Plants and mycorrhizal symbionts acquire substantial soil nitrogen from gaseous ammonia transport

Summary Nitrogen (N) is an essential nutrient that limits plant growth in many ecosystems. Here we investigate an overlooked component of the terrestrial N cycle – subsurface ammonia (NH 3 ) gas transport and its contribution to plant and mycorrhizal N acquisition. We used controlled mesocosms, soil incubations, stable isotopes, and imaging to investigate edaphic drivers of NH 3 gas efflux, track lateral subsurface N transport originating from 15 NH 3 gas or 15 N‐enriched organic matter, and assess plant and mycorrhizal N assimilation from this gaseous transport pathway. NH 3 is released from soil organic matter, travels belowground, and contributes to root and fungal N content. Abiotic soil properties (pH and texture) influence the quantity of NH 3 available for subsurface transport. Mutualisms with arbuscular mycorrhizal (AM) fungi can substantially increase plant NH 3 ‐N uptake. The grass Brachypodium distachyon acquired 6–9% of total plant N from organic matter‐N that traveled as a gas belowground. Colonization by the AM fungus Rhizophagus irregularis was associated with a two‐fold increase in total plant N acquisition from subsurface NH 3 gas. NH 3 gas transport and uptake pathways may be fundamentally different from those of more commonly studied soil N species and warrant further research.