Nitrogen fixation of common bean (Phaseolus vulgaris L.) as affected by mineral nitrogen supply at different growth stages

Abstract

The effect of mineral nitrogen applications at different growth stages on N₂ fixation, nodulation and shoot dry weight of two cultivars of common bean (Phaseolus vulgaris L.) was investigated in a greenhouse experiment. Mineral nitrogen was split into two unequal doses (5 and 25 mg N kg^-1 soil). The main fertilizer dose was applied at sowing or at the vegetative stage, V6. N₂ fixation was determined by the ¹⁵N dilution method combined with the single treatment method. By physiological maturity, shoot dry weights and grain yields did not differ between cultivars, but cv. Puebla fixed more N₂ than cv. Negro Argel. The timing of the main nitrogen application (25 mg N kg^-1 soil) did not affect final shoot dry weights and grain yields. At the V6 vegetative stage, 28 days after the application of 25 mg N kg^-1 soil, the nodule numbers of cv. Puebla and cv. Negro Argel were 14% and 82%, respectively, of the nodule numbers of plants which received 5 mg N kg^-1 soil at sowing. Also, nodule dry weight and acetylene reducing activity per plant at V6 were reduced following the application of 25 mg N kg^-1 soil at sowing. However, when 25 mg N kg^-1 soil were applied at sowing and 5 mg N kg^-1 soil at the V6 stage, 55% and 45%, respectively, of the final (R8) nitrogen accumulation in the shoots of cv. Puebla and cv. Negro Argel was derived from N₂ fixation. Less N was derived from the atmosphere (45% and 35% in the shoots of cv. Puebla and cv. Negro Argel at R8, respectively) when 5 mg N kg^-1 soil was applied at sowing and 25 mg N kg^-1 soil at V6. The delayed onset of N₂ fixation of the Phaseolus vulgaris-Rhizobium symbiosis as a consequence of the main nitrogen application at sowing was less detrimental to the quantity of symbiotically fixed nitrogen of both cultivars than the effect of 25 mg N kg^-1 soil during vegetative growth.