Abstract:
Quantifying biologically fixed nitrogen (BNF) by
legumes through the 15N 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 15N content of nodules, roots,
and shoot components was analyzed separately to determine
the weighted whole plant 15N fractionation during N2 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±
1.4‰ suggesting a variable isotope fractionation during N2
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 15N 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 essen
