Soils are an essential resource for our societies and at the cross road of major challenges: food security, renewable energy provision, water security, biodiversity maintenance and climate change. Indeed, soils contain twice as much carbon as the atmosphere and could then contribute to mitigating climate change by storing more carbon in the form of soil organic matter. This has been recently emphasized by the 4p1000 initiative proposed at the COP21, which fosters the preservation and increase of the organic matter content of soils for food security, adaptation to and mitigation of climate change Plant biomass is the main source of soil organic matter and lignocellulosic biomasses the main source of organic carbon to soils. Maintaining or increasing the inputs of plant biomass to soils is therefore crucial, at the local scale for soil fertility and biodiversity and other ecosystem services, and at the global scale for climate change mitigation. This can be achieved in agricultural soils through an increased primary production with adequate agricultural practices (e.g. cover crops, agroforestry), while ensuring sufficient plant residue return to the soil. Recent results show that increasing plant biomass inputs to soils is more effective to increase soil organic carbon stocks than no tillage practices. How much biomass should be returned to soil to maintain or increase its carbon stocks and its fertility? What are the effects on soil fertility and soil carbon stocks of returning more or less processed biomass to soil (e.g. straw, compost, digestate..) ? Where should the biomass be returned to soil in priority at the landscape scale? These are questions and challenges to be explored for an optimized management of lignocellulosic biomasses in a bioeconomy addressing global challenges