Demystifying the agronomic and environmental N performance of grain legumes across contrasting soil textures of central Germany

Abstract

Grain legume cultivation has multifaceted benefits in farmland and its on-going expansion in Europe contributes to the targeted self-sufficiency in protein production for human and livestock nutrition. However, there is limited information on key environmental processes such as the nitrogen (N) cycle. To address this knowledge gap, N input and output pathways were investigated among six grain legumes across two experimental farms (Gladbacherhof and Oberfeld) in central Germany, between 2020 and 2022. Percentage of N derived from atmospheric fixation (%Ndfa), was estimated at peak biomass production stage through the ¹⁵N natural abundance method, and ranged from 8.9% to 93.9%. Grain legumes at Oberfeld were found to have a higher %Ndfa (median 75.4%) than at Gladbacherhof (54.5%) with winter varieties i.e., winter pea (Pisum sativum) and winter faba (Vicia faba), found to be more efficient at acquiring Ndfa in comparison to soya (Glycine max). At Gladbacherhof, the N export in the form of harvested grain was higher than the N input resulting from biological N fixation while at Oberfeld, an opposite trend was observed.The potential of winter legumes to reduce nitrate leaching compared to plots which were left fallow over winter and then cropped with spring legumes, was measured. Nitrate leaching was found to be low at both sites, ranging from 1.1 to 3.4 kg N ha⁻¹ at Gladbacherhof, and from 1.0 to 1.5 kg N ha⁻¹ at Oberfeld, with no significant differences across treatments. N₂O emissions were measured from November 2020 to October 2021. Daily emissions averaged 0.033 mg N₂O-N m⁻¹ h⁻¹ whereas the cumulative emissions ranged from 0.8 to 1.6 kg N₂O-N ha⁻¹ (November 2020 – April 2021), and from 0.7 to 2.1 kg N₂O-N ha⁻¹ (April 2021 – October 2021).Selecting for grain legume varieties that can fix considerable amounts of atmospheric N₂ can be an important criterion for inclusion in crop rotations. However, the drawbacks of N losses through nitrate leaching and nitrous oxide emissions, as well as N exports through grains, should be accounted for, to ensure efficient field-scale N management.