title: The role of stoichiometric flexibility in modelling forest ecosystem responses to nitrogen fertilization.
abstract: The response of the forest carbon (C) balance to changes in nitrogen (N) deposition is uncertain, partly owing to diverging representations of N cycle processes in dynamic global vegetation models (DGVMs). Here, we examined how different assumptions about the degree of flexibility of the ecosystem's C : N ratios contribute to this uncertainty, and which of these assumptions best correspond to the available data. We applied these assumptions within the framework of a DGVM and compared the results to responses in net primary productivity (NPP), leaf N concentration, and ecosystem N partitioning, observed at 22 forest N fertilization experiments. Employing flexible ecosystem pool C : N ratios generally resulted in the most convincing model-data agreement with respect to production and foliar N responses. An intermediate degree of stoichiometric flexibility in vegetation, where wood C : N ratio changes were decoupled from leaf and root C : N ratio changes, led to consistent simulation of production and N cycle responses to N addition. Assuming fixed C : N ratios or scaling leaf N concentration changes to other tissues, commonly assumed by DGVMs, was not supported by reported data. Between the tested assumptions, the simulated changes in ecosystem C storage relative to changes in C assimilation varied by up to 20%.
authors.affiliation: Biogeochemical Integration Department, Max Planck Institute for Biogeochemistry, Hans-Knöll-Str. 10, D-07745, Jena, Germany.|International Max Planck Research School (IMPRS) for Global Biogeochemical Cycles, Hans-Knöll-Str. 10, D-07745, Jena, Germany.
authors.forename: Johannes
authors.initials: J
authors.lastname: Meyerholt
authors.affiliation: Biogeochemical Integration Department, Max Planck Institute for Biogeochemistry, Hans-Knöll-Str. 10, D-07745, Jena, Germany.
authors.forename: Sönke
authors.initials: S
authors.lastname: Zaehle
mesh_terms: D002244:Carbon / Q000378:metabolism*; D057486:Carbon Cycle; D005308:Fertilizers; D065928:Forests; D008954:Models, Biological; D009584:Nitrogen / Q000378:metabolism*; D058458:Nitrogen Cycle; D018515:Plant Leaves / Q000378:metabolism; D018517:Plant Roots / Q000378:metabolism; D012987:Soil / Q000737:chemistry; D014197:Trees / Q000378:metabolism*; D014934:Wood / Q000378:metabolism
chemical_list: D005308:Fertilizers; D012987:Soil; D002244:Carbon; D009584:Nitrogen
keywords: carbon-nitrogen cycle coupling; ecosystem modelling; fertilization; forest carbon balance; nitrogen cycle; plant stoichiometry