Forests Find More Nitrogen in the Soils of a Warming World
Melillo, J.M., Butler, S., Johnson, J., Mohan, J., Steudler, P., Lux, H., Burrows, E., Bowles, F., Smith, R., Scott, L., Vario, C., Hill, T., Burton, A., Zhou, Y.-M. and Tang, J. 2011. Soil warming, carbon-nitrogen interactions, and forest carbon budgets. Proceedings of the National Academy of Sciences USA 108: 9508-9512.
In a long-term (seven-year) effort designed to further explore these closely related phenomena, Melillo et al. (2011) measured changes in net carbon storage in both trees and soil in a mixed hardwood forest ecosystem in central Massachusetts (USA) in response to a 5°C increase in soil temperature imposed on a 30 x 30-m tract of land that was heated by a matrix of heating cables buried at a depth of 10 cm and spaced 20 cm apart, comparing the results from that tract of land with those they obtained on a non-heated 30 x 30-m tract of similar land.
The fifteen researchers report that the soil warming of their study resulted in carbon losses from the soil; but they say that it simultaneously stimulated carbon gains in the woody tissues of the trees. Altogether, over the seven years of the experiment, they indicate that "the cumulative warming-induced net flux of carbon has been from the forest to the atmosphere," but they note that "the magnitude of the flux has diminished over time as a result of the increase in tree growth rate in the heated area." And they state that in the seventh year of the study, "warming-induced soil carbon losses were almost totally compensated for by plant carbon gains in response to warming," which phenomenon they attributed to "warming-induced increases in nitrogen availability."
Melillo et al. conclude that "although warming has resulted in a net positive feedback to the climate system, the magnitude of the feedback has been substantially dampened by the increase in storage of carbon in vegetation." And if their study were to continue, and if the trend established over its first seven years were to continue, one could expect to see the sign of the feedback change from positive to negative, perhaps as soon as the next year or two, and to grow more negative from that point in time, with the long-term climate feedback ultimately proving to be negative, demonstrating the extreme importance of long-term studies of this nature.
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