Paper Reviewed
Huang, S., Jia, X., Zhao, Y., Bai, B. and Chang, Y. 2017. Elevated CO2 benefits the soil microenvironment in the rhizosphere of Robinia pseudoacacia L. seedlings in Cd- and Pb-contaminated soils. Chemosphere 168: 606-616.

Soil contamination by heavy metals is a significant issue in many parts of the world, including China, where more than 20 million ha of land are presently contaminated (Huang et al., 2009). Such contamination significantly impairs plant growth by reducing photosynthesis and altering soil biogeochemical processes. Some plants, however, are more tolerant than others in coping with metal toxicity and are thus frequently utilized for their phytoremediation potential to clean up contaminated soils. Black locust (Robinia pseudoacacia) is one such plant. Yet, according to Huang et al. (2017), "no studies" have examined its response to both cadmium (Cd) and lead (Pb) toxicity under elevated CO2 concentrations that are predicted for the future.

To remedy this situation the five Chinese researchers sowed black locust seeds in pots in open-top chambers maintained under either ambient (385 ppm) or elevated (700 ppm) CO2 concentrations. Various subsets of the pots contained soils that were treated with different levels of Cd (0.2, 1.2 or 5.2 mg kg^-1 dry soil) and Pb (15.6 or 515.6 mg kg^-1 dry soil) contamination. After 30, 60 and 90 days of plant emergence the scientists collected samples from each treatment and performed a series of analyses to determine the effects of elevated CO2 on the rhizosphere microenvironment (which is characterized by interactions between roots, soil and microorganisms) of the locust seedlings in the Cd- and Pb-contaminated soils.

So what did their analysis reveal?

Huang et al. say that "overall, elevated CO2 benefited the microenvironment in the rhizosphere of R. pseudoacacia seedlings under Cd and Pb stress by increasing the concentration of dissolved organic carbon and organic compounds, microbial population, and microbial and enzyme activity." What is more, they add that "elevated CO2 was associated with greater removal of heavy metals from rhizosphere soils relative to ambient CO2" (see figure below), which "improvements in the microenvironment suggest that elevated CO2 could benefit soil fertility in the rhizosphere of R. pseudoacacia seedlings under heavy metal stress."

These positive effects of elevated CO2 should represent welcomed relief for managers of marginal soils and lands that suffer from heavy metal toxicity.

Figure 1. The removal ratio of Cd and Pb by R. pseudoacacia in rhizosphere soils under different treatments. Cd0, Cd1, and Cd5 represent 0.2, 1.2, and 5.2 mg Cd Kg-1 dry soil, respectively. Pb represents 515.6 mg Pb kg-1 dry weight soil. Cd0 also represents the control for Pb0 treatment (no added Cd or Pb); therefore, the label "Pb0" in the figures was replaced by "Cd0". The Pb-only treatment is labeled "Cd0/Pb" because no Cd0 was added. Adapted from Huang et al. (2017).

Reference
Huang, Y.Z., Hu, Y. and Liu, Y.X. 2009. Combined toxicity of copper and cadmium to six rice genotypes (Oryza sativa L.). Journal of Environmental Sciences 21: 647-653.