For one thing, lignin tends to inhibit the biodegradation of organic materials. Hence, if atmospheric CO2 enrichment results in the production of more lignin in plant tissues, the ongoing rise in the air's CO2 content could lead to enhanced carbon sequestration in the world's soils, because plant-produced organic matter supplied to soils would be more resistant to decomposition; and the end result of this phenomenon would be a slowing of the rate-of-rise of the air's CO2 content that could significantly reduce the magnitude of CO2-induced global warming.

So, does atmospheric CO2 enrichment really lead to greater lignin concentrations in plant tissues?

A couple of papers we have reviewed actually found just the opposite, i.e., that plant lignin concentrations tended to decline in response to increases in the air's CO2 concentration. In one of these studies - a five-year open-top chamber experiment - Cotrufo and Ineson (2000), found that a doubling of the air's CO2 content decreased lignin concentrations in twigs of beech seedlings by approximately 12%. Nevertheless, after 42 months of incubation in native forest soils, the decomposition rate of the CO2-erniched twigs was still 5% less than that of the twigs grown in ambient air. Likewise, in an open-top chamber study of an agricultural crop (tall fescue), Newman et al. (2003) found that doubling the air's CO2 content reduced forage lignin concentration by 14%.

At the other end of the spectrum, a number of experiments have produced CO2-induced increases in plant lignin concentrations.

In a two-year study of ash and sycamore seedlings grown in closed-top chambers or solar domes at CO2 concentrations of either 350 ppm (ambient) or 600 ppm (enriched), Cotrufo et al. (1998) observed greater concentrations of lignin in the litter produced by both species; and after one year of incubating the litter from the two CO2 treatments in bags placed within a forest soil, the bags containing the litter produced by the CO2-enriched trees of both species had about 30% more dry mass remaining in them than did the bags containing the litter produced by the ambient-treatment trees. In addition, woodlouse arthropods consumed 16% less litter in the CO2-enriched chambers than in the ambient-treatment chambers.

Higher plant lignin concentrations were also observed in oak seedlings growing in doubled-CO2 air (700 ppm as opposed to 350 ppm) in the controlled-environment greenhouse study of Staudt et al. (2001), as well as in trembling aspen seedlings growing in doubled-CO2 air in the open-top chamber study of Tuchman et al. (2003). And in an open-top chamber study of a real-world 30-year-old mixed-stand of various Mediterranean forest species growing near the coast of central Italy, De Angelis et al. (2000) found that doubled-CO2 air increased leaf-litter lignin concentrations by 18%, which phenomenon was accompanied by an 8% reduction in the initial loss of mass from the decomposing CO2-enriched litter and a 4% reduction one year later.

In the agricultural realm, higher lignin concentrations have additionally been observed by Booker et al. (2000) in the roots of CO2-enriched cotton, which is actually a perennial woody plant, as well as by Booker et al. (2005) in CO2-enriched soybean, which is an annual herbaceous plant. Their findings led them to conclude that "one result of increased residue production and higher levels of recalcitrant material such as lignin being added to the soil is that soil carbon sequestration should increase, a response anticipated to occur with increasing concentrations of atmospheric CO2."

More often than not, however, experiments of this type have tended to find no significant changes in the tissue lignin concentrations of plants grown in CO2-enriched air. The studies in this category that we have reviewed are those of Hirschel et al. (1997) dealing with plants growing in an alpine grassland, a lowland calcareous grassland and a lowland wet tropical rainforest, Booker and Maier (2001) and Finzi and Schlesinger (2002) dealing with loblolly pine trees, Penuelas et al. (2002) dealing with three species of shrub in Pisa, Italy, and Billings et al. (2003) dealing with four species of shrub in the Mojave Desert, Nevada, USA.

So what's the bottom line with respect to the terrestrial biosphere as a whole? A meta-analytic review that considers this question from a number of different perspectives was conducted by Norby et al. (2001). Based on a total of 46 experimental observations, they determined that elevated atmospheric CO2 concentrations increased leaf-litter lignin concentrations by an average of 6.5%. However, these increases in lignin content occurred only in woody species. Moreover, leaf-litter lignin concentrations were not affected by elevated CO2 when plants were grown in open-top chambers, FACE plots or in the proximity of CO2-emitting springs. And in analyzing a total of 101 observations, Norby et al. found that elevated CO2 had no consistent effect on leaf litter decomposition rate in any experimental setting.

