The Paris Agreement reached at the COP 21 in 2015 signals the new centrality of carbon sinks, such as soils, as key means of offsetting anthropogenic greenhouse gas emissions and enabling a 'net' global climate balancing. My communication addresses the rising focus of climate science and policy on soil, including the interpellation and promotion of soil as enhanced carbon sink. Starting from the calculation that increasing the amount of carbon contained in soil by 0.4% per year would allow for offsetting annual anthropogenic greenhouse gas emissions at the planetary scale, the "4 per 1000: Soils for Food Security and Climate" action was launched by the French government and currently has more than 200 signatories internationally. By accounting for the genesis and on-going developments of the 4 per 1000 Initiative, my communication shall contribute to a critical understanding of the requalification of soil in terms of climatic 'services' provision and bio-geo-engineering potential. Drawing on a grounded investigation including interviews with soil and climate scientists and policy-makers, I shall suggest that current soil carbon sequestration projects rely on a rather restrictive vision of soil as a global stock of carbon that we could monitor, measure, model, and enhance, which tends to ignore the actual diversity of soils, the complexity of their agency and temporality (i.e. soil releasing as well as sequestering carbon) and major issues of soil conservation. It will unpack the risks associated to the development of a new promissory climatic regime relying on soil-based negative emission technologies.