The recycling of organic wastes in agriculture contributes to a circular economy by returning to the soil nutrients and reducing the need of mineral-based fertilisers. An agronomical and environmental evaluation of a series of biochars prepared from a range of urban and agricultural wastes was performed by soil incubation experiments and pot trials. The impact of biochar addition (alone, or in combination with either mineral or organic fertiliser) on soil N, P and micronutrients was studied, as well as the potential limitations for their agricultural use (associated to phytotoxicity and presence of potentially toxic metals). The type and origin of feedstock only had a minor impact on the response of biochar in soil and its interaction with the most important nutrient cycles. The presence of ashes in biochars prepared from urban and pre-treated organic wastes caused an increase in the availability of N and P in soil, compared to raw lignocellulosic biochar. All tested biochars exhibited favourable properties as soil amendments and no phytotoxic effects or negative impacts on soil nutrient dynamics were observed during the soil incubation experiments. Their agricultural use is only limited by the presence of potentially toxic metals in biochars prepared from feedstocks of urban origins.