15N‐Determined Time Course for N2Fixation in Two Cultivars of Field Bean1

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Abstract
Dinitrogen (N2) fixation was measured for two bean cultivars, ‘Aurora’ and ‘Kentwood’, in a 2‐year field experiment, at four growth stages and two levels (10 and 40 kg ha−1) of fertilizer N1as Ca(NO3)2. Both 15N isotope dilution and modulation parameters were used to determine if nitrogenase activity at these growth stages could explain Aurora's superiority in symbiotic N2fixation. Nodules were actively reducing C2H2at the first trifoliate stage and acetylene‐reducing activity (ARA) increased until anthesis or various stages of pod filling, depending on the cultivar and the year. The addition of fertilizer N at 40 kg ha −1did not alter the pattern of ARA over time although nitrogenase activity was affected. Although nodules were reducing low levels of C2H2at physiological maturity, no activity was observed at harvest maturity. Acetylene‐reducing activity peak activities of 120 and 151 nmol C2H4min−1plant−1for Aurora and 69 and 192 nmol C2H4min−1plant−1for Kentwood in 1981 and 1982, respectively, were similar to values observed for other temperate zone beans. Only a single peak of ARA was observed. The two bean cultivars received similar proportions of their plant N requirements from the atmosphere, averaging 72% in 1981 but only 54% in the cooler year, 1982. Plants apparently had the ability to alter their assimilation of different N sources (air, soil, and fertilizer) to compensate for the lower %Ndfa in the cooler year, 1982. Soil and fertilizer N uptake was particularly important to plant yield in early growth stages until N2 fixation was actively established. Aurora fixed 124 and 114 kg N ha−1(at the 10 kg N ha−1rate) and 124 and 100 kg N ha−1(40 kg N ha−1) in 1981 and 1982, respectively. Kentwood fixed 70 and 97 kg N ha−1(10 kg N ha−1) and 93 and 79 kg N ha−1(40 kg N ha−1) in 1981 and 1982. This superiority of Aurora was due to a longer vegetative phase supporting N2fixation. The ARA severely underestimated N2fixation (as determined by 15N‐isotope dilution) and we believe that N2fixation in beans has been underrated solely because of the inadequacies of ARA. ‘Galt’ barley served as an excellent non‐fixing control plant for 15N isotope dilution to measure N2fixation in these bean cultivars because its pattern of assimilating N from the soils was similar to that of the beans at all growth stages.