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Title: High ozone increases soil perchlorate but does not affect foliar perchlorate content

Author
item GRANTZ, DAVID - University Of California
item JACKSON, ANDREW - Texas Tech University
item VU, HAI-BANG - University Of California
item Burkey, Kent
item MCGRATH, MARGARET - Cornell University
item HARVEY, GREGORY - United States Air Force

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/13/2014
Publication Date: 7/14/2014
Citation: Grantz, D., Jackson, A., Vu, H., Burkey, K.O., Mcgrath, M., Harvey, G. 2014. High ozone increases soil perchlorate but does not affect foliar perchlorate content. Journal of Environmental Quality. 43:1460-1466. doi: 10.2134/jeq2013.11.0464.

Interpretive Summary: Perchlorate salts are detected throughout the environment. Perchlorate ions at high levels are considered toxic to mammals, including humans, because perchlorate disrupts thyroid metabolism by interfering with the uptake of iodide. Natural sources of perchlorate include atmospheric oxidation of chloride mediated by lightning, ultraviolet radiation, or tropospheric ozone. In addition to natural atmospheric sources, a number of anthropogenic point sources have been identified that include industrial and military applications, consumer products such as fireworks and highway flares, and in Chilean nitrate fertilizer which contains high natural concentrations of perchlorate. The current ambient distribution of perchlorate does not correspond with the abundance of known sources, suggesting that additional sources may remain to be identified. We tested the hypothesis that plants exposed to high levels of ozone may synthesize perchlorate. Soybean, cotton, bush bean, sorghum and maize were exposed for 9 weeks to high ozone concentrations similar to levels found in mega-cities or associated with stratospheric incursions into the troposphere. No evidence was found to suggest that plant metabolism is capable of perchlorate biosynthesis. Significantly, however, high ozone concentrations increased perchlorate in the soil through unknown soil surface reactions that require further investigation as a potential source of perchlorate in the environment.

Technical Abstract: Ozone (O3) is implicated in the natural source inventory of perchlorate (ClO4-), a hydrophilic salt that migrates to ground water and interferes with uptake of iodide in mammals, including humans. Tropospheric O3 is elevated in many areas. We previously showed (Grantz et al., 2013; Environmental Pollution) that controlled O3 exposure at near-ambient concentrations did not increase foliar ClO4-. However, under laboratory conditions, O3 has been shown to oxidize chloride (Cl-) to ClO4-. Plants tissues contain Cl- and exhibit numerous responses to O3 exposure, including those involving redox reactions. We hypothesized that O3 exposure of vegetation at elevated concentrations associated with stratospheric incursion and with developing mega-cities (up to 204 nL L-1, 12 hour mean; 320 nL L-1 maximum), may increase foliar ClO4-. This would contribute to observed occurrence of ClO4- in environments in which obvious point sources are absent. At these high O3 concentrations we demonstrate an increase in ClO4- concentration in surface soil that was linearly related to O3 concentration. There was no relationship of foliar ClO4- with either O3 exposure or with O3 dose (stomatal uptake of O3). Accumulation of ClO4- varied substantially between species at low O3, but this was not related to soil surface ClO4- or to ClO4- concentration following exposure to O3. Tropospheric O3 contributes to environmental ClO4- through direct interaction with surface soil, but does not induce increased foliar concentrations of ClO4-.