Filipe Natalio  - Department of Plant & Environmental Sciences, Weizmann Institute of Science, Rehovot 7610001, Israel; Kimmel Center for Archeological Science, Weizmann Institute of Science, Rehovot 7610001, Israel. 
Tomas P Corrales  - Departamento de Física, Universidad Técnica Federico Santa María, Casilla 110-V, Valparaíso, Chile. 
Yishay Feldman  - Chemical Services Unit, Weizmann Institute of Science, Rehovot 7610001, Israel. 
Beni Lew  - Institute of Agricultural Engineering, The Volcani Center, Agricultural Research Organization, P.O. Box 15159, Rishon LeTzion 7528809, Israel. 
Ellen R Graber - Institute of Soil, Water and Environmental Sciences, The Volcani Center, Agricultural Research Organization, P.O. Box 15159, Rishon LeTzion 7528809, Israel.

Global warming has prompted a search for new materials that capture and sink carbon dioxide (CO₂). Biochar is a derivative of biomass pyrolysis and a carbon sink mainly used to improve crop production. This work explores the underlying mechanism behind biochar's electric conductivity using a wide range of feedstocks and its combination with a binder (gypsum). This gypsum-biochar composite exhibits decreased density and flexural moduli with increasing biochar content, particularly after 20% w/w. Gypsum-biochar drywall-like composite prototypes display increasing shielding efficiency mostly in the microwave range as a function of biochar content, differing from other conventional metal (copper) and synthetic carbon-based materials. This narrow range of electromagnetic interference (EMI) shielding is attributed to natural alignment (isotropy) of the carbon ultrastructure (e.g., lignin) induced by heat and intrinsic interconnectivity in addition to traditional phenomena such as dissipation of surface currents and polarization in the electric field. These biomass-derived products could be used as sustainable lightweight materials in a future bio-based economy.