Impacts on Soil Health of Soil Improvers Derived from Agri-Food Processing Residues: a Systematic Review with a Focus on European Field Studies

The circular bioeconomy, centred on the principles of "Reduce, Reuse, and Recycle", is central to achieving the goals of the European Green Deal and the "Farm to Fork" strategy. The key approach, within this framework, is the valorisation of food processing side-streams through various processing technologies to produce fertilisers that, when applied to the soil, act as potential "soil improvers". When applied to soil, these materials affect not only the physical, chemical, and biological properties of soil but also impact greenhouse gas emissions, nutrient leaching, and the levels of contaminants or pathogens. A systematic review of field studies conducted in European soils between 2014 and 2024 was performed with stringent inclusion and exclusion criteria, which allowed the identification of suitable articles in three independent steps. The main soil improvers derived from various agri-food side-streams were biochar, compost, and digestate. Their effects, along with the potential risks associated with their use in agricultural fields, were carefully evaluated and discussed. This review forms part of the DeliSoil project "Delivering safe, sustainable, tailored & societally accepted soil improvers from circular food production for boosting soil health" funded under the European Union "A Soil Deal for Europe". This systematic review showed that soil improvers enhance soil health by improving microbial diversity, organic matter, and water retention, and by boosting plant growth. However, potential risks require careful management, and long-term studies are needed to fully assess their environmental and agronomic impacts.