Forests Find More Nitrogen in the Soils of a Warming World

Authors Melillo et al. (2011) write that "soil warming experiments conducted in a variety of ecosystems, including forests, have shown short-term losses of soil carbon as CO₂," as well as "acceleration of nitrogen cycling rates, leading to an increase in the availability of nitrogen to the vegetation (Peterjohn et al., 1994; Rustad and Fernandez, 1998; Luo et al., 2001; Shaw and Harte, 2001; Melillo et al., 2002; Eliasson et al., 2005)," and they state that "the principles of ecosystem stoichiometry (Melillo and Gosz, 1983; Rastetter et al., 1992; Sterner and Elser, 2002) suggest that, in forest ecosystems, the redistribution of a relatively small amount of this newly available nitrogen from the soil to the trees could result in a substantial increase in carbon storage in woody tissues."

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.

Additional References
Eliasson, P.E., McMurtrie, R.E., Pepper, D.A., Stromgren, M., Linder, S. and Agren, G.I. 2005. The response of heterotrophic CO₂ flux to soil warming. Global Change Biology 11: 167-181.

Luo, Y., Wan, S., Hui, D. and Wallace, L.L. 2001. Acclimatization of soil respiration to warming in a tall grass prairie. Nature 413: 622-625.

Melillo, J.M. and Gosz, J.R. 1983. Interactions of biogeochemical cycles in forest ecosystems. In: Bolin, B. and Cook, R.B. (Eds.). The Major Biogeochemical Cycles and Their Interactions. John Wiley & Sons, New York, New York, USA, pp. 177-222.

Melillo, J.M., Steudler, P.A., Aber, J.D., Newkirk, K., Lux, H., Bowles, F.P., Catricala, C., Magill, A., Ahrens, T. and Morrisseau, S. 2002. Soil warming and carbon-cycle feedbacks to the climate system. Science 298: 2173-2176.

Peterjohn, W.T., Melillo, J.M., Steudler, P.A., Newkirk, K.M., Bowles, F.P. and Aber, J.D. 1994. The response of trace gas fluxes and N availability to elevated soil temperatures. Ecological Applications 4: 617-625.

Rastetter, E.B., McKane, R.B., Shaver, G.R. and Melillo, J.M. 1992. Changes in C storage by terrestrial ecosystems: How C-N interactions restrict responses to CO₂ and temperature. Water, Air and Soil Pollution 64: 327-344.

Rustad, L.E. and Fernandez, I.J. 1998. Soil warming: Consequences for foliar litter decay in a spruce-fir forest in Maine, USA. Soil Science Society of America Journal 62: 1072-1081.

Shaw, R.M. and Harte, J. 2001. Response of nitrogen cycling to simulated climate change: Differential responses along a subalpine ecotone. Global Change Biology 7: 193-210.

Sterner, R.W. and Elser, J.J. 2002. Ecological Stoichiometry: The Biology of Elements from Molecules to the Biosphere. Princeton University Press, Princeton, New Jersey, USA.