Plant carbon-nutrient interactions control CO sub(2) exchange in Alaskan wet sedge tundra ecosystems

• Published in: Ecology (Durham) 2000-02, Vol.81 (2), p.453-469
• Main Authors: Johnson, L C, Shaver, G R, Cades, D H, Rastetter, E, Nadelhoffer, K, Giblin, A, Laundre, J, Stanley, A
• Format: Article
• Language: English
• Subjects: Cyperaceae; Freshwater; USA, Alaska
• ISSN: 0012-9658

We explored the long-term (8-yr) effects of separate field manipulations of temperature and nutrient availability on carbon balance in wet sedge tundra near the Arctic Long Term Ecological Research (LTER) site at Toolik Lake, Alaska. Our goals were (1) to assess the relative importance of chronic warming (with field greenhouses) and increased N and P availability (by fertilization) in controlling gross ecosystem photosynthesis, ecosystem respiration (plant plus heterotrophic respiration), and ultimately ecosystem C balance; and (2) to attempt to partition ecosystem responses to these treatments between plant and soil contributions. We present results of the effects of these manipulations on whole-system CO sub(2) exchange over seasonal and diel cycles, and on nonrhizosphere soil microbial respiration using in situ soil incubations. Wet sedge control plots were, at best, a weak sink for carbon even during the peak growing season. Chronic nutrient additions of N + P shifted wet sedge carbon balance to a strong sink throughout the growing season; nutrient availability regulated seasonal and diel CO sub(2) exchanges in these two wet sedge ecosystems. The N + P plots had significantly higher photosynthesis and ecosystem respiration in spite of the unanticipated effect of similar to 30% reduction in thaw depth in these plots, apparently due to a twofold increase in litter accumulation insulating the soil surface and/or possible shading from greater plant cover in these plots. These results highlighted the prevailing importance of nutrient-carbon interactions in controlling ecosystem processes and ecosystem C balance in arctic tundra. In contrast, warming had only subtle effects on CO sub(2) exchanges. Increased temperatures in the warmed plots had little effect on instantaneous rates of photosynthesis or respiration. After eight years of chronic warming with an average 5.6 degree C higher air temperature over the growing season and a 40-200% increase in net N mineralization rate, it was surprising that warming did not have more profound effects on CO sub(2) exchange and plant cover. If there were an effect of warming, increased temperatures might cause early canopy development and lengthen the growing season, rather than directly affect instantaneous rates of photosynthesis. Based on photosynthesis-light response curves developed from the early- and late-season diel measurements, we demonstrated that the main effect of warming was to accelerate the development o