title: Anthropogenic nitrogen deposition in boreal forests has a minor impact on the global carbon cycle.
abstract: It is proposed that increases in anthropogenic reactive nitrogen (Nr ) deposition may cause temperate and boreal forests to sequester a globally significant quantity of carbon (C); however, long-term data from boreal forests describing how C sequestration responds to realistic levels of chronic Nr deposition are scarce. Using a long-term (14-year) stand-scale (0.1 ha) N addition experiment (three levels: 0, 12.5, and 50 kg N ha(-1)  yr(-1) ) in the boreal zone of northern Sweden, we evaluated how chronic N additions altered N uptake and biomass of understory communities, and whether changes in understory communities explained N uptake and C sequestration by trees. We hypothesized that understory communities (i.e. mosses and shrubs) serve as important sinks for low-level N additions, with the strength of these sinks weakening as chronic N addition rates increase, due to shifts in species composition. We further hypothesized that trees would exhibit nonlinear increases in N acquisition, and subsequent C sequestration as N addition rates increased, due to a weakening understory N sink. Our data showed that understory biomass was reduced by 50% in response to the high N addition treatment, mainly due to reduced moss biomass. A (15) N labeling experiment showed that feather mosses acquired the largest fraction of applied label, with this fraction decreasing as the chronic N addition level increased. Contrary to our hypothesis, the proportion of label taken up by trees was equal (ca. 8%) across all three N addition treatments. The relationship between N addition and C sequestration in all vegetation pools combined was linear, and had a slope of 16 kg C kg(-1)  N. While canopy retention of Nr deposition may cause C sequestration rates to be slightly different than this estimate, our data suggest that a minor quantity of annual anthropogenic CO2 emissions are sequestered into boreal forests as a result of Nr deposition.
authors.affiliation: Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, SE-901 83, Umeå, Sweden.
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Gundale
authors.forename: Fredrik
authors.initials: F
authors.lastname: From
authors.forename: Lisbet H
authors.initials: LH
authors.lastname: Bach
authors.forename: Annika
authors.initials: A
authors.lastname: Nordin
mesh_terms: D018533:Biomass; D044002:Bryophyta / Q000254:growth & development / Q000378:metabolism; D057965:Carbon Sequestration; D006802:Human Activities; D009584:Nitrogen / Q000378:metabolism*; D028222:Picea / Q000254:growth & development / Q000378:metabolism; D006109:Poaceae / Q000254:growth & development / Q000378:metabolism; D012987:Soil / Q000737:chemistry; D013548:Sweden; D014197:Trees / Q000254:growth & development / Q000378:metabolism*; D029800:Vaccinium myrtillus / Q000254:growth & development / Q000378:metabolism
chemical_list: D012987:Soil; D009584:Nitrogen
keywords: 15N labeling; Nitrogen deposition; carbon sequestration; carbon sink; dwarf shrubs; feather mosses; productivity; soil nitrogen uptake; understory vegetation