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The δ 15 N natural abundance technique is frequently used to quantify the proportion of legume nitrogen derived from atmospheric fixation (%Ndfa). This method compares the δ 15 N of a legume with a nonlegume reference plant to estimate the %Ndfa. For accurate estimations, it is recommended to pair each legume plot with a nonlegume reference plant to account for spatial variation. However, this pairing scheme is not feasible in large screening trials. Here, we tested the feasibility of eliminating reference plants to screen a faba bean (Vicia faba L.) population for %Ndfa. The first experiment screened 63 faba bean genotypes for %Ndfa by comparing estimates derived from an adjacent wheat (Triticum aestivum L.) reference plant in a nearest neighbor scheme (NN) with an arbitrary δ 15 N reference value (AR) of 7‰. Average and standard deviation of the genotypes' %Ndfa were 58 ± 13% in 2019 and 72 ± 8% in 2020 using the NN model and 61 ± 11% in 2019 and 75 ± 7% in 2020 using the AR model. The AR model was able to identify a majority of the high and low %Ndfa genotypes in both years. In a second study, wheat and a dicot weed within the field were used as reference plants to estimate the %Ndfa. The weed reference plant resulted in 3 to 13% higher %Ndfa values compared with the wheat in three out of four testing environments, but the variation did not alter ranking of the genotypes for %Ndfa.

The growing regional and global interests in legume crops for cover cropping and sustainable agriculture has provided new opportunities for growers to incorporate faba bean (Vicia faba L.) into their production systems. Much of faba bean's potential, especially in the western United States, is rooted in its high N fixation potential compared with other cultivated legumes. However, faba bean production is currently hindered by existing markets, regional agronomic extension guides, and availability of regionally adapted cultivars. This experiment began the effort to bridge one of the three gaps by evaluating 63 regionally and globally derived plant materials for agronomic and N fixation traits in four environments in California's Central Valley. The experiment observed significant (P ≤ .05) genotype × environment interactions for all faba bean N and agronomic except % of N derived from the atmosphere, harvest index, and 100-seed weight. Further, we found opportunities to leverage global germplasm to increase or decrease the 100-seed weight and days to flowering in locally adapted plant materials. Finally, this experiment identified trait-trait-associations that provide both opportunities and challenges to improving screening efficiencies (e.g., link between N yield and 100-seed weight in Chico2019 [r = 0.57; P ≤ .001] and in Fresno2020 [r = 0.51; P ≤ .001]). This work sets a framework for regional faba bean cultivar development work in the western United States. INTRODUCTIONA deeply intertwined interest in sustainable food systems and compatible specialty markets continues to grow in the western United States which opens the door for the plant breeders to reassess the potentials of once orphaned food crops. Among crop species, legumes have great potential for sustainable agricultural systems due to their value for human consumption Abbreviations: %Ndfa, N derived from the atmosphere.This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

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