However, almost all of the studies that have been conducted to date on woody species - where there is some evidence of an increase in leaf-litter lignin concentration (and there thus remains a possibility of CO2-enhanced carbon sequestration) - have been of short duration compared to the life spans of various forest components; and we will not know the long-term consequences of modest CO2-induced increases in woody-plant lignin concentrations for soil carbon sequestration until a number of experiments of much longer duration than any conducted so far have been concluded. Nevertheless, as Booker et al. (2005) remind us, because there is also considerably more litter produced by almost all plants in CO2-enriched air, soil carbon sequestration should continue to increase as the air's CO2 content continues to rise.

References
Billings, S.A., Zitzer, S.F., Weatherly, H., Schaeffer, S.M., Charlet, T., Arnone III, J.A. and Evans, R.D. 2003. Effects of elevated carbon dioxide on green leaf tissue and leaf litter quality in an intact Mojave Desert ecosystem. Global Change Biology 9: 729-735.

Booker, F.L. and Maier, C.A. 2001. Atmospheric carbon dioxide, irrigation, and fertilization effects on phenolic and nitrogen concentrations in loblolly pine (Pinus taeda) needles. Tree Physiology 21: 609-616.

Booker, F.L., Prior, S.A., Torbert, H.A., Fiscus, E.L., Pursley, W.A. and Hu, S. 2005. Decomposition of soybean grown under elevated concentrations of CO2 and O3. Global Change Biology 11: 685-698.

Booker, F.L., Shafer, S.R., Wei, C.-M. and Horton, S.J. 2000. Carbon dioxide enrichment and nitrogen fertilization effects on cotton (Gossypium hirsutum L.) plant residue chemistry and decomposition. Plant and Soil 220: 89-98.

Cotrufo, M.F., Briones, M.J.I. and Ineson, P. 1998. Elevated CO2 affects field decomposition rate and palatability of tree leaf litter: importance of changes in substrate quality. Soil Biology and Biochemistry 30: 1565-1571.

Cotrufo, M.F. and Ineson, P. 2000. Does elevated atmospheric CO2 concentration affect wood decomposition? Plant and Soil 224: 51-57.

De Angelis, P., Chigwerewe, K.S. and Mugnozza, G.E.S. 2000. Litter quality and decomposition in a CO2-enriched Mediterranean forest ecosystem. Plant and Soil 224: 31-41.

Finzi, A.C. and Schlesinger, W.H. 2002. Species control variation in litter decomposition in a pine forest exposed to elevated CO2. Global Change Biology 8: 1217-1229.

Hirschel, G., Korner, C. and Arnone III, J.A. 1997. Will rising atmospheric CO2 affect leaf litter quality and in situ decomposition rates in native plant communities? Oecologia 110: 387-392.

Newman, J.A., Abner, M.L., Dado, R.G., Gibson, D.J., Brookings, A. and Parsons, A.J. 2003. Effects of elevated CO2, nitrogen and fungal endophyte-infection on tall fescue: growth, photosynthesis, chemical composition and digestibility. Global Change Biology 9: 425-437.

Norby, R.J., Cotrufo, M.F., Ineson, P., O?Neill, E.G. and Canadell, J.G. 2001. Elevated CO2, litter chemistry, and decomposition: a synthesis. Oecologia 127: 153-165.

Penuelas, J., Castells, E., Joffre, R. and Tognetti, R. 2002. Carbon-based secondary and structural compounds in Mediterranean shrubs growing near a natural CO2 spring. Global Change Biology 8: 281-288.

Staudt, M., Joffre, R., Rambal, S. and Kesselmeier, J. 2001. Effect of elevated CO2 on monoterpene emission of young Quercus ilex trees and its relation to structural and ecophysiological parameters. Tree Physiology 21: 437-445.

Tuchman, N.C., Wahtera, K.A., Wetzel, R.G. and Teeri, J.A. 2003. Elevated atmospheric CO2 alters leaf litter quality for stream ecosystems: an in situ leaf decomposition study. Hydrobiologia 495: 203-211.