Compost and biochar are increasingly considered to improve crop growth and soil functioning in agriculture. However, their combined application has shown contrasting results, probably resulting from the use of different biochar/compost ratios and divergent (synergistic or antagonist) impacts on nutrient availability, especially nitrogen (N). We aimed to elucidate how biochar/compost mixtures affect nutrient availability and plant growth. We hypothesised that biochar and compost will have a synergistic effect, which will depend on the biochar/compost ratio, consequently impacting nutrient uptake and biomass of plants. In this context, ryegrass was grown on agricultural soil amended with five compost/biochar ratio mixtures with and without N fertilisation. We followed soil fertility parameters, soil microbial carbon (C) and N, nutrient uptake, and plant growth. Results showed that irrespective of their ratio, biochar and compost mixtures had no effect on microbial biomass but increased soil nitrate concentration, suggesting that, despite their high C/N ratios, amendments increased N availability while preventing microbial immobilisation. Plant biomass and nutrient uptake improvements depended on the biochar/compost mixing ratio. Plant stoichiometric analysis revealed that a mixture containing less biochar than compost reduced N limitation and was even more effective in stimulating plant growth than mineral N fertiliser. The beneficial effects of biochar and compost on plant growth were strengthened when used in combination with N fertilisation. In conclusion, we demonstrated synergistic effects between biochar and compost, predominantly driven by their mixing ratio, to reduce N limitation in the soil towards a more nutrient-equilibrated system and highlighted their potential use as a sustainable alternative or supplement to mineral fertilisers.
