixed N. A significant (r³ = 0.99) linear inverse relationship occurred between fertilizer-N uptake and symbiotic N fixation. Increasing quantities of fertilizer uptake into fababean tissues decreased plant N derived from fiiation. Fababeans were as adept as barley in extracting available soil and fertilizer-N, and evidence indicates fababeans preferentially feed from soil and fertilizer N sources.