title: Plant community change mediates the response of foliar δ(15)N to CO 2 enrichment in mesic grasslands.
abstract: Rising atmospheric CO2 concentration may change the isotopic signature of plant N by altering plant and microbial processes involved in the N cycle. CO2 may increase leaf δ(15)N by increasing plant community productivity, C input to soil, and, ultimately, microbial mineralization of old, (15)N-enriched organic matter. We predicted that CO2 would increase aboveground productivity (ANPP; g biomass m(-2)) and foliar δ(15)N values of two grassland communities in Texas, USA: (1) a pasture dominated by a C4 exotic grass, and (2) assemblages of tallgrass prairie species, the latter grown on clay, sandy loam, and silty clay soils. Grasslands were exposed in separate experiments to a pre-industrial to elevated CO2 gradient for 4 years. CO2 stimulated ANPP of pasture and of prairie assemblages on each of the three soils, but increased leaf δ(15)N only for prairie plants on a silty clay. δ(15)N increased linearly as mineral-associated soil C declined on the silty clay. Mineral-associated C declined as ANPP increased. Structural equation modeling indicted that CO2 increased ANPP partly by favoring a tallgrass (Sorghastrum nutans) over a mid-grass species (Bouteloua curtipendula). CO2 may have increased foliar δ(15)N on the silty clay by reducing fractionation during N uptake and assimilation. However, we interpret the soil-specific, δ(15)N-CO2 response as resulting from increased ANPP that stimulated mineralization from recalcitrant organic matter. By contrast, CO2 favored a forb species (Solanum dimidiatum) with higher δ(15)N than the dominant grass (Bothriochloa ischaemum) in pasture. CO2 enrichment changed grassland δ(15)N by shifting species relative abundances.
authors.affiliation: Grassland, Soil and Water Research Laboratory, USDA-Agricultural Research Service, Temple, TX, 76502, USA, wayne.polley@ars.usda.gov.
authors.forename: H Wayne
authors.initials: HW
authors.lastname: Polley
authors.forename: Justin D
authors.initials: JD
authors.lastname: Derner
authors.forename: Robert B
authors.initials: RB
authors.lastname: Jackson
authors.forename: Richard A
authors.initials: RA
authors.lastname: Gill
authors.forename: Andrew C
authors.initials: AC
authors.lastname: Procter
authors.forename: Philip A
authors.initials: PA
authors.lastname: Fay
mesh_terms: D044822:Biodiversity; D018533:Biomass; D002244:Carbon / Q000378:metabolism; D002245:Carbon Dioxide / Q000737:chemistry*; D065948:Grassland; D009584:Nitrogen / Q000378:metabolism*; D058458:Nitrogen Cycle; D009587:Nitrogen Isotopes / Q000378:metabolism; D018515:Plant Leaves / Q000378:metabolism*; D010944:Plants / Q000378:metabolism*; D006109:Poaceae / Q000254:growth & development / Q000378:metabolism; D012987:Soil / Q000737:chemistry*; D012988:Soil Microbiology; D032322:Solanum / Q000254:growth & development / Q000378:metabolism; D013781:Texas
chemical_list: D009587:Nitrogen Isotopes; D012987:Soil; D002245:Carbon Dioxide; D002244:Carbon; D009584:Nitrogen