Revised belowground factor reduces seed yield-based estimates of faba bean N2 fixation

The incorporation of legumes, particularly faba bean ( Vicia faba L.), into cropping systems is pivotal for sustainable agriculture due to their ability to fix atmospheric nitrogen (N). Despite its significance, the quantification of biological nitrogen fixation (BNF) in faba bean remains challenging, particularly regarding the belowground (BG) contribution. This study updates the BG factor for BNF estimation using data from 42 primary studies reporting on both, measurements of above and belowground fixation. Total BNF based on dry matter seed yields was then recalculated with a comprehensive dataset of 329 primary studies. A BG factor of 1.15 was derived, lower than widely used values, resulting in a total BNF estimate of 49 kg N per Mg dry matter seed yield. As a case study, the new estimation was applied for German national statistics data and compared with the legal framework for nutrient balances. This revealed an 18% overestimation of BNF and a twofold overestimation of nitrogen balances in official calculations, with implications for nutrient management policies. Our findings highlight the necessity of revising BNF estimates to improve the accuracy of rotational nitrogen budgeting and nutrient balance assessments. Faba beans remain a valuable crop for reducing synthetic N inputs, but precise quantification of their BNF potential is critical for informed decision-making and achieving sustainable agriculture.