Endocrine disrupting compounds (EDCs) are emerging contaminants of concern because they can have detrimental effects on reproduction and development in wildlife and humans at very low concentrations. Surface runoff from manure-fertilized fields represents a significant, uncontrolled source of EDCs in the environment. The EDCs in manure include naturally-occurring hormones (e.g., 17β-estradiol [17β-E2], estrone [E1], progesterone [P4]) and synthetic pharmaceuticals and their metabolites (e.g., α-zearalanol [zeranol, α-ZAL], zearalanone [ZAN], melengestrol acetate [MGA], 17β-trenbolone, trendione). Several mechanisms may attenuate EDCs from agricultural runoff, including soil sorption, uptake by plants, and degradation by plants and microbes.
The soil sorption behavior of EDCs (17β-E2, E1, MGA, and α-ZAL) was assessed in batch sorption studies with three Midwestern agricultural soils. The EDCs sorbed rapidly and the isotherms were largely linear. Sorption behavior was modeled using a large matrix of single-parameter linear free energy relationships (sp-LFERs), based on octanol-water partition coefficients, and more recently-developed polyparameter LFERs (pp-LFERs), based on sorbent and solute properties such as polarizability, hydrogen bonding, and molar volume. Overall, sp-LFERs better fit the experimental results, although the pp-LFERs were more accurate in select cases.
The uptake and transformation of one class of EDCs, estrogens, by plants was assessed using maize seedlings because it is ubiquitous as a crop. The uptake of estrogens (17β-E2, E1, α-ZAL, and ZAN) from hydroponic solutions by maize seedlings was very rapid, with concentrations decreasing by 85 to 100% after two weeks of exposure. Estrogens sorbed rapidly to root tissues, although not in concentrations sufficient to explain the observed removal from solution.
Reversible transformations between reduced estrogens (17β-E2 and α-ZAL) and oxidized estrogens (E1 and ZAN) were observed in maize-exposed samples, with reduced and oxidized forms both present in hydroponic solutions and root tissues. However, only the reduced form of each pair was detected in shoot tissues. Incubation of estrogens with crude enzyme extracts of root tissues resulted in products of both oxidations and reductions, although at low concentrations. On the other hand, only products of reductions were observed after incubation of estrogens with shoot tissue enzyme extracts, and those products were formed at concentrations between one and three orders of magnitude higher than in root tissue enzyme extracts. In solutions inoculated with maize-associated aerobic microbes, estrogens were quickly removed from solution, and only oxidation products were observed. Thus, I propose that the observed reversible transformation of estrogens is due to oxidations performed by aerobic microbes and reductions catalyzed by plant enzymes.
While plants can impact the environmental fate of EDCs, EDCs may also affect the growth and biochemistry of crop plants. I examined 1) the germination and growth of several crop species (bean, maize, onion, pea, spinach, and tomato) in the presence of estrogens and 2) the germination, growth, and enzyme activity of maize in the presence of several EDCs (E1, 17β-E2, fulvestrant [an anti-estrogen], MGA, P4, α-ZAL, and ZAN) . While some differences were statistically significant, no clear patterns emerged. With improvements in the experimental technique, this warrants further examination because impacts on crop growth may negatively affect crop production and agricultural economies.