title: Global patterns of nitrogen limitation: confronting two global biogeochemical models with observations.
abstract: 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 (15) N 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.
authors.affiliation: Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, NY, USA; National Center for Atmospheric Research, Boulder, CO, USA.
authors.forename: R Quinn
authors.initials: RQ
authors.lastname: Thomas
authors.forename: Sönke
authors.initials: S
authors.lastname: Zaehle
authors.forename: Pamela H
authors.initials: PH
authors.lastname: Templer
authors.forename: Christine L
authors.initials: CL
authors.lastname: Goodale
mesh_terms: D064751:Ammonium Compounds; D057486:Carbon Cycle; D005308:Fertilizers; D008962:Models, Theoretical; D009566:Nitrates; D009584:Nitrogen; D058458:Nitrogen Cycle; D014197:Trees
chemical_list: D064751:Ammonium Compounds; D005308:Fertilizers; D009566:Nitrates; D009584:Nitrogen
keywords: carbon cycling; nitrogen cycling; nitrogen limitation; terrestrial biogeochemistry - climate feedbacks