Overview of isotope modelling in the context of the Nitrogen cycle

The nitrogen (N) cycle is a complex interplay of different processes including the mineralization of soil organic matter, uptake of N by plants, microbial N immobilization, and nitrification and denitrification. These key processes result in the formation of various N-containing species, some of which have detrimental environmental effects. Unfortunately, some processes yield the same molecules, for instance nitrous oxide, complicating clear source partitioning. Process-based biogeochemical models are increasingly being used to assess the fate of N species in the environment. However, to reflect the complexity of N cycling, these models often include numerous mathematical descriptions of processes that depend on pool sizes, reaction rate constants, soil temperature, soil moisture or oxygen concentration. It has remained a challenge to determine a sufficient set of these quantities in high temporal resolution or for a given specific measurement site, resulting in a scarcity of easily available validation variables compared to a surplus of modeled quantities.

The fundamental processes of isotopic fractionation and mixing result in the characteristic isotopic compositions of the various N species at natural abundance level. In this context, the natural abundance isotopic compositions can be used to determine relative contributions of different processes to – for instance – nitrous oxide production or as an integrating validation quantity, since the soil ¹⁵N enrichment reflects N loss pathways on a time scale of decades. Similarly, labelling studies using fertilizers enriched in ¹⁵N have been used to study the allocation of fertilizers to soil and plants or to determine N₂ emission. This is a significant, yet poorly understood component of N budgets and is extremely challenging to measure.

Thus, the aforementioned characteristics of the ¹⁵N isotopic composition make it a powerful tool for improving our understanding of the N cycle and for testing process-based biogeochemical models. In this presentation, we provide an overview of different model types used in the context of the N cycle, focusing specifically on isotope mixing models and process-based isotope models. We demonstrate how natural abundance ¹⁵N and ¹⁵N tracing studies can be employed to interpret measurement data, identify model weaknesses, refine models and reduce uncertainty of modeled N₂O emissions.