Different Species and Cultivars of Broad Beans, Lupins, and Clovers Demonstrated Varying Environmental Adaptability and Nitrogen Fixation Potential When Cultivated as Green Manures in Northeastern Portugal
The success of growing legumes as green manure depends on their spatial and temporal
integration within agroecosystems, which minimizes competition with cash crops, and on their
nitrogen (N) fixation potential. This study evaluated seven legume species for biomass production,
N fixation, and suitability for use in cropping systems in northern Portugal. Oats (Avena sativa L.)
were grown to estimate the N fixation using the difference method, as a non-legume reference crop is
required for this purpose, and oats are widely grown in the region. The study was conducted over
four cropping cycles (2021–2024) in two climate zones across four land plots. The results indicated
that the biomass production and N fixation varied by the species/cultivar and cropping cycle, which
was significantly influenced by spring precipitation. Broad beans (Vicia faba L.) failed to develop
in one cycle on highly acidic soil (pH 4.9), showing negative N fixation values when calculated
by the difference method. Conversely, the lupins maintained a relatively high level of N fixation
across all the conditions, demonstrating strong environmental adaptability. Thus, the N fixation
values across the four cycles ranged from −5.4 to 419.4 kg ha−1 for broad bean (cv. Favel), while
yellow lupin (Lupinus luteus L.) exhibited average values between 204.0 and 274.0 kg ha−1. The
percentage of N derived from the atmosphere (%Ndfa) ranged from −13.3 to 91.6, −39.4 to 85.8, 83.8
to 94.7, 74.9 to 94.3, 72.8 to 92.2, 23.1 to 75.8, and 11.7 to 21.7 for these species/cultivars. Due to their
environmental adaptability, biomass production, and N fixation capacity, these legumes could be
used as green manure in inter-rows of woody crops or in summer annual crops like tomatoes and
maize, grown in winter as an alternative to fallow land. The lupins showed strong promise due to
their environmental resilience.
