Global patterns of nitrogen limitation: confronting two global biogeochemical models with observations

• Published in: Global change biology 2013-10, Vol.19 (10), p.2986-2998
• Main Authors: Thomas, R. Quinn, Zaehle, Sönke, Templer, Pamela H., Goodale, Christine L.
• Format: Article
• Language: English
• Subjects: Ammonium Compounds; Animal and plant ecology; Animal, plant and microbial ecology; Biogeochemistry; Biological and medical sciences; Carbon Cycle; carbon cycling; Fertilizers; Fundamental and applied biological sciences. Psychology; General aspects; Models, Theoretical; Nitrates; Nitrogen; Nitrogen Cycle; nitrogen cycling; nitrogen limitation; Synecology; terrestrial biogeochemistry - climate feedbacks; Trees; Vegetation
• ISSN: 1354-1013; 1365-2486
• DOI: 10.1111/gcb.12281

Projections of future changes in land carbon (C) storage using biogeochemical models depend on accurately modeling the interactions between the C and nitrogen (N) cycles. Here, we present a framework for analyzing N limitation in global biogeochemical models to explore how C‐N interactions of current models compare to field observations, identify the processes causing model divergence, and identify future observation and experiment needs. We used a set of N‐fertilization simulations from two global biogeochemical models (CLM‐CN and O‐CN) that use different approaches to modeling C‐N interactions. On the global scale, net primary productivity (NPP) in the CLM‐CN model was substantially more responsive to N fertilization than in the O‐CN model. The most striking difference between the two models occurred for humid tropical forests, where the CLM‐CN simulated a 62% increase in NPP at high N addition levels (30 g N m−2 yr−1), while the O‐CN predicted a 2% decrease in NPP due to N fertilization increasing plant respiration more than photosynthesis. Across 35 temperate and boreal forest sites with field N‐fertilization experiments, we show that the CLM‐CN simulated a 46% increase in aboveground NPP in response to N, which exceeded the observed increase of 25%. In contrast, the O‐CN only simulated a 6% increase in aboveground NPP at the N‐fertilization sites. Despite the small response of NPP to N fertilization, the O‐CN model accurately simulated ecosystem retention of N and the fate of added N to vegetation when compared to empirical 15N tracer application studies. In contrast, the CLM‐CN predicted lower total ecosystem N retention and partitioned more losses to volatilization than estimated from observed N budgets of small catchments. These results point to the need for model improvements in both models in order to enhance the accuracy with which global C‐N cycle feedbacks are simulated.