What is envisioned are communities where transportation is based on compressed air engines, the compressors being also used to remove nitrogen from the air for agricultural fertilizer, and carbon dioxide being removed as well as the advanced stage of the plan for use of compressors.
Nitrogen, oxygen, and carbon dioxide are able to removed from air with existing technologies. The same compressor for the air cars could be used for compression in a pressure swing absorption plant. The nitrogen can be used making fertilizer, oxygen can be utilized as explained in the next paragraph, and carbon dioxide can be sequestered (thus Carbon Negative Communities) by means of being used for green houses...the plants would sequester the CO2.
Landfills gas is approximately 50% methane. Methane can be converted into hydrogen and carbon, and the hydrogen and oxygen from the pressure swing absorption plant can generate electricity by means of fuel cells as well as water as a byproduct.
The water from the fuel cell can be used for the green houses. All in all, the communities may be able to create a negative carbon footprint.