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Coastal « green tides » - algal blooms caused by eutrophication - are a common occurrence throughout the world where rivers drain areas of intensive agriculture towards relatively closed bays. In Britanny (France) blooms of algea from the genus Ulva have been observed since the 1970’s, and despite four successive action plans to tackle their cause, still cause health, environmental and economic issues every summer.Biochar is known to affect nitrogen dynamics in soils, reducing nitrate leaching and N2O emissions, in addition to increase the stable carbon pool of the soil. The greenhouse gases balance of a biochar system is strongly influenced by biomass source, and to a lower extent by transportation of the feedstock biomass and finished biochar [1]. Residual biomass that can be used as feedstock for biochar production is subjected to competing use, e.g. for animal bedding, incorporation to the soil, or anaerobic digestion. Local biochar systems based on residual biomass with little to no other uses (waste) are therefore likely to be more environmentally and economically sustainable.In the SyBio project, we are investigating the biogeochemical consequences and technical and social feasibility of a local biochar system based on green algea biomass that is collected on the sandy shores of Britanny during algal blooms, as an alternative to direct application of algea to the soil or landfilling, the most common disposal options at present. In particular, we studied the effect of Ulva biochar on nutrient cycle and greenhouse gases emissions in the soil plant-system. We compared the effects of the biochar with that of direct application of algea, and also tested the combination of biochar and compost. We hypothesized that biochar application would reduce nitrate leaching and N2O emissions relative to the control situations and direct algea application, while plant growth and biomass yield would remain unaffected.We carried out a climate chamber experiment, growing spring barley (Hordeum vulgare) in three soils with contrasting texture under a 10 years averaged climate typical of Britanny. We measured soil CO2 and N2O emissions, nitrogen forms in the soil solution and drainage water, and plant photosynthesis at regular intervals throughout the growing period. We will present the results of this experiment as well as insights from other aspects of the project.[1] Lehmann, J., Cowie, A., Masiello, C.A. et al. Biochar in climate change mitigation. Nat. Geosci. 14, 883–892 (2021). https://doi-org.insu.bib.cnrs.fr/10.1038/s41561-021-00852-8