Heterotrophic nitrification of organic N and its contribution to nitrous oxide emissions in soils

• N2O production processes and the related N pools in soils are summarized.
• Heterotrophic nitrification of organic N contributes to N2O emission from soils.
• Amendment of the HIP model by an organic N pathway is proposed.

Nitrous oxide (N2O) is one of the key trace gases playing an important role in global climate change. Soils are the most important source of global N2O. A large number of studies have been conducted to quantify soil-based N2O emissions, including processes of N2O production, microbial mechanisms of N2O production, and the prediction of N2O emissions via various modeling approaches on various spatial scales. However, a considerable uncertainty still exists regarding the spatial and temporal variability of N2O emissions in natural and managed habitats. In this review, we summarize the nitrogen (N) pools related with N2O production and the methods quantifying the gross heterotrophic nitrification of organic N and its contribution to N2O emissions in soils, with the aim to derive a simplified conceptual approach for N2O emissions. We show that with current stable isotopes techniques such a quantification is possible and can propose more information to understand N2O emissions from a wide range of soil and ecosystems. The gross heterotrophic nitrification of organic N rate may be generally significant in acidic forest soils with high C/N ratio. However, the contribution of heterotrophic nitrification of organic N process to total N2O emissions seems to be dependent on soil pH, C/N ratio and land use. Therefore, we propose introducing N2O production via heterotrophic nitrification of organic N into the conceptual “hole-in-the-pipe” (HIP) model of N2O emission, originally developed by Firestone and Davidson (1989).