title: Thirteen decades of foliar isotopes indicate declining nitrogen availability in central North American grasslands.
abstract: *Humans are increasing both the deposition of reactive nitrogen (N) and concentrations of atmospheric CO(2) on Earth, but the combined effects on terrestrial ecosystems are not clear. In the absence of historical records, it is difficult to know if N availability is currently increasing or decreasing on regional scales. *To determine the nature and timing of past changes in grassland ecosystem dynamics, we measured the composition of stable carbon (C) and N isotopes in leaf tissue from 545 herbarium specimens of 24 vascular plant species collected in Kansas, USA from 1876 to 2008. We also parameterized a simple model of the terrestrial N cycle coupled with a stable isotope simulator to constrain processes consistent with observed patterns. *A prolonged decline in foliar N concentrations began in 1926, while a prolonged decline in foliar delta(15)N values began in 1940. Changes in the difference between foliar and atmospheric C isotopes reveal slightly increased photosynthetic water use efficiency since 1876. *The declines in foliar N concentrations and foliar delta(15)N suggest declining N availability in these grasslands during the 20th century despite decades of anthropogenic N deposition. Our results are consistent with progressive-nitrogen-limitation-type hypotheses where declines in N availability are driven by increased ecosystem N storage as a result of increased atmospheric CO(2).
authors.affiliation: Department of Geography, Kansas State University, Manhattan, KS 66506, USA.
authors.forename: Kendra K
authors.initials: KK
authors.lastname: McLauchlan
authors.affiliation: Division of Biology, Kansas State University, Manhattan, KS 66506, USA.
authors.forename: Carolyn J
authors.initials: CJ
authors.lastname: Ferguson
authors.affiliation: Department of Geography, Kansas State University, Manhattan, KS 66506, USA.
authors.forename: Iris E
authors.initials: IE
authors.lastname: Wilson
authors.affiliation: Division of Biology, Kansas State University, Manhattan, KS 66506, USA.
authors.forename: Troy W
authors.initials: TW
authors.lastname: Ocheltree
authors.affiliation: Division of Biology, Kansas State University, Manhattan, KS 66506, USA.
authors.forename: Joseph M
authors.initials: JM
authors.lastname: Craine
mesh_terms: D001272:Atmosphere; D002247:Carbon Isotopes / Q000032:analysis*; D017753:Ecosystem; D006801:Humans; D007615:Kansas; D016014:Linear Models; D008954:Models, Biological; D009584:Nitrogen / Q000032:analysis*; D058458:Nitrogen Cycle; D009587:Nitrogen Isotopes / Q000032:analysis*; D010788:Photosynthesis; D018515:Plant Leaves / Q000737:chemistry*; D006109:Poaceae / Q000737:chemistry* / Q000502:physiology
chemical_list: D002247:Carbon Isotopes; D009587:Nitrogen Isotopes; D009584:Nitrogen