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Effects of Elevated CO2, O3, and Moisture on Soil Microbial Biomass and Respiration
Islam, K.R., Mulchi, C.L. and Ali, A.A.  2000.  Interactions of tropospheric CO2 and O3 enrichments and moisture variations on microbial biomass and respiration in soil.  Global Change Biology 6: 255-265.

What was done
Wheat (Triticum aestivum) and soybean (Glycine max) plants were grown in a standard crop rotation from seed to maturity in open-top chambers, while being subjected to two soil moisture regimes and various atmospheric combinations of ambient and elevated (500 ppm) CO2 and ambient and elevated (ambient + 35 ppb) O3 concentrations, to determine the effects of these parameters on soil microbial characteristics.

What was learned
After two years of wheat-soybean crop rotations, total organic carbon contents in soils were not significantly affected by soil moisture or atmospheric CO2 or O3 enrichment.  However, the size of easily mineralizable soil carbon pools increased by 22% with atmospheric CO2 enrichment, when averaged across both soil moisture regimes, while they remained unaffected by O3 enrichment.  Carbon in soil microbial biomass increased by 15% in response to elevated CO2, while it decreased by 17% in response to elevated O3, with respect to ambient CO2 and O3 control values averaged across both soil moisture regimes.  Nevertheless, when subjected to elevated CO2 and O3 concentrations simultaneously, soil microbial biomass carbon contents were not significantly different from dry soil ambient controls and were only 10% lower than wet soil ambient controls.  Thus, atmospheric CO2 enrichment completely and partially alleviated the detrimental effects of elevated O3 on soil microbial carbon contents in dry and wet soils, respectively.

There were also some interesting findings concerning metabolic parameters.  Elevated CO2 decreased soil microbial maintenance respiration by 10%, regardless of soil moisture, while elevated O3 increased it by 17%, with respect to control values observed at ambient CO2 and O3 concentrations.  However, in the combined elevated CO2 and O3 treatment, elevated CO2 did not reduce the higher respiration values caused by elevated O3 alone.  Similarly, microbial biomass carbon loses were 1.3 times less in CO2-enriched environments than they were in ambient CO2 environments, regardless of soil moisture, while they were 1.75 times higher in elevated O3 environments.  In the combined case, however, the elevated CO2 concentration partially alleviated the high microbial biomass carbon losses caused by elevated O3 alone.

What it means
As mankind continues to utilize fossil fuels to produce energy, it is likely that atmospheric CO2 and O3 concentrations will continue to rise.  The results of this study suggest that these phenomena will increase the amount of easily mineralizable carbon pools in soils, thus making it easier for microbes and fungal organisms to obtain soil carbon for facilitating their development and growth.  Although elevated O3 can negatively impact soil microbial carbon content and its loss from soils, elevated CO2 tends to ameliorate those detrimental effects, thus allowing soil microbial carbon contents to remain higher for longer periods of time.  This should allow microbes and fungal organisms to better perform their various functions, some of which improve soil structure and mineral availability to plants.

Reviewed 15 June 2000