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Following the Paris agreement (COP21), France set a carbon neutrality objective by 2050. Given its contribution to national GHG emissions (20%), the French agricultural sector is expected to play its part in achieving this ambitious target. It can act on three levers: N2O and CH4 emissions reduction, renewable energy production, and carbon storage in biomass and soil, the latter being less studied. The recent controversy on the 4 per 1000 initiative has also emphasized the need for a quantitative assessment of the potential for and cost of additional C storage in agricultural soils at the national level. Eight carbon storing practices were identified based on a literature review: expansion of cover crops; mobilization of new C inputs (not already spread on agricultural soils under current management practices); expansion of temporary grasslands (instead of silage maize); agroforestry; hedges; moderate intensification of extensive grasslands (+50kgN/ha); animal grazing instead of mowing and grass cover of vineyard. We assessed and mapped their potential for additional carbon storage in soils at a very fine spatial scale (1 km2), using a crop model (STICS) and a grassland model (PASIM). The additional C storage was calculated as the difference between the simulated soil C stock under i) the C storing practices and ii) the current management practices (i.e. the baseline), after a 30 years period. Using public statistics, we then assessed for each C storing practice: its potential applicability, its implementation cost, and its efficiency at the regional level. Using an economic model (BANCO), we finally compute the cost-effective allocation of the additional carbon storage effort, i.e. the uptake level of each storing practice in each region that minimizes the total cost of achieving a given total additional C storage target in mainland France. By varying this national storage target, we are able to depict a marginal carbon storage cost curve. Our results show a potential for additional C storage of about 8.43 Mt C/year for a total cost of 2.3 bn€/yr, which would compensate 6.8% of national GHG emissions and 41% of the agricultural sector emissions. This potential is mainly found in arable soils, where initial soil C stocks are low. Moreover, 96% of this potential can be achieved at a lower cost than 250€/tCO2e (the 2030 target carbon price in France) and for a total cost of 1.6 bn€/yr, which corresponds to only 18% of the current French budget for the Common Agricultural Policy. Reaching high soil C storage targets implies the full implementation of cover crops and agroforestry, together with the expansion of temporary grasslands in crop rotations. The costs and potential for additional C storage vary both between practices and across regions. This is why there is no “one-size-fits- all” solution, rather a combination of good practices at the right place. Our results thus provide useful information for the design of efficient climate change mitigation policies. Besides highlighting the potential for additional C storage in arable soils, where initial stocks are low, our study also underlines the need to maintain soil C stocks where they are high, i.e. in grasslands and forest soils.