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

• Published in: Global Change Biology
• Main Authors: Gundale, Michael J., From, Fredrik, Bach, Lisbet H., Nordin, Annika
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2013
• Subjects: Northern Sweden
• Online Access: http://dx.doi.org/10.1111/gcb.12422; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fgcb.12422; https://onlinelibrary.wiley.com/doi/pdf/10.1111/gcb.12422

Abstract It is proposed that increases in anthropogenic reactive nitrogen (N r ) 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 N r 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 N r deposition may cause C sequestration rates to be slightly different than this estimate, our data suggest that a minor quantity of annual anthropogenic CO 2 emissions are sequestered into boreal forests as a result of N r deposition.