Synthesis report: European-scale trade-off analysis of reduced mineral nitrogen fertilization

Reducing the use of chemical fertilizers is one of the objectives of the Farm to Fork Strategy (F2F) of the European Commission to reduce environmental impacts of agricultural production. To assess potential trade-offs of reduced mineral nitrogen (N) fertilization on multiple ecosystem services of soils we set up a modelling framework that allows to quantify effects on yields, soil organic carbon (SOC) storage, nitrous oxide (N2O) emissions and nitrate (NO3) losses. We applied the SOMMIT index tool that was developed within the EJP SOIL project SOMMIT for trade-off analysis (Calone et al. 2024). This allows a quantitative assessment of the trade-offs among yields, SOC, N2O and NO3, presenting a comprehensive evaluation of reduced N fertilization. We calculated a SOMMIT index for each NUTS2 region in Europe and for each of the six most important crops (wheat, maize, barley, rapeseed, sugar beet, potato).

Mean SOMMIT index values differed only slightly between the different crop types, but showed high spatial variability for all crops. We identified regions with very low SOMMIT index values, suggesting that high trade-offs for yields and SOC storage occur and/or that reductions in N losses were low. In other cases, SOMMIT index values were high demonstrating that low trade-offs for yields and SOC storage and/or significant reductions in N losses are expected if mineral N fertilization would be reduced by 20 %. This variability in SOMMIT index values that we found emphasizes the unequal distribution of risks associated with reduced N fertilization for both crop yield and SOC storage and underlines the importance of region-specific approaches.

We conclude that a general implementation of reduced mineral N fertilization bares risks for crop production and SOC storage. Tools including multiple ecosystem services of soils as the SOMMIT index in combination with European-scale modelling can help identify regions with strong trade-offs as well as regions, where efforts to reduce negative impacts of high N loads could be prioritized. Such a stepped approach could help gain experience and produce valuable results for adaptive management policies as these are expanded to more vulnerable regions.