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Carbon Sequestration in Soils:
Where Measurements Lead, Theory Is Sure to Follow

Agren and Bosatta (2002) state at the outset of their recently published paper in Soil Biology & Biochemistry that "global warming has long been assumed to lead to an increase in soil respiration and, hence, decreasing soil carbon stores." Indeed, this dictum reigned as gospel for many years, for a host of laboratory experiments seemed to suggest that nature would just not allow more carbon to be sequestered in the soils of a warming world. As one experiment after another recently began to suggest otherwise, however, theory was forced to change in order to accommodate reality.

The old-school view of things began to unravel in 1999, when two important studies presented evidence refuting the long-standing orthodoxy. Abandoning the laboratory for the real world of nature, Fitter et al. (1999) heated natural grass ecosystems by 3C and found that the temperature increase had "no direct effect on the soil carbon store." Even more astounding, Liski et al. (1999) showed that carbon storage in the soils of both high- and low-productivity boreal forests in Finland actually increased with warmer temperatures along a natural temperature gradient.

The following year saw more of the same. Johnson et al. (2000) warmed natural Arctic tundra ecosystems by nearly 6C for eight full years and still found no significant effect of that major temperature increase on ecosystem respiration. Likewise, Giardina and Ryan (2000) analyzed organic carbon decomposition data derived from the forest soils of 82 different sites on five continents, reporting the amazing fact that "despite a 20C gradient in mean annual temperature, soil carbon mass loss ... was insensitive to temperature."

What was theory to do? It had to change. What is more, it had to change fast. And it did. The very next year, Thornley and Cannell (2001) ventured forth gingerly with what they called "an hypothesis" concerning the matter. Specifically, they proposed the idea that warming may increase the rate of certain physico-chemical processes that transfer organic carbon from less-stable to more-stable soil organic matter pools, thereby enabling the better-protected organic matter to avoid, or more strongly resist, decomposition. Then, they developed a dynamic soil model in which they demonstrated that if their thinking was correct, long-term soil carbon storage would appear to be insensitive to a rise in temperature, even if the respiration rates of all soil carbon pools rose in response to warming, as they indeed do.

The new paper by Agren and Bosatta appears to be an independent parallel development of much the same concept, although they describe the core idea in somewhat different terms and upgrade the concept from what Thornley and Cannell called "an hypothesis" to what they refer to as the continuous-quality "theory." Quality, in this context, refers to the degradability of soil organic matter; and continuous quality expresses the idea that there is a wide-ranging continuous spectrum of soil organic carbon "mini-pools," each of which possess differing degrees of resistance to decomposition.

The continuous quality theory thus states that soils from naturally higher temperature regimes will have soil organic matter "continuous quality" distributions that contain relatively more organic matter in carbon pools that are more resistant to degradation and are consequently characterized by lower rates of decomposition, which has been observed experimentally to be the case by Grisi et al. (1998). In addition, it states that this shift in the distribution of soil organic matter qualities, i.e., the higher-temperature-induced creation of more of the more-difficult-to-decompose organic matter, will counteract the decomposition-promoting influence of the higher temperatures, so that the overall decomposition rate of the totality of organic matter in a higher-temperature soil is either unaffected or reduced.

Actually, it's all pretty simple when one thinks about it; and we wonder why someone never thought about it before. The reason, of course, is that the prior concept was even more simple, and there were no embarrassing real-world data to upset the apple cart.

The upshot of it all? One can always create a theory to explain the data; but one cannot - unless one is truly dishonest - do the opposite.

Dr. Sherwood B. Idso Dr. Keith E. Idso

References
Agren, G.I. and Bosatta, E. 2002. Reconciling differences in predictions of temperature response of soil organic matter. Soil Biology & Biochemistry 34: 129-132.

Fitter, A.H., Self, G.K., Brown, T.K., Bogie, D.S., Graves, J.D., Benham, D. and Ineson, P. 1999. Root production and turnover in an upland grassland subjected to artificial soil warming respond to radiation flux and nutrients, not temperature. Oecologia 120: 575-581.

Giardina, C.P. and Ryan, M.G. 2000. Evidence that decomposition rates of organic carbon in mineral soil do not vary with temperature. Nature 404: 858-861.

Grisi, B., Grace, C., Brookes, P.C., Benedetti, A. and Dell'abate, M.T. 1998. Temperature effects on organic matter and microbial biomass dynamics in temperate and tropical soils. Soil Biology & Biochemistry 30: 1309-1315.

Johnson, L.C., Shaver, G.R., Cades, D.H., Rastetter, E., Nadelhoffer, K., Giblin, A., Laundre, J. and Stanley, A. 2000. Plant carbon-nutrient interactions control CO2 exchange in Alaskan wet sedge tundra ecosystems. Ecology 81: 453-469.

Liski, J., Ilvesniemi, H., Makela, A. and Westman, C.J. 1999. CO2 emissions from soil in response to climatic warming are overestimated - The decomposition of old soil organic matter is tolerant of temperature. Ambio 28: 171-174.

Thornley, J.H.M and Cannell, M.G.R. 2001. Soil carbon storage response to temperature: an hypothesis. Annals of Botany 87: 591-598.