title: Elevated CO2 promotes long-term nitrogen accumulation only in combination with nitrogen addition.
abstract: Biogeochemical models that incorporate nitrogen (N) limitation indicate that N availability will control the magnitude of ecosystem carbon uptake in response to rising CO2 . Some models, however, suggest that elevated CO2 may promote ecosystem N accumulation, a feedback that in the long term could circumvent N limitation of the CO2 response while mitigating N pollution. We tested this prediction using a nine-year CO2 xN experiment in a tidal marsh. Although the effects of CO2 are similar between uplands and wetlands in many respects, this experiment offers a greater likelihood of detecting CO2 effects on N retention on a decadal timescale because tidal marshes have a relatively open N cycle and can accrue soil organic matter rapidly. To determine how elevated CO2 affects N dynamics, we assessed the three primary fates of N in a tidal marsh: (1) retention in plants and soil, (2) denitrification to the atmosphere, and (3) tidal export. We assessed changes in N pools and tracked the fate of a (15) N tracer added to each plot in 2006 to quantify the fraction of added N retained in vegetation and soil, and to estimate lateral N movement. Elevated CO2 alone did not increase plant N mass, soil N mass, or (15) N label retention. Unexpectedly, CO2 and N interacted such that the combined N+CO2 treatment increased ecosystem N accumulation despite the stimulation in N losses indicated by reduced (15) N label retention. These findings suggest that in N-limited ecosystems, elevated CO2 is unlikely to increase long-term N accumulation and circumvent progressive N limitation without additional N inputs, which may relieve plant-microbe competition and allow for increased plant N uptake.
authors.affiliation: Department of Biology, Villanova University, 800 Lancaster Avenue, Villanova, PA, 19085, USA.
authors.forename: Melissa A
authors.initials: MA
authors.lastname: Pastore
authors.affiliation: Smithsonian Environmental Research Center, 647 Contees Wharf Road, Edgewater, MD, 21037, USA.
authors.forename: J Patrick
authors.initials: JP
authors.lastname: Megonigal
authors.affiliation: Department of Biology, Villanova University, 800 Lancaster Avenue, Villanova, PA, 19085, USA.|Smithsonian Environmental Research Center, 647 Contees Wharf Road, Edgewater, MD, 21037, USA.
authors.forename: J Adam
authors.initials: JA
authors.lastname: Langley
mesh_terms: D001272:Atmosphere / Q000737:chemistry; D002245:Carbon Dioxide / Q000378:metabolism*; D017753:Ecosystem; D008396:Maryland; D009584:Nitrogen / Q000378:metabolism*; D058458:Nitrogen Cycle; D009587:Nitrogen Isotopes; D010944:Plants / Q000378:metabolism*; D012987:Soil / Q000737:chemistry; D055871:Tidal Waves; D053833:Wetlands
chemical_list: D009587:Nitrogen Isotopes; D012987:Soil; D002245:Carbon Dioxide; D009584:Nitrogen
keywords: CO2 enrichment; brackish marsh; denitrification; isotopic biogeochemistry; nitrogen pollution; nitrogen retention and loss