Researchers at Syracuse University have confirmed that fertilizer and pesticide applications to croplands have replaced coal-fired power plants as the leading human source of sulfur to the environment.
Their findings have major health and environmental implications and could spark a new wave of research into the global sulfur cycle, says Charles Driscoll, University Professor of Environmental Systems as well as Distinguished Professor of Civil and Environmental Engineering.
“High application of sulfur in agriculture constitutes a major yet understudied environmental concern,” says Driscoll, based in the College of Engineering and Computer Science. “Since sulfur is not a major nutrient, few people outside of the agricultural community understand how much is added to croplands via fertilizers, pesticides and soil conditioners.”
Writing in Nature Geoscience (Springer Nature, 2020), Driscoll and colleagues from the University of Colorado (CU) Boulder and Southern Illinois University maintain that human modification of the sulfur cycle is far from over, despite a major reduction in atmospheric sulfur emissions since the 1970s.
The team looked at targeted additions of sulfur to various crops across the United States, including corn in the Midwest, sugarcane in Florida and wine grapes in California. They found that sulfur has been entering the nation’s soil and waterways at rates 10 times higher than amounts in the environment in the 1960s and 1970s, during the days of acid rain.
Linked to the degradation of forest and aquatic systems across the United States and Europe, acid rain is caused by the reaction of sulfur dioxide and nitrogen oxide with water molecules in the air, producing corrosive sulfuric acid. The Clean Air Act and its amendments have reduced atmospheric sulfur deposition to near preindustrial levels.
“Although sulfur is important to the production and health of crops, it can have detrimental effects to agricultural soils and downstream waters,” says Driscoll, adding that sulfur is a naturally occurring element.
When rocks erode, they release sulfur, which combines with oxygen to form sulfate. During photosynthesis, sulfate is absorbed by plants, which, in turn, are consumed by animals.
The sulfur cycle continues when decomposing plants and animals release sulfur back into the environment. Afterward, sulfur returns to the soil—and can be mobilized through water runoff.
According to Driscoll, airborne sulfur-containing particles may lead to excess amounts of acid in lakes and streams, while damaging trees and forest soils. Meanwhile, sulfur in surface water runoff contributes, through a microbial process, to the formation of methylmercury, a neurotoxin found in high levels in fish.
Airborne concentrations of sulfur—which occur when sulfur is sprayed as a pesticide in areas like the grape-rich Napa Valley—can have human health consequences, too.
Driscoll and the paper’s lead author, Eve-Lyn Hinckley, believe now is the time to apply lessons learned from the study of other fertilizers, namely those containing nitrogen and phosphorous, to high sulfur use in agriculture.
They encourage farmers and state officials to work together to communicate findings and develop appropriate application rates to allow for sound crop production, while minimizing environmental impacts. “We need more research to quantify the extent of the effects discussed in our paper,” Driscoll says.
“Sulfur in agriculture is not going away,” says Hinckley, an assistant professor of environmental studies at CU Boulder. “We have an opportunity to bring science and practice together to create variable solutions that protect long-term environmental, economic and human health goals.”
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