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.