Explanation

Anaerobic decomposition of the organic fraction of municipal solid waste by methanogenic bacteria in landfills produces "landfill gas," roughly half methane and half carbon dioxide. Methane is therefore the characteristic gas released from urban landfills, and it is also a potent greenhouse gas, which is why landfill-gas capture projects exist. Nitrogen and oxygen are the background gases of air, not landfill products, and hydrogen is not generated in bulk by this process.