title: Anthropogenic nitrogen deposition enhances carbon sequestration in boreal soils.
abstract: It is proposed that carbon (C) sequestration in response to reactive nitrogen (Nr ) deposition in boreal forests accounts for a large portion of the terrestrial sink for anthropogenic CO2 emissions. While studies have helped clarify the magnitude by which Nr deposition enhances C sequestration by forest vegetation, there remains a paucity of long-term experimental studies evaluating how soil C pools respond. We conducted a long-term experiment, maintained since 1996, consisting of three N addition levels (0, 12.5, and 50 kg N ha(-1) yr(-1) ) in the boreal zone of northern Sweden to understand how atmospheric Nr deposition affects soil C accumulation, soil microbial communities, and soil respiration. We hypothesized that soil C sequestration will increase, and soil microbial biomass and soil respiration will decrease, with disproportionately large changes expected compared to low levels of N addition. Our data showed that the low N addition treatment caused a non-significant increase in the organic horizon C pool of ~15% and a significant increase of ~30% in response to the high N treatment relative to the control. The relationship between C sequestration and N addition in the organic horizon was linear, with a slope of 10 kg C kg(-1) N. We also found a concomitant decrease in total microbial and fungal biomasses and a ~11% reduction in soil respiration in response to the high N treatment. Our data complement previous data from the same study system describing aboveground C sequestration, indicating a total ecosystem sequestration rate of 26 kg C kg(-1) N. These estimates are far lower than suggested by some previous modeling studies, and thus will help improve and validate current modeling efforts aimed at separating the effect of multiple global change factors on the C balance of the boreal region.
authors.affiliation: Department of Forest Ecology and Management, Swedish University of Agricultural Sciences (SLU), Umeå, SE-901, Sweden.
authors.forename: Nadia I
authors.initials: NI
authors.lastname: Maaroufi
authors.affiliation: Umeå Plant Science Center (UPSC), Department of Forest Genetics and Plant Physiology, Swedish University of Agricultural Sciences, Umeå, SE-901 83, Sweden.
authors.forename: Annika
authors.initials: A
authors.lastname: Nordin
authors.affiliation: Department of Forest Ecology and Management, Swedish University of Agricultural Sciences (SLU), Umeå, SE-901, Sweden.
authors.forename: Niles J
authors.initials: NJ
authors.lastname: Hasselquist
authors.affiliation: Department of Ecology and Environmental Science (EMG), Umeå University, Umeå, SE-901 87, Sweden.
authors.forename: Lisbet H
authors.initials: LH
authors.lastname: Bach
authors.affiliation: Department of Ecology and Environmental Science (EMG), Umeå University, Umeå, SE-901 87, Sweden.
authors.forename: Kristin
authors.initials: K
authors.lastname: Palmqvist
authors.affiliation: Department of Forest Ecology and Management, Swedish University of Agricultural Sciences (SLU), Umeå, SE-901, Sweden.
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Gundale
mesh_terms: D002244:Carbon / Q000032:analysis*; D057965:Carbon Sequestration; D017753:Ecosystem; D009584:Nitrogen / Q000032:analysis*; D012987:Soil / Q000737:chemistry; D012988:Soil Microbiology; D013548:Sweden; D066188:Taiga
chemical_list: D012987:Soil; D002244:Carbon; D009584:Nitrogen
keywords: boreal forest; boreal soil; carbon sequestration; carbon sink; nitrogen deposition; soil C pool; soil respiration