Carbon-nitrogen interactions on land at global scales: current understanding in modelling climate biosphere feedbacks

Interactions between the terrestrial carbon (C) and nitrogen (N) cycles shape the response of ecosystems to global change. The limitation of ecosystem C storage due to N availability, and the response of N(2)O emissions to environmental conditions and N addition have been intensively studied at the site level. However, their contribution to biosphere-climate interactions at regional to global scales remains unclear. A growing number of global terrestrial biogeochemical models provide a means to scale ecological understanding of the nitrogen cycle to regional and global scales with the ultimate aim to investigate the magnitude of nitrogen cycling effects on global biogeochemistry, as well as their indirect consequences for biogeophysical land-atmosphere interactions. Key challenges to modelling the coupled terrestrial carbon-nitrogen cycles arise from the need to account for micro-scale processes to represent and quantify important N fluxes, uncertainties in the representation of key carbon-nitrogen cycle couplings at the ecosystem scale, and vagaries in the available observations to constrain global models. The new generation of carbon-nitrogen cycle models suggests that reactive nitrogen deposition is associated with a moderate increase in current terrestrial carbon sequestration, providing a small climate cooling effect. The models further unanimously demonstrate that nitrogen cycling reduces both global carbon sequestration due to CO(2) fertilisation and the carbon losses associated with climate change on land, in sum leading to an acceleration of carbon accumulation in the atmosphere relative to C-cycle only models. A recent study furthermore suggests a moderate positive interaction between terrestrial N(2)O emissions and recent climatic changes, although the atmospheric increase in N(2)O over the last few decades appears to be mostly associated with anthropogenic Nr additions to the terrestrial biosphere. At least some of these models can be used to assess the biogeophysical effects of N cycling through altered albedo and changed sensible and latent heat fluxes, but no study so far has assessed these consequences explicitly