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Global Warming: How It May Impact Soil Carbon Storage by Reducing the Frequency and Severity of Freezing
Since biological activity generally increases with rising temperatures - especially when the initial temperature is below the freezing point of water - one might think that global warming would enhance rates of soil microbial respiration, leading to increases in the soil-to-air flux of CO2. However, as Neilsen et al. (2001) note, "overwinter processes account for a significant portion (20-70%) of annual ecosystem carbon and nitrogen cycling and soil-atmosphere trace gas fluxes." Hence, it is not immediately apparent what the ultimate consequences of warming-induced reductions in the frequency and severity of freezing would really be; and to thus find out for themselves, Neilsen et al. conducted an experiment.

They began by collecting samples of soil from a northern hardwood-dominated forest in New Hampshire, USA. These samples, from nearly pure stands of sugar maple (Acer saccharum Marshall) and yellow birch (Betula alleghaniensis Britton), were placed in small vessels and either maintained at the normal laboratory temperature of 20-25C or subjected to mild and severe freezes of -3 and -13C, respectively, for ten days, after which all samples were kept at the normal laboratory temperature for 23 additional days. The evolution of CO2 from the soils was measured at the beginning and end of the full 33-day period and at three other times during the course of the experiment.

Freezing had a significant effect on CO2 evolution from the soils. Cumulative 33-day totals of respiration (in units of mg carbon per kg of soil) for the soil samples taken from the maple stand were 1497, 2120 and 3882 for the control and -3 and -13C temperature treatments, respectively, which numbers represent carbon loss enhancements (relative to that of the control) of 42 and 159% for the -3 and -13C treatments, respectively, or an increased carbon loss of 13 1% for each degree C below freezing. For the soil samples taken from the birch stand, the corresponding respiration numbers were 1734, 2866 and 5063, representing carbon loss enhancements of 65 and 192% for the -3 and -13C treatments, respectively, or an increased carbon loss of 18 3% for each degree C below freezing.

It can be readily appreciated how these research results relate to the subject of global warming effects on soil carbon sequestration. As temperatures gradually warm over the course of many years and different climate zones move poleward in latitude and upward in elevation, regions that experienced many hard freezes in the past will experience less of them in the future. Other regions will experience a shift from hard freezes to mild freezes. Still other regions that experienced mild freezes in the past will experience fewer - or none - in the future. And in all of these situations, together with every permutation that falls somewhere between them, there will be a tendency for less carbon to be released to the atmosphere, which means more will remain sequestered in the soil. And as old Ben Franklin would probably say were he alive today, "a gram of carbon saved is a gram of carbon earned."

In addition to carbon, Neilsen et al. similarly studied the loss of nitrogen from the soils upon freezing and thawing. In this case, however, the -3C treatment had little to no impact on nitrogen loss; but in response to the -13C treatment, nitrogen loss from the soil of the maple stand was enhanced by 39% while that from the soil of the birch stand was enhanced by 34%. Hence, in summing up their findings, the researchers say "the results from this study suggest that changes in climate and snow cover that increase the frequency of soil freezing events could increase nitrogen and carbon losses from northern hardwood forest ecosystems."

In a warming world, of course, just the opposite happens; both soil carbon and nitrogen are conserved. Furthermore, as we have indicated in earlier essays of this series (see Notes below), greater soil nitrogen contents tend to enhance soil carbon storage. Once again, therefore, we have yet another example - and this one double-barreled, no less - of how nature works to maintain earth's temperature within an equable range conducive to the continued existence of life.

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

References
Neilsen, C.B., Groffman, P.M., Hamburg, S.P., Driscoll, C.T., Fahey, T.J. and Hardy, J.P. 2001. Freezing effects on carbon and nitrogen cycling in northern hardwood forest soils. Soil Science Society of America Journal 65: 1723-1730.

Carbon Sequestration Commentaries that highlight the importance of nitrogen to soil carbon storage:

Atmospheric CO2, Soil Nitrogen and Plant Phenolics: How Their Interaction Influences Carbon Sequestration in Soils

Atmospheric Nitrogen Deposition: Its Long-Term Impact on Carbon Sequestration in Soils

CO2 and Nitrogen Effects on Soil Carbon Sequestration: The Whole is Often Greater Than the Sum of Its Parts