Biological nitrogen fixation and its potential to reduce greenhouse gas emissions in a lentil-cereal intercropping system in southern Germany

Legume-cereal intercropping is promoted as a strategy to reduce reliance on synthetic nitrogen (N) fertilizer and mitigate agricultural greenhouse gas (GHG) emissions, yet the magnitude of these benefits in different management contexts remains poorly quantified. This study assessed biological nitrogen fixation (BNF), grain yield and quality, partial N balance, and potential GHG emission savings in lentil grown in cereal intercrops (barley and oats) over two growing seasons (2024, 2025) at two sites in southern Germany: an experimental station (Ihinger Hof, IHO, 48°44'39.4" N, 8°55'25.8" E) and a commercial farm (Bad Windsheim, BWI, 49°29'36.2" N, 10°23'11.1" E). The proportion of plant N derived from atmospheric fixation (%Ndfa) was estimated with the 15 N natural abundance method, and belowground fixed N was estimated from aboveground fixed N with a species-specific conversion coefficient. GHG savings from avoided synthetic fertilizer use were estimated with an IPCC Tier 1 approach. Lentil yield, companion crop yield, thousand-grain weight, protein content, and several BNF parameters showed significant interactions, and results are reported for each site and season. At IHO, grain %Ndfa was high in both years (92.6–95.2%), whereas at BWI it was lower and more variable (70.8–79.6%). The partial N balance was positive in all four site-years. Estimated GHG savings from N fixed in shoots and roots ranged from 166 to 407 kg CO 2 -eq ha -1 , with the highest values at IHO in 2024. These estimates assume that the fixed N is available to the following crop, and root N was estimated rather than measured. Two sites and two seasons were sampled and no sole-crop lentil was included, so the results are an initial, site-specific reference point. They indicate that lentil grown in cereal intercrops can contribute N inputs and potential GHG savings in southern Germany, and they call for further work across more sites, years, and cropping sequences.