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Can an EPA-Certified Air Pollutant Counteract the Botanical Harm Caused by a Bona Fide Soil Pollutant?
Volume 13, Number 45: 10 November 2010

In an illuminating paper recently published in the Journal of Hazardous Materials, Jia et al. (2010) write that "mining and smelting, disposal of sewage sludge and the use of cadmium rich phosphate fertilizers (Wagner, 1993; Liu et al., 2007) have contaminated a large proportion of the agricultural land throughout the world, causing an increase in the soil concentration of many heavy metals," while further noting that "as one of the most toxic environmental pollutants (Zhang et al., 2009) cadmium (Cd) has a strong influence on metabolic activities of crop plants by inducing a number of physiological changes, such as growth inhibition, changes in water and ion metabolism, photosynthesis inhibition, enzyme activity changes, and free radical formation (Ekmekci et al., 2008)," stating that "even at relatively low concentrations cadmium can exert strong toxic effects on crops (Seregin and Ivanov, 2001)."

In a study designed to see to what extent the ongoing rise in the air's CO2 content might be able to ameliorate this host of soil-pollutant-induced problems, the seven scientists grew Italian and perennial ryegrass (Lolium mutiflorum and L. perenne) in pots filled with soil from a long-term experimental rice field in Guangdong Province, China, which they treated so as to contain either 0, 25 or 100 mg Cd per kg soil. Then, they fertilized the soils so as to contain 150 mg N/kg, 100 mg P/kg and 150 mg K/kg, after which (once the ryegrass seeds had sprouted) the pots were taken outdoors and distributed among six open-top chambers, three of which (one each for the three different soil cadmium concentrations) were maintained at the ambient atmospheric CO2 concentration of 375 ppm and three of which were maintained at an elevated CO2 concentration of 810 ppm from 0800 to 1700 hours throughout all 58 days of the summer study, during which time, as well as at its conclusion, they measured a number of plant physiological processes and parameters.

In following these procedures, Jia et al. found that elevated CO2 significantly increased both net photosynthesis and plant water use efficiency, which led to increases in both shoot and root biomass at harvest. More specifically, they determined that "when compared with the ambient CO2 control, the increase in total biomass due to elevated CO2 was about 32 and 31% for L. multiflorum and L. perenne, respectively, grown on the control soil; 37 and 45% on soil amended with 25 mg/kg Cd; [and] 46 and 52% on soil spiked with 100 mg/kg Cd, respectively." And on top of these very positive results, they found that compared to the ambient CO2 control, under elevated CO2 both Lolium species had decreased Cd concentrations in their shoots and roots, reporting that "the decreased magnitude of Cd concentration in L. multiflorum and L. perenne grown on soil spiked with 25 mg/kg Cd was 10.3 and 3.8% for the shoots, and 18.6 and 14.7% for the roots, respectively; [while] for those [plants] grown on soil spiked with 100 mg/kg Cd, it was 8.4 and 8.9% for the shoots, and 12.5 and 13.9% for the roots, respectively."

Clearly, these results are of the "best of both worlds" type; for in response to atmospheric CO2 enrichment, both ryegrass species produced more root and shoot biomass; and that more abundant plant material contained reduced concentrations of a toxic soil pollutant, all due to the presence of a trace gas of the atmosphere that the U.S. Environmental Protection Agency has labeled a dangerous air pollutant. Yet of that air pollutant, as the U.S. Supreme Court has also called it, Jia et al. say that "given expected global increases in CO2 concentration, elevated CO2 may help plants better survive in contaminated soil and reduce the food safety risk due to CO2-induced reduction and dilution in heavy metal concentration."

So what does it all mean? It means that Shakespeare was indeed correct when he wrote that "a rose by any other name would smell as sweet."

Sherwood, Keith and Craig Idso

Ekmekci, Y., Tanyolac, D. and Ayhan, B. 2008. Effects of cadmium on antioxidant enzyme and photosynthetic activities in leaves of two maize cultivars. Journal of Plant Physiology 165: 600-611.

Jia, Y., Tang, S., Wang, R., Ju, X., Ding, Y., Tu, S. and Smith, D.L. 2010. Effects of elevated CO2 on growth, photosynthesis, elemental composition, antioxidant level, and phytochelatin concentration in Lolium mutiflorum and Lolium perenne under Cd stress. Journal of Hazardous Materials 180: 384-394.

Liu, Y., Wang, X., Zeng, G., Qu, D., Gu, J., Zhou, M. and Chai, L. 2007. Cadmium-induced oxidative stress and response of the ascorbate-glutathione cycle in Bechmeria nivea (L.), Gaud. Chemosphere 69: 99-107.

Seregin, I.V. and Ivanov, V.B. 2001. Physiological aspects of cadmium and lead toxic effects on higher plants. Russian Journal of Plant Physiology 48: 523-544.

Wagner, G.J. 1993. Accumulation of cadmium in crop plants and its consequences to human health. Advances in Agronomy 51: 173-212.

Zhang, F.Q., Zhang, H.X., Wang, G.P., Xu, L.L. and Shen, Z.G. 2009. Cadmium-induced accumulation of hydrogen peroxide in the leaf apoplast of Phaseolus aureus and Vicia sativa and the roles of different antioxidant enzymes. Journal of Hazardous Materials 168: 76-84.