title: Nitrogen deposition and greenhouse gas emissions from grasslands: uncertainties and future directions.
abstract: Increases in atmospheric nitrogen deposition (Ndep) can strongly affect the greenhouse gas (GHG; CO2, CH4, and N2O) sink capacity of grasslands as well as other terrestrial ecosystems. Robust predictions of the net GHG sink strength of grasslands depend on how experimental N loads compare to projected Ndep rates, and how accurately the relationship between GHG fluxes and Ndep is characterized. A literature review revealed that the vast majority of experimental N loads were higher than levels these ecosystems are predicted to experience in the future. Using a process-based biogeochemical model, we predicted that low levels of Ndep either enhanced or reduced the net GHG sink strength of most grasslands, but as experimental N loads continued to increase, grasslands transitioned to a N saturation-decline stage, where the sensitivity of GHG exchange to further increases in Ndep declined. Most published studies represented treatments well into the N saturation-decline stage. Our model results predict that the responses of GHG fluxes to N are highly nonlinear and that the N saturation thresholds for GHGs varied greatly among grasslands and with fire management. We predict that during the 21st century some grasslands will be in the N limitation stage where others will transition into the N saturation-decline stage. The linear relationship between GHG sink strength and N load assumed by most studies can overestimate or underestimate predictions of the net GHG sink strength of grasslands depending on their N baseline status. The next generation of global change experiments should be designed at multiple N loads consistent with future Ndep rates to improve our empirical understanding and predictive ability.
authors.affiliation: Department of Plant Biology, University of Illinois at Urbana-Champaign, Urbana, IL, 61801, USA.|Institute of Genomic Biology, University of Illinois at Urbana-Champaign, Urbana, IL, 61801, USA.|Energy Biosciences Institute, University of Illinois at Urbana-Champaign, Urbana, IL, 61801, USA.
authors.forename: Nuria
authors.initials: N
authors.lastname: Gomez-Casanovas
authors.affiliation: Department of Forest, Rangeland, and Fire Sciences, University of Idaho, Moscow, ID, 83844, USA.
authors.forename: Tara W
authors.initials: TW
authors.lastname: Hudiburg
authors.affiliation: Department of Plant Biology, University of Illinois at Urbana-Champaign, Urbana, IL, 61801, USA.|Energy Biosciences Institute, University of Illinois at Urbana-Champaign, Urbana, IL, 61801, USA.|Global Change and Photosynthesis Research Unit, Agricultural Research Service, USDA, Urbana, IL, 61801, USA.
authors.forename: Carl J
authors.initials: CJ
authors.lastname: Bernacchi
authors.affiliation: National Renewable Ecology Laboratory, Colorado State University, Ft. Collins, CO, 805523, USA.
authors.forename: William J
authors.initials: WJ
authors.lastname: Parton
authors.affiliation: Department of Plant Biology, University of Illinois at Urbana-Champaign, Urbana, IL, 61801, USA.|Institute of Genomic Biology, University of Illinois at Urbana-Champaign, Urbana, IL, 61801, USA.|Energy Biosciences Institute, University of Illinois at Urbana-Champaign, Urbana, IL, 61801, USA.
authors.forename: Evan H
authors.initials: EH
authors.lastname: DeLucia
mesh_terms: D000393:Air Pollutants / Q000032:analysis*; D002245:Carbon Dioxide / Q000032:analysis*; D065948:Grassland; D008697:Methane / Q000032:analysis*; D008962:Models, Theoretical; D009584:Nitrogen / Q000032:analysis*; D009609:Nitrous Oxide / Q000032:analysis*; D035501:Uncertainty
chemical_list: D000393:Air Pollutants; D002245:Carbon Dioxide; D009609:Nitrous Oxide; D009584:Nitrogen; D008697:Methane
keywords: CH
  4; N2O; grassland; methane; net ecosystem CO2 exchange; net ecosystem productivity; nitrogen deposition; nitrogen fertilization; nitrous oxide; uncertainty