Abstract
Nutrient availability in the arctic is expected to increase in the next century due to accelerated decomposition associated with warming and, to a lesser extent, increased nitrogen deposition. To explore how changes in nutrient availability affect ecosystem carbon (C) cycling, we used radiocarbon to quantify changes in belowground C dynamics associated with long-term fertilization of graminoid-dominated tussock tundra at Toolik Lake, Alaska. Since 1981, yearly fertilization with nitrogen (N) and phosphorus (P) has resulted in a shift to shrub-dominated vegetation. These combined changes have altered the quantity and quality of litter inputs, the vertical distribution and dynamics of fine roots, and the decomposition rate of soil organic C. The loss of C from the deep organic and mineral soil has more than offset the C accumulation in the litter and upper organic soil horizons. In the litter and upper organic horizons, radiocarbon measurements show that increased inputs resulted in overall C accumulation, despite being offset by increased decomposition in some soil pools. To reconcile radiocarbon observations in the deeper organic and mineral soil layers, where most of the ecosystem C loss occurred, both a decrease in input of new root material and a dramatic increase of decomposition rates in centuries-old soil C pools were required. Therefore, with future increases in nutrient availability, we may expect substantial losses of C which took centuries to accumulate.
Highlights
Artic tundra soils hold at least 5–6% of the world‘s soil carbon, recent estimates suggest that this amount is at least six times higher (IPCC 2001; Horwath 2007)
It is expected to affect soil C storage both directly, through temperature responses in microbial decomposition, and indirectly, through feedbacks associated with nutrient availability, as well as, changing surface energy balance and plant species composition (Chapin and others 1995; Hobbie and Chapin 1998; Dormann and Woodin 2002; Weintraub and Schimel 2005; Van Wijk and others 2004)
Many of these feedbacks are positive, such as the observed decreases in albedo (Chapin and others 2005) and increases in snow depth during the winter months (Sturm and others 2001, 2005), which lead to further increases in air and/or soil temperatures. Another important set of feedbacks is related to the faster decomposition of organic matter in warmer soils leading to increased nutrient availability2 y)1), higher productivity, and changes in plant community composition (Chapin and others 1995; Mack and others 2004)
Summary
Artic tundra soils hold at least 5–6% of the world‘s soil carbon, recent estimates suggest that this amount is at least six times higher (IPCC 2001; Horwath 2007). Arctic tundra soils are warming rapidly (Overpeck and others 1997; ACIA 2004) As this warming continues, it is expected to affect soil C storage both directly, through temperature responses in microbial decomposition, and indirectly, through feedbacks associated with nutrient availability, as well as, changing surface energy balance and plant species composition (Chapin and others 1995; Hobbie and Chapin 1998; Dormann and Woodin 2002; Weintraub and Schimel 2005; Van Wijk and others 2004). Shrub soils have been found to have higher rates of mineralization than their chemistry would predict (Weintraub and Schimel 2003)
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