Linked Cycles

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The ARC LTER uses plastic-covered greenhouses, shade frames, and fertilizer addition in long-term manipulations of tundra ecosystems. Effects of N fertilizers are shown in the middle ground of this photo, where the vegetation is taller, denser, more productive, less species rich, and dominated by the deciduous dwarf shrub Betula nana.

Our understanding of tundra biogeochemistry has been transformed by long term research by the ARC LTER showing that the carbon and nitrogen cycles are strongly linked and interactive at all steps in the cycle of organic matter. Changes in the arctic carbon cycle in response to climate change cannot be understood or predicted without considering carbon-nutrient interactions.

The basis for the ARC LTER’s contribution to this transformation is a series of long-term experimental manipulations of nutrient availability, air temperature, and insolation (sunlight) in the common tundra types at Toolik Lake, combined with focused studies of key carbon-cycling processes like photosynthesis, plant and soil respiration, and organic matter decomposition and nutrient mineralization. ARC researchers have made additional analyses and predictions using simulation models to explore the implications of the research for whole ecosystems and at larger geographic and longer time scales. Among the most important findings are that increased carbon and nitrogen inputs to tundra ecosystems do not necessarily lead to increased carbon and nitrogen storage because losses of these elements by decomposition may be even greater. Research on decomposition of soil organic matter indicates that, unexpectedly, soil microbes as well as plants in these systems may be strongly nitrogen-limited.

The results from the Arctic are consistent with research in other ecosystems around the globe suggesting that changes in the global carbon cycle are strongly linked to other element cycles, especially that of nitrogen. These strong stoichiometric constraints indicate, for example, that increased carbon dioxide concentration in the atmosphere will have less impact on global photosynthesis than we expected 30 years ago, and that indirect impacts of climate change on carbon cycling, such as the stimulation of organic matter turnover and nutrient mineralization by climatic warming, may be more important than direct impacts on carbon cycling processes.

After 20 years of fertilizer addition at Toolik Lake, the C and N content of both plants and the upper soil horizons had doubled while C and N stocks in lower soil horizons had decreased. Losses in deeper soil were greater so whole system C and N stocks decreased
Mack et al. 2004
For further reading: 
Shaver, G. R., W. D. Billings, F. S. Chapin, III, A. E. Giblin, K. J. Nadelhoffer, W. C. Oechel and E. B. Rastetter 1992. Global change and the carbon balance of arctic ecosystems. BioScience 42:433-441.
McKane, R. B., E. B. Rastetter, G. R. Shaver, K. J. Nadelhoffer, A. E. Giblin, and J. A. Laundre. Climatic effects on tundra carbon storage inferred from experimental data and a model. Ecology 78:1170-1187.
Mack, M.C., E.A.G. Schuur, M.S. Bret-Harte, G.R. Shaver, and F.S. Chapin, III. 2004. Ecosystem carbon storage in arctic tundra reduced by long-term nutrient fertilization. Nature 431: 440-443
For further information: 
Gaius R Shaver
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