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Anthropogenic nitrogen deposition in boreal forests has a minor impact on the global carbon cycle

Authors

  • Michael J. Gundale,

    Corresponding author
    1. Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, Umeå, Sweden
    • Correspondence: Michael J. Gundale, tel: +46 90 786 84 27, fax: +46 90 786 81 63 e-mail: Michael.Gundale@slu.se

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  • Fredrik From,

    1. Umeå Plant Science Centre, Department of Forest Genetics and Plant Physiology, Swedish University of Agricultural Sciences, Umeå, Sweden
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  • Lisbet H. Bach,

    1. Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, Umeå, Sweden
    2. Department of Ecology and Environmental Science, Umeå University, Umeå, Sweden
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  • Annika Nordin

    1. Umeå Plant Science Centre, Department of Forest Genetics and Plant Physiology, Swedish University of Agricultural Sciences, Umeå, Sweden
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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 15N 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.

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