Importance of correct B value determination to quantify biological N2 fixation and N balances of faba beans (Vicia faba L.) via 15N natural abundance

Quantifying biologically fixed nitrogen (BNF) by legumes through the N-15 natural abundance techniques requires correct determination of a so-called B value. We hypothesized that significant variations in B values exist between faba bean (Vicia faba L.) varieties having consequences for BNF and N balance calculations. We experimentally determined B values for a range of faba bean varieties and quantified to what extent variety has an effect on B values and hence BNF quantification. Seeds of six faba bean varieties released in Ethiopia were inoculated with Rhizobium fabae strain LMG 23997-19 and grown in vermiculite with an N-free nutrient solution in a growth room until full flowering. Total N and N-15 content of nodules, roots, and shoot components was analyzed separately to determine the weighted whole plant N-15 fractionation during N-2 fixation, i.e., the B value. Owing to its large seed size and high N content, a correction for seed N was carried out. We then calculated the percentage of N derived from air (%Ndfa), BNF, and N balance for faba beans grown in the field using three B value scenarios (variety specific B value corrected for seed N, variety specific B value without seed N correction, and a literature derived B value). Whole plant seed N corrected B values were significantly different (P < 0.05) between varieties and varied between +0.5 +/- 0.4 and -1.9 +/- 1.4aEuro degrees suggesting a variable isotope fractionation during N-2 fixation. The %Ndfa was significantly (P < 0.05) different between varieties (59 +/- 4.2-84 +/- 4.5 %) using seed N corrected B values. BNF (218 +/- 26.2-362 +/- 34.7 kg N ha(-1)) was significantly (P < 0.05) different between varieties for corrected and uncorrected B values. Soil N balance did not result in statistically significant (P > 0.05) difference between varieties for all three B value scenarios. Use of inappropriate B values masked the difference between varieties and affected their ranking in terms of BNF, resulting from an over- to underestimation of 15 and 19 %, respectively. When applying the N-15 natural abundance technique to compare BNF of legume accessions, we recommend determining a B value for each accession. For legumes with large seeds such as faba beans, it is moreover essential to account for seed N when determining the B value.