Location: Crop Systems & Global Change
Title: Phytotoxicity of zinc and manganese to seedlings grown in soil contaminated by zinc smelting Authors
Submitted to: Environmental Pollution
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 15, 2013
Publication Date: August 20, 2013
Citation: Beyer, W.N., Green, C.E., Beyer, M., Chaney, R.L. 2013. Phytotoxicity of zinc and manganese to seedlings grown in soil contaminated by zinc smelting. Environmental Pollution. 179:167-176. Interpretive Summary: Zinc smelter emissions caused severe disruption of forest ecosystems near the smelters at Palmerton, PA. We previously reported that in still living forest areas near the smelter, tree seedlings (undergrowth) were not becoming established. Severe soil contamination coupled with continuing acidic rainfall appear to be making these contaminated forest soils increasingly phytotoxic over time. Thus, we conducted greenhouse experiments to test the response of different tree species to mixtures of contaminated and non-contaminated forest soils to characterize plant species responses under controlled conditions, and soil and foliar contamination levels associated with phytotoxicity. Each of the tree species suffered severe phytotoxicity in the very strongly acidic contaminated forest soils when the contaminated soil comprised as little as 10% of the soil mixture. Plant analysis showed that both zinc and manganese were present at phytotoxic levels in leaves depending on the plant species, while although cadmium was increased, it was below phytotoxic levels. Based on analysis of regional soils, it is now clear that Mn emissions occurred in addition to the Zn, Cd, Pb and other previously identified contaminants. Because these forest soils are naturally very strongly acidic, Zn and Mn solubility is increased and the highly plant available soluble forms are readily absorbed by the plant roots, causing phytotoxicity. Because of the extremely low pH, Mn oxides are reduced to divalent Mn, the form which can be phytotoxic; only when pH is raised above 5.4 can the oxidation of Mn back to Mn dioxide reduce Mn phytoavailability and toxicity. Because soybean has distinct symptoms of Zn, Cd and Mn phytotoxicity, soybean was also grown on the mixtures of contaminated and non-contaminated soils. Soybean suffered both chlorosis indicative of Zn phytotoxicity, and “crinkle leaf” characteristic of Mn phytotoxicity. Over time, severe necrosis resulted as leaf cells died. In addition, one test was conducted with red oak to examine whether addition of limestone to raise soil pH could alleviate the severe Zn and Mn phytotoxicity. Six levels of pH were established with a mixture of % contaminated and % non-contaminated soil. At the existing pH, severe phytotoxicity resulted, while as pH rose toward 6, growth improved and at higher pH the plants appeared free of symptoms. Raising soil pH to 7 reduced growth; foliar Cu levels fell very strongly with increasing pH likely because the highly organic soil binds Cu even more strongly as pH rises. Taken together, these studies show that the existing contaminated forest soils which are displaying poor growth on new seedlings near the Palmerton Zn smelters are so acidic and so contaminated by Zn, Mn and Cd that phytotoxicity results and strongly limits tree seedling growth. Tests with soybean showed that both Zn and Mn were phytotoxic; different plant species had varied levels of apparent Zn and Mn phytotoxicity. In addition, red oak appeared to suffer Ca deficiency under toxic soil conditions. Because of the multi-element contamination of the soils, remediation will require raising pH and addition of fertilizer nutrients so that the forest death resulting from the smelter contamination does not spread even more widely surrounding the smelter.
Technical Abstract: Historic emissions from two zinc smelters have damaged the forest on Blue Mountain near Palmerton, Pennsylvania, USA. Seedlings of soybeans and five tree species were grown in a greenhouse in a series of mixtures of smelter-contaminated and reference soils. As little as 10% Palmerton soil mixed with reference soil killed or greatly stunted seedlings of most species, listed in order of decreasing sensitivity as red maple (Acer rubrum), gray birch (Betula populifolia), red northern oak (Quercus rubra), soybean (Glycine max), chestnut oak (Quercus prinus), and white pine (Pinus strobus). Zinc was the principal cause of the phytotoxicity to the tree seedlings, although Mn and Cd may also have been phytotoxic in the most contaminated soil mixtures. Exposed soybeans also showed symptoms of Mn toxicity (crinkle leaf). The naturally low soil pH and fertility of the Hazleton soil on the ridgetop contributed to the high phytotoxicity. The concentrations of Zn in soil and plant tissues that were associated with 50% decreases in leaf and root weights were calculated as phytotoxic thresholds (PT50). Phytotoxic thresholds that were based on soil Zn concentrations extracted with 0.01M strontium nitrate solution were considered more applicable to other sites than were thresholds based on soil Zn concentrations extracted with nitric acid. A test of the effect of liming on remediation of the Zn and Mn phytotoxicity caused a striking decrease in Sr-extractable metals and demonstrated that liming was critical to remediation and restoration. Excessive liming, however (pH > 7), appeared to reduce soil fertility, possibly by reducing the available Cu concentration in these soils with low total Cu levels. Calcium deficiency or Zn/Mn induced Ca deficiency seemed to play a role in the observed phytotoxicity. Calcium was marginally deficient in the reference Hazelton soil and decreased to deficient concentrations in plants with higher Zn concentrations. Growth was more closely correlated with the ratio of Ca to Zn concentrations in leaves than with the Zn concentration alone.