Using conceptual theory and observations to evaluate mechanisms of N limitation in Earth system models
Nitrogen (N) limitation of the terrestrial carbon (C) cycle is widespread across the globe. Consequently, most Earth system models now include representations of C-N interactions to improve predictions of carbon uptake and climate change. These global models with C-N cycles have shown that both the terrestrial C sink and climate are sensitive to the availability of N, but differ markedly in their predicted sensitivity. In order to evaluate and improve model predictions of N limitation, new datasets and conceptual approaches are needed. Here, I discuss how a range of data sources can be used to evaluate multiple aspects of the coupled C and N cycles in Earth system models. I also discuss how, in light of a paucity of global data on C-N interactions, conceptual theory of N limitation can be valuable for evaluation.
Using simulations from multiple Earth system models, I demonstrate that N addition experiments, 15N tracer studies, small watershed scale N input-output budgets, and forest responses to N deposition gradients can all be used to evaluate different processes in coupled C-N interactions. For example, comparing the output of two models to N addition experiments and forest C responses across N deposition gradients helped simultaneously evaluate N retention capacity and non-N limited productivity. Finally, I show that Earth system models have used a range of approaches to represent the conceptual theory of N limitation. Models particularly differ in how they simulate N losses that are unable to be controlled by plants, a strong driver of N limitation, and in how they simulate N fixation. In conclusion, the high spatial and temporal variability of the N cycle and the lack of globally integrative datasets for Earth system model evaluation necessitates the use of multiple data sources and conceptual theory to describe the quality of coupled C-N cycle predictions.