Application of a triple 15N tracing technique to elucidate N transformations in a UK grassland soil

•Heterotrophic nitrification is an important source of N2O at low WFPS.•Similar contributions of nitrification, heterotrophic nitrification and denitrification occurred at medium WFPS level.•High N2O emissions result via nitrification coupled denitrification at medium & high WFPS.•Abiotic factors need to be considered as well as biotic factors to understand the cause of emissions.