Fire-derived organic matter is present in the Earth’s soil, sediment, atmosphere, and water. We investigated interactions of pyrogenic organic matter (PyOM) with ammonia (NH3) gas, which makes up much of the Earth’s reactive nitrogen (N) pool. Here we show that PyOM’s NH3 retention capacity under ambient conditions can exceed 180 mg N g-1 PyOM carbon, resulting in a material with a higher N content than any unprocessed plant material and most animal manures. As PyOM is weathered, NH3 retention increases 4-fold, with more than half of the N retained through chemisorption rather than physisorption. Near-edge X-ray absorption fine structure and nuclear magnetic resonance spectroscopy reveal that a variety of covalent bonds form between NH3-N and PyOM, more than 10% of which may be contained in heterocyclic structures. We estimate that through these mechanisms soil PyOM stocks could retain more than 600-fold annual NH3 emissions from agriculture, and conclude that PyOM could exert an important and unaccounted-for control on global N cycling. This dataset supports the findings of this study.