The nitrogen cycle during the Anthropocene

In the last century, the strong increase in anthropogenic emissions and agricultural activities brought about a tripling in atmospheric nitrogen deposition (AND) and a doubling in river input of nutrients (RIN) rates to the ocean. There is growing evidence for a strong fingerprint of increased AND and RIN on aquatic systems. The impact of AND on ocean biogeochemistry, and measurable tracers have been simulated in global models (e.g. Yang et al. submitted) but not in the realm of internal variability and recent climate change. Additionally, impact of RIN have only been assessed in localized sites such as estuaries and lakes, but its impact on global ocean biogeochemistry remains mostly unstudied. In this study, we address these gaps by running a suite of forced historical experiments or 'hindcasts' with the ocean-ice components of the global earth system model CESM from 1948 to 2009. Forcing fields include transient heat fluxes, freshwater fluxes, momentum fluxes as well as transient nitrogen deposition and transient river input of nutrients to the ocean. We investigate detectability of changes in N* and δ15N with respect to background variability as well as the nitrogen cycle response to the anthropogenic perturbation.