Fire-derived organic matter retains ammonia through covalent bond formation.

Fire-derived organic matter, often referred to as pyrogenic organic matter (PyOM), is present in the Earth's soil, sediment, atmosphere, and water. We investigated interactions of 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 sixfold, 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 contained heterocyclic structures. We estimate that through these mechanisms soil PyOM stocks could retain more than 600-fold annual NH3 emissions from agriculture, exerting an important control on global N cycling.