Quantification of symbiotically fixed N2 by faba bean and allocation of 15N among above- and below-ground components of faba bean, canola, and barley on a Gray Luvisol

This study was carried out to estimate N-15 allocation into different components of the above- and below-ground plant parts, and to quantify biological N-fixation in faba bean using N-15 dilution method, and both canola and barley as reference crops. Four samplings were conducted during the crop growing season, both above- and below-ground plant parts, and soil were sampled. The above-ground plant was fractionated into leaves, stems, shell, and seeds for faba bean and canola; and dead-leaves, live-leaves, stems, sheaths, husks, and seeds for barley. Roots were extracted from soil by root-washing technique and further separated from dead debris by water floatation and hand-picking. N-15 excess of faba bean above-ground parts was lower than its roots, but the reverse was true for the non-legumes (canola and barley). N-15 fractionation was apparently taking place in various plant parts. Reproductive organs of faba bean had lower N-15 excess than the vegetative ones, but reproductive organs of non-legumes (canola and barley) had higher N-15 excess than the vegetative ones. N-fixation in faba bean was quantified by using N-15 dilution method with either canola or barley as a reference crop, the quantity of N derived from atmospheric fixation was 183-199 kg N/ha/yr in the above-ground parts of faba bean and 18-22 kg N/ha/yr in faba bean roots by September 1, when faba beans were not fully matured. Either canola or barley can be a valid reference crop for N-fixation estimation in N-15 dilution method. Total difference method agreed with N-15 dilution method with less than 10% variation in this study. A peak of N-fixation was observed after faba bean flowering and the rate of N-fixation during this period was 4.0-4.7 kg N /ha/day. This study provides the key inofmration for the quantity of N-fixation from atmosphere in faba bean growing on this soil of Canada.