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.

The Impact of Moss Species and Biomass on the Growth of Pinus sylvestris Tree Seedlings at Different Precipitation Frequencies

“…The values we used in the 80% moss biomass classes (500 g/m 2 ) were comparable to moss biomass values of P. schreberi and H. splendens measured in 140 y-old uncut forest stands in Eastern Finland, while moss biomass during the first seven years after clear-cutting was about 50% of this value [18]. It is also known that mosses can be strongly affected by global change factors, such as atmospheric N deposition [44,58], climatic warming and changes in precipitation regimes [2,26]. As we have shown in this study, changes in precipitation can drastically affect moss growth and the amount of moss biomass present in the forest.…”

“…This was done by placing a resin capsule (PST1 capsule, Unibest Bozeman, Bozeman, MT, USA) containing approximately 1 g of mixed bed ionic resins in the humus layer of each pot at the beginning of the experiment [44]. These capsules were removed after 18 weeks and later extracted with three separate extractions of 10 mL 1 M KCl, and run for NH 4 + , NO 3 − and PO 4 3− on an Autoanalyzer 3 (SEAL Analytical).…”

Representing sub-grid scale variations in nitrogen deposition associated with land use in a global Earth System Model: implications for present and future nitrogen deposition fluxes over North America

“…Once in the atmosphere, N can be transported over long distances before it is removed by dry and wet deposition, providing greater N inputs to otherwise pristine regions (e.g., national parks, boreal forests) (Paulot et al, 2014;Malm et al, 2016). Since N can be a limiting nutrient, the increase in N deposition may promote ecosystem productivity, (Townsend et al, 1996;Magnani et al, 2007;Pregitzer et al, 2008;Reay et al, 2008;Dezi et al, 2010;Wårlind et al, 2014;Devaraju 30 et al, 2015) especially in boreal regions (Högberg, 2012;Gundale et al, 2014;Fleischer et al, 2015).…”

Genotypic variability in Populus tremula L. affects how anthropogenic nitrogen enrichment influences litter decomposition

“…Scandinavia) increasingly apply fertilizers to forests (Lindkvist et al 2011). These two types of anthropogenic N inputs can have numerous consequences in N-limited boreal forests, such as increase net primary productivity and aboveground carbon (C) stocks (De Vries et al 2006;Gundale et al 2014), alter species composition and community structure (Bobbink et al 1998 and references therein; Nordin et al 1998;Strengbom et al 2003;Nordin et al 2006), and reduce diversity (Nordin et al 2005;Bobbink et al 2010;Meunier et al 2016). While impacts of N enrichment on aboveground properties are well studied, relatively little is known about how soil processes respond to N enrichment (but see e.g.…”