*2.1. Materials Used*

Biochar samples made from *Populus tremuloides* (commonly known as poplar) were provided by Green Carbon Nanostructures Corporation (Hartland, WI, USA). Three poplar biochar samples were provided: (1) an unadulterated biochar control, designated "BC control"; (2) a biochar sample that was air-oxidized at 300 ◦C for 30 min designated "BC air", and (3) a biochar sample treated with carbon dioxide at 300 ◦C for 30 min designated "BC CO2". An N-339 CB (Vulcan M) was supplied by the Cabot Corporation (Alpharetta, GA, USA). Styrene-butadiene rubber (SBR) provided by Michelin (Greenville, SC, USA) was used as the rubber matrix for composite samples. Lauric acid (98%) was provided by Aldrich Chemical Company (Milwaukee, WI, USA). Precipitated stearic acid-coated calcium carbonate (SA-CC) nanopowder was bought from US Research Nanomaterials, Inc. (Houston, TX, USA) and used as a dry-milling aid.
