Back in the 1980s, sulfur, a crucial ingredient for plants, was as free as air, falling down from the dirty skies into farmers' fields.
However, the nutrition also generated acid rain and sparked chemistry that resulted in higher mercury in fish.
Regulations reduced the amount of sulfur in the air, but crops still need the nutrient in the Midwest, where sulfur-hungry maize and soybean fields proliferated.
Air quality improvements lead to more sulfur fertilizer use
"We find a definite rise in sulfur fertilizer usage corresponding with a drop in atmospheric deposition," said Eve-Lyn Hinckley, a CIRES fellow, University of Colorado Boulder ecologist, and lead author of a new evaluation of sulfur fertilizer use. "From 1985 to 2015, we produced the first time series of sulfur fertilizer data," as per ScienceDaily.
Farmers began spraying sulfur right after it ceased falling from the sky, according to Hinckley and her colleague Charles Driscoll of Syracuse University, who published their findings in the journal Communications Earth & Environment in late December.
Furthermore, they discovered that sulfur fertilization grew rapidly, significantly surpassing the development in the usage of other nutrients such as nitrogen and phosphorus.
This is critical because sulfur's effects may be severe: the chemical can effectively make heavy metals, like poisonous mercury, more "mobile" and hence more likely to enter fish.
According to Hinckley, it is not yet apparent how much sulfur fertilizer affects the mercury cycle.
"It's the same type of sulfur that was in acid rain, but that was a diffuse, widespread atmospheric deposition, whereas this is intense, targeted applications in much larger amounts."
She and her colleagues are already investigating the link, looking at the potential interaction between agricultural sulfur runoff and downstream methylmercury formation stimulation.
Despite the environmental concerns, farmers require sulfur fertilizers, according to Hinckley, and her research does not support banning them.
In the case of other nutrients that, when given in excess of crop requirements, might cause environmental harm, such as nitrogen and phosphorus, agricultural professionals have learned how to reduce damage by selected timing and quantities of application.
That is the purpose of her sulfur research, she stated. "We want to assist people in using it wisely and understanding what it does to the environment."
Read more: 120 Countries Pledge to Fight Mercury Pollution Around the World
Mercury in the Environment
Mercury's hazardous effects are determined by its chemical form and the method of exposure. The most poisonous form of mercury is methylmercury [CH3Hg], as per USGS.
It has an impact on the immune system, changes the genetic and enzyme systems, and harms the neural system, including coordination and the perceptions of touch, taste, and sight.
Methylmercury is especially harmful to developing embryos, which have five to ten times the sensitivity of adults. This is often ingested, and it is more quickly absorbed and excreted than other forms of mercury.
When vapors of elemental mercury, Hg(0), the form emitted by broken thermometers, are breathed over time, they induce tremors, gingivitis, and excitability. Mercury concentrations in fish and animals are high enough in certain locations of the United States to pose a concern to wildlife.
However, proving cause and effect in field research is challenging since other factors that may contribute to the biological impact under investigation (for example, reproductive success) are sometimes impossible to control.
Scientists have observed harmful effects in the field at mercury amounts that are toxic in the lab, while controlled lab investigations have shown toxic effects at mercury quantities that are typical in particular habitats.
Reduced loon chick development has been seen in Wisconsin lakes when mercury contents in eggs reach lethal levels in laboratory tests.
The behavior of young great egrets can be altered by dietary mercury quantities characteristic of the Everglades.
Studies on mallards, great egrets, and other aquatic birds have revealed that defensive enzymes are less efficient after mercury exposure.
Analyses of such molecular indications show that mercury is doing havoc on diving ducks in San Francisco Bay, herons and egrets in Nevada's Carson River, and heron embryos in Mississippi River colonies.
Finally, additional pollutants influence mercury toxicity. When selenium, another potentially hazardous mineral, is present in the diet, methylmercury can be more detrimental to bird embryos.
Related article: Ducks Contaminated By Mercury Pollution Found to Be More Prone to Developing Bird Flu