Ecosystem carbon storage in arctic tundra reduced by long-term nutrient fertilization

Abstract

Global warming is predicted to be most pronounced at high latitudes, and observational evidence over the past 25 years suggests that this warming is already under way. One-third of the global soil carbon pool is stored in northern latitudes, so there is considerable interest in understanding how the carbon balance of northern ecosystems will respond to climate warming. Observations of controls over plant productivity in tundra and boreal ecosystems have been used to build a conceptual model of response to warming, where warmer soils and increased decomposition of plant litter increase nutrient availability, which, in turn, stimulates plant production and increases ecosystem carbon storage. Here we present the results of a long-term fertilization experiment in Alaskan tundra, in which increased nutrient availability caused a net ecosystem loss of almost 2,000 grams of carbon per square meter over 20 years. We found that annual aboveground plant production doubled during the experiment. Losses of carbon and nitrogen from deep soil layers, however, were substantial and more than offset the increased carbon and nitrogen storage in plant biomass and litter. Our study suggests that projected release of soil nutrients associated with high-latitude warming may further amplify carbon release from soils, causing a net loss of ecosystem carbon and a positive feedback to climate warming.

Two Decades of Experimental Manipulations of Heaths and Forest Understory in the Subarctic

“…1a) soil C was unchanged following a decade of warming (but increased by fertilizer addition) ; Table 1). This contrasts with expectations of increased warming-induced losses of C to the atmosphere from tundra ecosystems (Mack et al 2004). While more CO 2 is emitted from the system in warmed plots (Illeris et al 2004), the soil C stock at least to 15 cm depth is unchanged .…”

“…While more CO 2 is emitted from the system in warmed plots (Illeris et al 2004), the soil C stock at least to 15 cm depth is unchanged . This suggests that a major part of the respired C is derived from recently fixed labile C from litter and root exudates (Grogan et al 2001), and another part possibly from deeper soil layers, emphasizing that gains in plant C pools must be compared to potential losses in soil pools in the entire soil horizon in order to estimate the long-term fate of C storage and release (Mack et al 2004).…”

“…The balance between uptake and release determines whether an ecosystem is a sink or source of CO 2 , and this C balance can be altered by environmental perturbations and climate change. In general, it is expected that climate change stimulates plant biomass production and photosynthetic uptake of CO 2 , but increases loss of C from soils (Mack et al 2004).…”

Nitrogen:phosphorous supply ratio and allometry in five alpine plant species

“…Usually, grasses become increasingly dominant under long‐term N addition due to their greater ability to take up nutrients relative to forbs (Chapin, Shaver, Giblin, Nadelhoffer, & Laundre, 1995; Mack, Schuur, Bret‐Harte, Shaver, & Chapin, 2004; Shaver et al., 2001). In our study, the three grasses exhibited significantly higher M A and M B , and greater leaf area, than forbs under all unbalanced N:P supply ratios (Figure 3a–c).…”

Storage and Turnover of Organic Matter in Soil

“…Changes in vegetation allocation strategy, litter quality, and soil microbes can lead to large C losses belowground that more than offset C gains associated with increased aboveground productivity (Mack et al 2004). Any benefits of N-deposition are expected to reach a saturation point, after which productivity levels off, and eventually diminishes due to other nutrient limitations or increased susceptibility to stresses such as pollution, frost damage, or disease (Agren and Bosatta 1988;Aber et al 1989).…”