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Effects of Elevated CO2 on Litter Decomposition in Desert Ecosystems
Reference
Weatherly, H.E., Zitzer, S.F., Coleman, J.S. and Arnone III, J.A.  2003.  In situ litter decomposition and litter quality in a Mojave Desert ecosystem: effects of elevated atmospheric CO2 and interannual climate variability.  Global Change Biology 9: 1223-1233.

Background
The authors recount how "early hypotheses suggested that CO2-induced reductions in green tissue quality" -- which are typically equated with increases in foliage carbon (C) to nitrogen (N) ratio -- "would translate into reductions in litter quality, which would slow litter decomposition and litter-mediated nutrient recycling in the soil (e.g. Melillo, 1983; Strain and Bazzaz, 1983)."  However, in contradiction of this concept, which posits that subsequent lower N availability would serve as a negative feedback to further plant productivity, they report that studies from a range of natural and/or nutrient-limited and mesic ecosystems indicate that although increases in the air's CO2 content nearly always result in increases in green leaf C:N, they typically do not cause large changes in leaf litter C:N (e.g. Franck et al., 1997; Hirschel et al., 1997), further noting that the CO2-induced increases in leaf litter C:N that have been observed have generally been too small to affect decomposition (Norby et al., 2001).  Hence, to see if the same holds true for deserts and semi-arid lands, Weatherly et al. (2003) conducted an experiment at the Nevada Desert FACE Facility (NDFF) that had the potential to answer this question.

What was done
Naturally-senesced litter was collected from each of nine 25-meter-diameter FACE plots, six of which were exposed to ambient CO2 (367 ppm) and three of which were exposed to elevated CO2 (550 ppm) in both a wet year (1998, 306 mm precipitation) and a dry year (1999, 94 mm precipitation).  Five different types of litter samples (from different plants) were placed in "litterbags" and returned to the locations from which the litter had been collected.  Then, after four months and twelve months, the litterbags were retrieved and a number of different measurements made on their contents.

What was learned
Although CO2-induced reductions in litter quality were observed (increased C:N), the reductions were too small to materially affect rates of litter decomposition.  Year-to-year variations in precipitation, on the other hand, influenced both the quantity and quality of litter production, as well as species composition; and these changes, in sharp contrast to the inconsequential CO2-induced changes, profoundly affected litter decomposition, as well as litter-mediated ecosystem N and C cycling.

What it means
In light of their observations, the authors conclude that "decomposition in arid shrub-dominated systems will remain largely unchanged in a CO2-richer world," which is essentially what has been determined to be the case for mesic ecosystems.  Hence, we can also confidently conclude, as have Billings et al. (2003), that "potential increases in desert productivity with elevated CO2 thus may not be limited by reduced leaf litter quality," which finding could well place deserts "among the most responsive ecosystems to elevated CO2, with increases in productivity leading to potential increased C sequestration."

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.

Franck, V.M., Hungate, B.A., Chapin III, F.S. and Field, C.B.  1997.  Decomposition of litter produced under elevated CO2: dependence on plant species and nutrient supply.  Biogeochemistry 36: 223-237.

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.

Melillo, J.M.  1983.  Will increases in atmospheric CO2 concentrations affect litter decay rates?  The Ecosystems Center Annual Report.  Marine Biological Laboratory, Woods Hole, MA, USA.

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.

Strain, B.R. and Bazzaz, F.A.  1983.  Terrestrial plant communities. In: CO2 and Plants, Lemon, E.R. (Ed.).  Westview Press, Boulder, CO, USA, pp. 177-222.


Reviewed 31 December 2003