PARENT SESSION
Oral Session # 35: Biogeochemistry III: Arctic, Alpine, and Tundra Systems.
Presiding: A Hartley
Tuesday, August 5. 1:30 PM to 5:00 PM, SITCC Meeting Room 203.

Carbon and nitrogen allocation of tundra plants assessed through an ecosystem scale dual tracer experiment.

Sommerkorn, Martin^*,1, Nadelhoffer, Knute¹, Johnson, Loretta², Kling, George³, Rastetter, Edward¹, ¹ The Macaulay Institute, Aberdeen, UK² Kansas State University, Manhattan, KS, USA³ University of Michigan, Ann Arbor, MI, USA

ABSTRACT- Plant allocation determines the location, timing, and magnitude of inputs into ecosystem carbon (C) and nitrogen (N) pathways. In tundra systems, plant belowground production generally exceeds aboveground production. The fate of aboveground production differs greatly from that of belowground production. Plant allocation patterns can therefore have strong influence on the regulation of ecosystem biogeochemistry. To examine allocation patterns and controls on the fate of carbon allocated belowground, we labeled 1m², paired control and chronically fertilized wet sedge tundra plots with ¹⁵N and ¹⁴C on three dates during the growing season. On the fertilized plots, nine years of chronic N+P additions had altered the relative abundances of species and had increased aboveground plant biomass by a factor of 3, belowground biomass by a factor of 2. Through isotope analysis of harvested plant material and of respired CO₂, we found that 7 days after labeling, control plots had lost 60-75%, fertilized plots 30-40%, of the assimilated C as CO₂, indicating higher growth efficiency of fertilized plots. Root production ranged from 0.1-0.5 gCm^-2d^-1 and was higher in control plots than in fertilized plots. Root production peaked mid season in fertilized plots and late season in control plots, whereas leaf production of fertilized plots lagged behind that of control plots. At mid season, fertilized plants allocated 8 times more C to leaves and 20 times more to rhizomes compared to control plants. Fertilized plants were able to take up 5-8 times more N compared to control plants. They also allocated 7-9 times more N to rhizomes and up to 24 times more N to leaves compared to control plants. We conclude that chronically increased nitrogen availability shifts allocations patterns away from nutrient acquiring tissues and toward C acquiring tissues. The increase in N availability also enhances allocation toward storage and vegetative reproduction.

Key words: biogeochemistry, carbon, ecosystem, arctic