For one thing, phenolic compounds inhibit the biodegradation of organic materials (Freeman et al., 2001).  Hence, if atmospheric CO2 enrichment results in the production of more of these decay-resistant substances, one would expect the ongoing rise in the air's CO2 content would ultimately lead to improved carbon sequestration in the world's soils, for the plant-produced organic matter supplied to the soils would be more resistant to decomposition.  What do experiments reveal about this hypothesis?

For a long time the picture was rather muddled.  Many studies reported the expected increases in CBSC concentrations with experimentally-created increases in the air's CO2 content.  Others, however, could find no significant plant phenolic content changes; while a few even detected CO2-induced decreases in CBSC concentrations.  Although chaos thus reigned in this area for some time, Penuelas et al. (1997) finally brought order to the issue when they identified the key role played by soil nitrogen concentration.

In analyzing the results of several different studies, Penuelas et al. noticed that when soil nitrogen supply was less than adequate, some of the CBSC responses to a doubling of the air's CO2 content were negative, i.e., a portion of the studies indicated that plant CBSC concentrations declined as the air's CO2 content rose.  When soil nutrient supply was more than adequate, however, the responses were almost all positive, with plant CBSC concentrations rising in response to a doubling of the air's CO2 concentration.  In addition, when the CO2 content of the air was tripled, all CBSC responses - under both low and high soil nitrogen conditions - were positive.  The bottom line was thus fairly simple.  With a tripling of the air's CO2 content, nearly all plants exhibited increases in their production of CBSCs; but with only a doubling of the air's CO2 concentration, adequate nitrogen nutrition was needed to ensure a positive CBSC response.

An example of the possible significance of this nitrogen dependency of CBSC production for ultimate real-world soil carbon sequestration is provided by the study of Kaye et al. (2000), who assessed the carbon sequestering abilities of pure stands of either Eucalyptus or Albizia trees - as well as four different percentage mixtures of them - after 17 years of growth on abandoned sugarcane fields in Hawaii.  It had previously been documented by Bashkin and Binkley (1998) that planting monocultures of Eucalyptus trees on these fields did not alter the quantity of carbon they contained by any appreciable amount.  As ever greater proportions of nitrogen-fixing Albizia trees were interspersed among the Eucalyptus trees, however, soil carbon content rose in direct proportion to the fraction of Albizia trees in the mixed-species stands.

What makes these observations especially exciting is that atmospheric CO2 enrichment, in addition to enhancing plant growth, often stimulates nitrogen fixation in both woody (Olesniewicz and Thomas, 1999) and non-woody (Niklaus et al., 1998; Dakora and Drake, 2000) legumes.  Hence, as the air's CO2 content continues to rise, we can expect earth's nitrogen-fixing plants to become ever more proficient in this important enterprise.

What makes this knowledge even more exciting is that some of the extra nitrogen thus introduced into earth's ecosystems will likely be shared with non-nitrogen-fixing plants (Uselman et al., 1999), as could well have happened in the mixed Albizia-Eucalyptus stands of Kaye et al.'s experiment.  In addition, since the microorganisms responsible for nitrogen fixation are virtually omnipresent in nearly all natural ecosystems (Gifford, 1992) - and since atmospheric CO2 enrichment can directly stimulate the nitrogen-fixing activities of these microbes (Lowe and Evans, 1962) - it can be appreciated that the ongoing rise in the air's CO2 content will likely provide more nitrogen for the production of more CBSCs in all of earth's plants.  And with more decay-resistant materials infused throughout plant tissues, the plant-derived organic matter that is incorporated into the soil should remain there for longer periods of time, which is the very essence of superlative carbon sequestration.

References
Bashkin, M.A. and Binkley, D.  1998.  Changes in soil carbon following afforestation in Hawaii.  Ecology 79: 828-833.

Dakora, F.D. and Drake, B.G.  2000.  Elevated CO2 stimulates associative N2 fixation in a C3 plant of the Chesapeake Bay wetland.  Plant, Cell and Environment 23: 943-953.

Freeman, C., Ostle, N. and Kang, H.  2001.  An enzymic 'latch' on a global carbon store.  Nature 409: 149.

Gifford, R.M.  1992.  Interaction of carbon dioxide with growth-limiting environmental factors in vegetative productivity: Implications for the global carbon cycle.  Advances in Bioclimatology 1: 24-58.

Kaye, J.P., Resh, S.C., Kaye, M.W. and Chimner, R.A.  2000.  Nutrient and carbon dynamics in a replacement series of Euclayptus and Albizia trees.  Ecology 81: 3267-3273.

Lowe, R.H. and Evans, H.J.  1962.  Carbon dioxide requirement for growth of legume nodule bacteria.  Soil Science 94: 351-356.

Niklaus, P.A., Leadley, P.W., Stocklin, J. and Korner, C.  1998.  Nutrient relations in calcareous grassland under elevated CO2.  Oecologia 116: 67-75.

Olesniewicz, K.S. and Thomas, R.B.  1999.  Effects of mycorrhizal colonization on biomass production and nitrogen fixation of black locust (Robinia pseudoacacia) seedlings grown under elevated atmospheric carbon dioxide.  New Phytologist 142: 133-140.

Penuelas, J., Estiarte, M. and Llusia, J.  1997.  Carbon-based secondary compounds at elevated CO2.  Photosynthetica 33: 313-316.

Uselman, S.M., Qualls, R.G. and Thomas, R.B.  1999.  A test of a potential short cut in the nitrogen cycle: The role of exudation of symbiotically fixed nitrogen from the roots of a N-fixing tree and the effects of increased atmospheric CO2 and temperature.  Plant and Soil 210: 21-32.