Land-sea coordination is important for achieving carbon neutrality while maintaining ecosystem sustainability and food security. Excessive fertilizer inputs and low nutrient-use efficiency may constrain terrestrial carbon retention and increase nutrient export to coastal ecosystems. Biochar, owing to its carbon stability, porous structure, and nutrient-retention capacity, provides a potential material-based approach for coordinating terrestrial carbon management and coastal ecological restoration. Here, we develop a paired terrestrial-marine experimental framework to evaluate the effects of biochar at the terrestrial source and coastal receiving ends of the land-sea continuum. In the terrestrial component, an acidic-soil pot experiment was conducted in which biochar partially replaced chemical fertilizer during the cultivation of Shanghai pak choi. Preliminary results showed enhanced plant growth and higher soil mineral-associated organic carbon under biochar-amended fertilization, suggesting that partial fertilizer substitution may promote soil carbon stabilization while supporting crop production. In parallel, we established a controlled coastal water-column incubation system to evaluate the feasibility and performance of engineered biochar variants. These materials are benchmarked against diverse controls such as pristine biochars and olivine. We systematically quantify the vertical distribution patterns of physicochemical indicators, nutrient concentrations, total alkalinity, carbonate chemistry, chlorophyll-a, and fluorescent dissolved organic matter. These measurements aim to determine how the alkalinity and leachable organic matter derived from different biochar strategies influence phytoplankton development, microbial organic-matter processing, and their associated ecological responses across the water column. By integrating complementary terrestrial and marine experiments, this study links soil carbon stabilization and reduced fertilizer dependence with coastal carbonate-system regulation and biological carbon transformation. The paired framework provides a process-based basis for evaluating the opportunities, constraints, and ecological implications of biochar-enabled carbon management across the land-sea continuum.