Selection of different types of both N- and P-BBFs were screened for their effect on crop growth and agronomic efficiency in different climatic conditions. Potential risks related to their use, including environmental N and P losses, organic contaminants, heavy metals and dissemination of antibiotic resistance genes were assessed. Socio-economic impacts were analyzed and finally policy recommendations for replacing mineral fertilisers with BBFs were derived.
Two-year field trials conducted across Europe showed that both N- and P-BBFs can efficiently replace mineral counterparts. Average N fertiliser replacement value of N-BBFs was 71% across the experimental sites. The agronomic mineral replacement value of P-BBFs showed a larger variation compared to N-BBFs, but many of the P-BBFs are as efficient as common mineral P fertilisers. Production technologies had a major impact on the agronomic efficiency of the investigated BBFs, especially for P-BBFs. Considering these aspects, production technologies should potentially be adjusted accordingly to maximize the nutrient utilization from BBFs. European soils were found to have a high P status and majority of the agricultural lands are not P-responsive and up to 86% of the P requirement could be covered with BBFs.
A literature review and meta-analysis showed that solid and carbon rich BBFs had a positive effect on soil organic carbon content as compared to mineral fertilisers. Both climatic conditions and soil properties need to be considered when selecting the most beneficial BBFs. Furthermore, carbon depleting steps during BBF processing should be applied with caution and only if needed to eliminate organic pollutants and pathogens.
Content of harmful substances in the wide range of BBFs investigated were generally far below national EU member state benchmarks (pharmaceuticals, pesticides, PCBs, phthalates, dioxines, furans, PFAS, PAHs, plastics, heavy metals). Also, BBFs studied generally do not pose a risk of disseminating antibiotic resistance in the soil. BBFs are produced with a wide range of technologies, having different efficiencies for removing harmful substances. Therefore, the chemical composition of nutrient-rich side-streams needs to be known for targeting proper processing technologies for producing safe BBFs.
Due to the variable composition of P- and N-rich BBFs they may potentially induce very different losses (higher or lower) of N and P to the environment relative to their synthetic counterparts. For P leaching, P-BBFs are likely to cause lower P leaching losses than mineral P fertilisers, but soil and BBF properties need to be known for minimising P loss risk. For ammonia loss, some BBFs have very low risk, while others have a relatively high risk; for the latter, immediate soil incorporation should be recommended. For N leaching, BBF application rate (as determined by expected N fertiliser replacement value) is the major determinant factor - farmers should therefore not apply much higher total N rates with BBF to compensate for low N fertiliser value.
Replacing conventional mineral fertilisers with BBFs requires policy measures, as BBFs are typically not economically competitive without policy support. Applying investment support, subsidies for BBFs and gate fees on the potential biomasses to decrease the production costs of BBFs will enhance the competitiveness of BBFs. Targeting the policy instruments for regions with a high nutrient surplus may support the redistribution of BBFs to regions with high demand for nutrients.
