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Wednesday - February 07, 2007

From: Cleveland, OH
Region: Mid-Atlantic
Topic: Non-Natives
Title: Use of non-native Indian Mustard for reducing lead in soil
Answered by: Nan Hampton

QUESTION:

The EPA phytoremediation documents say lead contamination can be reduced with Brassica juncea:

"Successful Reduction of Lead Contamination. Phytoextraction was demonstrated at a site in Trenton New Jersey that had been used for the manufacture of lead acid batteries. Phytoextraction using Indian mustard (Brassica juncea) and ethylenediaminetetraacetic acid (EDTA) soil amendment reduced the average surface lead concentration by 13 percent in one growing season. The target soil concentration of 400 milligrams per kilogram (mg/kg) was achieved in approximately 72 percent of a 4,500 square-foot area. (Some of the reduction may be attributed to dilution as a result of tilling and spreading contaminants deeper into the soil column.)"

I don’t find it in your database of native plants. Is it native to Northeast Ohio? We have lots of lead contaminated land here. Maybe we should be planting mustard in our empty lots in this shrinking rust belt city… What do you think? No need for more exotic invasives…

ANSWER:

Indian Mustard, Brassica juncea, is not native to North America. It is an introduced species from central Asia and this is where it gets its "Indian" modifier. This is the cultivated plant that you can buy in your supermarket under the name of "mustard greens" and is a favorite crop in home vegetable gardens in the South.

Phytoremediation of soils with heavy metal contamination is a very active area of current research. The efficiency of uptake of contaminants by a wide variety of plants is under investigation. There are announcements of bioengineered transgenic plants with genes that significantly increase the amount of contaminants the plants can accumulate in their tissues:

Martinez, M. et al. An engineered plant that accumulates higher levels of heavy metals than Thlaspi caerulescens, with yields of 100 times more biomass in mine soils. Chemosphere, 2006 June, v. 64, issue 3, p. 478-485.

Gisbert, C. et al. A plant genetically modified that accumulates Pb (lead) is especially promising for phytoremediation. Biochemical and Biophysical Research Communications, Apr 4, 2003. v. 303 (2), p. 440-445.

To be a good candidate for phytoremediation of heavy metal contaminants, a plant needs to be efficient in uptake of the metal, sequester the contaminant in tissues that are easily harvested, and produce a large biomass for harvesting. From our perspective at the Wildflower Center, it is also desirable for the plant, or plants, to be native plants. Neither Thlaspi caerulescens, mentioned in the title above, nor the transgenic plant, Nicotiana glauca, are plants native to North America. (The fact that one of these is a transgenic plant brings up another controversial issue which won't be addressed in this answer.) In Table 4.2. Selected Lead Accumulating Plants (Section 4. Phytoremediation of Lead) from Northwestern University's A Resouce Guide: The Phytoremediation of Lead in Urban, Residential Soils, three native plants are listed:

Short Ragweed (Ambrosia artemisiifolia)
Seapink thrift (Armeria maritima)
Common Sunflower (Helianthus annuus)

Two of these, Short Ragweed and Common Sunflower, are native to Ohio, although I wouldn't necessarily recommend planting large fields of allergy-inducing ragweed in your neighborhood. Another article (Grist, Ray H. et al. Toxic metal uptake by sunflower, switchgrass, and Alyssum. Journal of the Pennsylvania Academy of Science. AUG 2005, v. 79, no. 1, pp. 29-34) offers another native plant, Switchgrass (Panicum virgatum), as a possibility, but concludes that the sunflower is more versatile than switchgrass or Alyssum (non-native).

Table 4.2. Selected Lead Accumulating Plants lists several cultivated garden plants—corn (Zea mays), wheat (Triticum aestivum), rutabaga, turnip (Brassica napus), ornamental kale and cabbage (Brassica oleracea), Indian mustard (Brassica juncea)—that, although not native, are not considered to be invasives. However, they do warrant a caution—plants that are efficient in accumulating lead, or other heavy metals, from the soil should not be used as food plants if they are grown in an area that has soils with a high level of lead contamination.

 

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