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Effects of Elevated CO2 and Soil Nitrogen on Decomposition of Cotton Residue
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.

What was done
Cotton (Gossypium hirsutum L. cv. Deltapine 51) plants were grown in pots placed within greenhouses receiving atmospheric CO2 concentrations of 390 and 720 ppm for two months prior to chemical defoliation and a 44-day incubation period to assess decomposition rates.  In addition, plants were subjected to low and high concentrations of soil nitrogen to determine any influence soil fertility may have on this process.

What was learned
Because the interactions between CO2 and nitrogen availability were complex and largely unpredictable, only the main effects of elevated CO2 will be discussed.  Litter collected from CO2-enriched plants generally contained greater concentrations of starch and soluble sugars and lesser amounts of nitrogen than what was measured in litter obtained from ambiently-grown plants.  Lignin concentrations, however, did not differ between atmospheric CO2 treatments, except in the case of roots, which had higher concentrations due to CO2 enrichment.

When measuring respiration rates during the incubation of plant litter to assess decomposition rates, the authors reported that litter generated in CO2-enriched environments decomposed at rates that were 10 to 14% slower than those observed for litter generated from plants exposed to ambient air.  As time progressed, however, these treatment differences became much smaller.

What it means
As anthropogenic emissions of CO2 add to its concentration in the atmosphere, it will likely affect the concentrations of starch, sugars and nitrogen within decaying litter of cotton plants.  However, the authors of this study concluded that these changes are not likely to substantially impact the decomposition of post-harvest cotton residues.  Nonetheless, they conclude that "the increased production of biomass (brought about by the fertilization effect of atmospheric CO2 enrichment) should add to the level of C stored in soils."  Thus, we can likely expect cotton, and perhaps other agricultural crops, to increase the amount of carbon they sequester within the soils beneath them as the air's CO2 content continues to rise.