The achievement of carbon neutrality till 2050 will require the deployment of negative carbon emission technologies like the production and soil incorporation of biochar, produced from pyrolyzed plant-based residues. The carbon sequestration potential of biochar (BC) from hazelnut (Corylus avellanaL.) shells (HNS) and sunflower (Helianthus annuusL.) husks (SFH) was evaluated when the biomass was carbonized in a fixed bed reactor, in a rotary kiln and in a screw reactor. In all tested reactors, higher temperatures (>500 degrees C) and longer retention times increased the C concentration and stability of the biochar, with negligible effects of the reactor type and feedstock. A national case study was developed for Austria concerning the potential use of SFH- and HNS-BC in combination with compost for reaching the "4 per mille" objective. An annual soil organic carbon increase of 2.5 Mt C would be needed, requiring amendment rates of 2.2 Mt C a(-1)for all annual crop areas and 0.3 Mt C a(-1)for all vineyards and orchards. If compost only were used, the annual cost would be about 200 EUR ha(-1)but short-term re-mineralization would have to be considered. If the more recalcitrant biochar were used only, about 2.3 t BC ha(-1)would be needed at a cost of 1400-1870 EUR ha(-1). The study shows in principle the feasibility of applying compost-biochar mixtures for achieving the "4 per mille" objective but in practice, supplemental soil management strategies for sequestering C will be required.