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Decomposition of Soybean Postharvest Residue: Contrasting Effects of Elevated O3 and CO2 Levels
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 O3Global Change Biology 11: 685-698.

What was done
A two-year open-top chamber study was conducted to determine the chemistry and decomposition rates of aboveground postharvest residues of soybean (Glycine max (L.) Merr. cv Essex) plants grown in reciprocal combinations of low and high atmospheric concentrations of O3 (21 and 74 nmol mol-1, respectively) and CO2 (370 and 714 ppm, respectively).

What was learned
In the words of the authors, "plant residue chemistry was generally unaffected by elevated CO2, except for an increase in leaf residue lignin concentration," which was also observed in the elevated O3 treatment.  Hence, they say that "the primary influence of elevated atmospheric CO2 and O3 concentrations on decomposition processes is apt to arise from effects on residue mass input, which is increased by elevated CO2 and suppressed by O3."  In this regard, they report that elevated O3 decreased aboveground postharvest residue by 15-46%, while elevated CO2 increased it by 28-56%; and in combination, the CO2 effect always predominated.  In the case of leaves, for example, elevating the air's O3 concentration dropped dry mass residue to only 54% of what it was under ambient conditions, while concurrently elevating the air's CO2 concentration boosted it to 124% of what it was in ambient air.  Corresponding results of 85% and 123% were obtained for petioles, 60% and 121% for stems, and 72% and 122% for husks.

What it means
Booker et al. state 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."  The results of their study additionally suggest that such should also occur in the face in concurrent increases in the air's O3 concentration, particularly since O3 additions to the atmosphere tend, as do CO2 additions, to increase residue lignin content.

Reviewed 15 June 2005