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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]