title: Global patterns and substrate-based mechanisms of the terrestrial nitrogen cycle.
abstract: Nitrogen (N) deposition is impacting the services that ecosystems provide to humanity. However, the mechanisms determining impacts on the N cycle are not fully understood. To explore the mechanistic underpinnings of N impacts on N cycle processes, we reviewed and synthesised recent progress in ecosystem N research through empirical studies, conceptual analysis and model simulations. Experimental and observational studies have revealed that the stimulation of plant N uptake and soil retention generally diminishes as N loading increases, while dissolved and gaseous losses of N occur at low N availability but increase exponentially and become the dominant fate of N at high loading rates. The original N saturation hypothesis emphasises sequential N saturation from plant uptake to soil retention before N losses occur. However, biogeochemical models that simulate simultaneous competition for soil N substrates by multiple processes match the observed patterns of N losses better than models based on sequential competition. To enable better prediction of terrestrial N cycle responses to N loading, we recommend that future research identifies the response functions of different N processes to substrate availability using manipulative experiments, and incorporates the measured N saturation response functions into conceptual, theoretical and quantitative analyses.
authors.affiliation: Key Laboratory of Ecosystem Network Observation and Modeling, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing, China.
authors.forename: Shuli
authors.initials: S
authors.lastname: Niu
authors.affiliation: The Natural History Museum of Denmark, University of Copenhagen, Universitetsparken 15, 2100, København Ø, Denmark.
authors.forename: Aimée T
authors.initials: AT
authors.lastname: Classen
authors.affiliation: Department of Forestry and Natural Resources, Department of Biological Sciences, Purdue Climate Change Research Center, Purdue University, West Lafayette, IN, USA.
authors.forename: Jeffrey S
authors.initials: JS
authors.lastname: Dukes
authors.affiliation: Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, S 90-183, Umeå, Sweden.
authors.forename: Paul
authors.initials: P
authors.lastname: Kardol
authors.affiliation: State Key Laboratory of Vegetation and Environment Change, Institute of Botany, Chinese Academy of Sciences, Beijing, China.
authors.forename: Lingli
authors.initials: L
authors.lastname: Liu
authors.affiliation: Department of Microbiology and Plant Biology, University of Oklahoma, Norman, OK, USA.
authors.forename: Yiqi
authors.initials: Y
authors.lastname: Luo
authors.affiliation: U.S. Forest Service, Northern Research Station, Durham, NH, USA.
authors.forename: Lindsey
authors.initials: L
authors.lastname: Rustad
authors.affiliation: Key Laboratory of Ecosystem Network Observation and Modeling, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing, China.
authors.forename: Jian
authors.initials: J
authors.lastname: Sun
authors.affiliation: The Ecosystem Center, Marine Biological Laboratory, Woods Hole, MA, USA.
authors.forename: Jianwu
authors.initials: J
authors.lastname: Tang
authors.affiliation: Department of Biology, Boston University, Boston, MA, USA.
authors.forename: Pamela H
authors.initials: PH
authors.lastname: Templer
authors.affiliation: Department of Forest Resources & Environmental Conservation, Virginia Tech, Blacksburg, VA, USA.
authors.forename: R Quinn
authors.initials: RQ
authors.lastname: Thomas
authors.affiliation: Key Laboratory of Ecosystem Network Observation and Modeling, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing, China.
authors.forename: Dashuan
authors.initials: D
authors.lastname: Tian
authors.affiliation: Research Group of Plant and Vegetation Ecology, Department of Biology, University of Antwerp, B-2610, Wilrijk, Belgium.
authors.forename: Sara
authors.initials: S
authors.lastname: Vicca
authors.affiliation: CSIRO Ocean and Atmosphere, PMB 1, Aspendale, Victoria, 3195, Australia.
authors.forename: Ying-Ping
authors.initials: YP
authors.lastname: Wang
authors.affiliation: Tiantong National Station of Forest Ecosystem &School of Ecological and Enivronmental Science, East China Normal University, Shanghai, 200062, China.|Research Center for Global Change and Ecological Forecasting, East China Normal University, Shanghai, 200062, China.
authors.forename: Jianyang
authors.initials: J
authors.lastname: Xia
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: D017753:Ecosystem; D008962:Models, Theoretical; D009584:Nitrogen / Q000032:analysis*; D058458:Nitrogen Cycle; D018521:Plant Physiological Phenomena; D010944:Plants / Q000378:metabolism; D012987:Soil / Q000737:chemistry*; D012988:Soil Microbiology
chemical_list: D012987:Soil; D009584:Nitrogen
keywords: Leaching; mineralisation; nitrification; nitrogen deposition; nitrogen loss; plant N uptake; saturation; soil N retention; threshold