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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. [ABSTRACT FROM AUTHOR]

Biochar and compost are increasingly considered sustainable amendments to improve soil fertility, while reducing agrochemical use. However, the efficiency of biochar, compost, and especially their mixtures under field conditions in temperate regions is still poorly studied. The objective of this study was to evaluate the effects of biochar/compost mixtures on crop yield and soil properties in French temperate cropping systems and to compare the amendment effects to soils receiving mineral potassium and phosphorus fertilization. To this end, green waste compost alone (8 t.ha−1) or in mixture with three contrasted biochars (8 t.ha−1 compost and 4 t.ha−1 biochar) were applied to maize-wheat cropping systems located in three major agricultural territories in France. Results showed that maize and wheat yields were predominantly site specific. Within each site, compost and biochar application led to similar yields and nutrient uptakes as compared to the mineral fertilization, suggesting that compost-biochar mixtures might be as efficient as mineral fertilizers to supply potassium and phosphorus, while biochar did not improve compost benefits to plant yield. Moreover, the effects of compost-biochar mixtures on soil organic carbon concentrations were site specific and led to no effect or increase by up to 53%. We conclude that compost-biochar mixtures may increase carbon content in soil and substitute phosphorus and potassium mineral fertilizers for crop production in temperate cropping systems, even though their effects are site specific. [ABSTRACT FROM AUTHOR]