The sequestering of carbon in the soils upon which woody plants grow has the potential to provide a powerful brake on the rate of rise of the air's CO2 content if the plant litter that is incorporated into those soils does not decompose more rapidly in a CO2-enriched atmosphere than it does in current ambient air. Hence, it is important to determine if this latter constraint is true or false; and in this summary we review this question with respect to litter produced by deciduous trees.
Scherzel et al. (1998) exposed seedlings of yellow poplar trees to elevated concentrations of atmospheric CO2 and O3 in open-top chambers for four full growing seasons, finding that rates of litter decomposition were similar for all treatments for the first five months of the study. Thereafter, however, litter produced in the elevated O3 and CO2 treatment decomposed at a significantly slower rate, such that after two years had passed, the litter from the elevated O3 and CO2 treatment contained approximately 12% more biomass than the litter from any other treatment.
Cotrufo et al. (1998) grew two-year-old ash and sycamore seedlings for one growing season in closed-top chambers maintained at atmospheric CO2 concentrations of 350 and 600 ppm. The high-CO2 air increased lignin contents in the litter produced from both tree species, which likely contributed to the decreased litter decomposition rates observed in the CO2-enriched chambers. After one-year of incubation, for example, litter bags from the CO2-enrihced trees of both species had about 30% more dry mass remaining in them than litter bags from the ambient trees. In addition, woodlouse arthropods consumed 16% less biomass when fed litter generated from seedlings grown at 600 ppm CO2 than when fed litter generated from seedlings grown in ambient air.
De Angelis et al. (2000) constructed large open-top chambers around 30-year-old mixed stands of naturally growing Mediterranean forest species (dominated by Quercus ilex, Phillyrea augustifolia, and Pistacia lentiscus) near the coast of central Italy. Half of the chambers were exposed to ambient air of 350 ppm CO2, while half were exposed to air of 710 ppm CO2; and after three years, the lignin and carbon concentrations of the leaf litter of all three species were increased by 18 and 4%, respectively, while their nitrogen concentrations were reduced by 13%. These changes resulted in a 20% CO2-induced increase in the carbon-to-nitrogen ratio of the leaf litter, which parameter is commonly used to predict decomposition rates, where larger ratios are generally associated with less rapid decomposition than smaller ratios. This case was no exception, with 4% less decomposition occurring in the leaf litter gathered from beneath the CO2-enriched trees than in the litter collected from beneath the trees growing in ambient air.
Cotrufo and Ineson (2000) grew beech seedlings for five years in open-top chambers fumigated with air containing either 350 or 700 ppm CO2. Subsequently, woody twigs from each CO2 treatment were collected and incubated in native forest soils for 42 months, after which they determined there was no significant effect of the differential CO2 exposure during growth on subsequent woody twig decomposition, although the mean decomposition rate of the CO2-enriched twigs was 5% less than that of the ambient-treatment twigs.
Conway et al. (2000) grew two-year-old ash tree seedlings in solardomes maintained at atmospheric CO2 concentrations of 350 and 600 ppm, after which naturally-senesced leaves were collected, inoculated with various fungal species, and incubated for 42 days. They found that the elevated CO2 significantly reduced the amount of nitrogen in the senesced leaves, thus giving the CO2-enriched leaf litter a higher carbon to nitrogen ratio than the litter collected from the seedlings growing in ambient air. This change likely contributed to the observed reductions in the amount of fungal colonization present on the senesced leaves from the CO2-enriched treatment, which would be expected to result in reduced rates of leaf decomposition.
King et al. (2001) grew aspen seedlings for five months in open-top chambers receiving atmospheric CO2 concentrations of 350 and 700 ppm. At the end of this period, naturally senesced leaf litter was collected, analyzed and allowed to decompose under ambient conditions for 111 days. Although the elevated CO2 slightly lowered leaf litter nitrogen content, it had no effect on litter sugar, starch or tannin concentrations. Thus, with little to no CO2-induced effects on leaf litter quality, it was not surprising there was no CO2-induced effect on litter decomposition.
Dilustro et al. (2001) erected open-top chambers around portions of a regenerating oak-palmetto scrub ecosystem in Florida, USA, and maintained them at CO2 concentrations of either 350 or 700 ppm, after which they incubated ambient- and elevated-CO2-produced fine roots for 2.2 years in the chamber soils, which were nutrient-poor and often water-stressed. They found that the elevated CO2 did not significantly affect the decomposition rates of the fine roots originating from either the ambient or CO2-enriched environments.
In summarizing the results of these seven studies of deciduous tree species, five are suggestive of slight reductions in litter decomposition rates under CO2-enriched growth conditions, while two show no effect. Hence, with deciduous trees exhibiting large growth enhancements in response to atmospheric CO2 enrichment (see our Editorial of 4 Dec 2002), we can expect to see large increases in the amounts of carbon they sequester in the soils on which they grow as the air's CO2 content continues to rise. And this phenomenon should slow the rate of rise of the atmosphere's CO2 concentration and thereby reduce the impetus for CO2-induced global warming.
References
Conway, D.R., Frankland, J.C., Saunders, V.A. and Wilson, D.R. 2000. Effects of elevated atmospheric CO2 on fungal competition and decomposition of Fraxinus excelsior litter in laboratory microcosms. Mycology Research 104: 187-197.
Cotrufo, M.F. and Ineson, P. 2000. Does elevated atmospheric CO2 concentration affect wood decomposition? Plant and Soil 224: 51-57.
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.
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.
Dilustro, J.J., Day, F.P. and Drake, B.G. 2001. Effects of elevated atmospheric CO2 on root decomposition in a scrub oak ecosystem. Global Change Biology 7: 581-589.
King, J.S., Pregitzer, K.S., Zak, D.R., Kubiske, M.E., Ashby, J.A. and Holmes, W.E. 2001. Chemistry and decomposition of litter from Populus tremuloides Michaux grown at elevated atmospheric CO2 and varying N availability. Global Change Biology 7: 65-74.
Scherzel, A.J., Rebbeck, J. and Boerner, R.E.J. 1998. Foliar nitrogen dynamics and decomposition of yellow-poplar and eastern white pine during four seasons of exposure to elevated ozone and carbon dioxide. Forest Ecology and Management 109: 355-366.