"What's really surprising about this result is that we didn't expect that this big loss of carbon from the soils would be stimulated by nitrogen alone. Everyone had assumed increased decomposition would be caused by increased temperatures, and the main effect of increased nitrogen would be to stimulate plant growth and store more carbon. We expected that fertilization by itself would lead to increased carbon storage."
"Instead, nitrogen seems to stimulate decomposition and promote carbon dioxide release to the atmosphere from the soils," Bret-Harte said.
The researchers found that although the aboveground portion of tundra plants doubled their productivity under fertilization and, as expected, stored more carbon, the losses of carbon and nitrogen from the deep-soil layers was substantial and more than offset the increased carbon stored in the aboveground plants and plant litter.
Because more than one-third of the world's soil carbon is stored in northern ecosystems boreal forest and Arctic tundra and is equivalent to two-thirds of the carbon found in the atmosphere, the loss of deep-soil carbon could mean an even greater increase in atmospheric carbon dioxide concentrations than is caused by fossil fuel burning.
"The paradigm is that decomposers (microbes) are always limited by carbon availability and almost never limited by nitrogen availability, but this project suggests that we don't understand decomposition as well as we thought we did. Better understanding of decomposition is necessary to be able to predict what will happen with climate warming in northern ecosystems."
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Contact: Marie Gilbert
marie.gilbert@uaf.edu
907-747-7412
University of Alaska Fairbanks
24-Sep-2004