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 potent