For the longest time, coal-fired power plants were the greatest source of reactive sulfur (think: acid rain) to the biosphere. A new study reveals that that fertilizer and pesticide applications to croplands have overtaken coal as the most significant source of sulfur to health and the environment. Rachel Carson must be weeping from the grave as agricultural sulfur contaminates human and wildlife ecosystems.
The study, published in the journal Nature Geoscience, is titled, “A Shift in Sulfur-Cycle Manipulation from Atmospheric Emissions to Agricultural Additions.” The authors outline how emissions of sulfur dioxide and reactive sulfur to the atmosphere have caused widespread health and environmental impacts. Yet calls to decrease sulfur emissions through anthropogenic modification of the sulfur cycle aren’t over.
Instead, the study indicated that high levels of sulfur are added to croplands as fertilizers and pesticides and constitute a major environmental disturbance. Long-term sulfur additions to crops have consequences for the health of soil and downstream aquatic ecosystems just as those impacted by acid rain.
Acid Rain’s Insidious Effects on Ecosystems
Acid rain results when sulfur dioxide (SO2) and nitrogen oxides (NOX) are emitted into the atmosphere and transported by wind and air currents. The SO2 and NOX react with water, oxygen, and other chemicals to form sulfuric and nitric acids. These then mix with water and other materials before falling to the ground.
Sulfur is a naturally occurring element that exists primarily in stable geologic forms and is an important plant nutrient. Through mining activities, including fossil fuel extraction as well as synthesis of fertilizers and pesticides, sulfur is brought into air, land, and water systems. It can react quickly, affect ecosystem health, and cycle toxic metals that pose a danger to wildlife and people.
Acid rain gained attention in the 1960s and 1970s when scientists linked degradation of forest and aquatic ecosystems across the northeastern US and Europe to fossil fuel emissions from industrial centers often hundreds of miles away. As it flows through the soil, acidic rain water can leach aluminum from soil clay particles and then flow into streams and lakes. The more acid that is introduced to the ecosystem, the more aluminum is released.
According to the EPA, at pH 5, most fish eggs cannot hatch. At lower pH levels, some adult fish die. Some acidic lakes have no fish. Acid rain also removes minerals and nutrients from the soil that trees need to grow. At high elevations, acidic fog and clouds can strip nutrients from trees’ foliage, leaving them with brown or dead leaves and needles. The trees are then less able to absorb sunlight, which makes them weak and less able to withstand freezing temperatures.
It was thought that the Clean Air Act and its Amendments, which regulated air pollution, would drive down sulfur levels in atmospheric deposition so that there would be no further threat such as was evident through acid rain to ecosystems.
“It seemed like the sulfur story was over,” said Eve-Lyn Hinckley, assistant professor of environmental studies at the University of Colorado, Boulder and lead author of the study. “But our analysis shows that sulfur applications to croplands in the US and elsewhere are often ten times higher than the peak sulfur load in acid rain. No one has looked comprehensively at the environmental and human health consequences of these additions.”
Agricultural Sulfur’s Detrimental Effects: Soils & Downstream Waters
Researchers from the University of Colorado, Boulder, University of Southern Illinois at Carbondale, and Syracuse University participated in this study. They examined trends in sulfur applications across multiple important crops in the US, including corn in the Midwest, sugarcane in Florida, and wine grapes in California. Their models of surface water sulfate export demonstrate that while areas like New England show declining trends in response to recovery from historic atmospheric deposition, sulfate export from agricultural areas is increasing.
“Although sulfur is applied to agricultural lands to improve the production and health of crops, it can have detrimental effects to agricultural soils and downstream waters, similar to what occurred in remote forest landscapes under acid rain,” concludes Charles Driscoll, a professor at Syracuse University and co-author of the study.
Driscoll says an example of the impacts of agricultural applications of sulfur is the enhanced formation of methylmercury in waters draining agricultural lands, such as the Everglades Agricultural Area in Florida. Methylmercury is a potent neurotoxin which accumulates in food chains leading to high concentrations in fish and increasing exposure of mercury to humans and wildlife that consume these fish.
The researchers predict that increasing trends will continue in many croplands around the world, including places like China and India that are still working to regulate fossil fuel emissions.
To date, much research has focused on understanding and regulating nitrogen and phosphorus fertilizers, which can cause eutrophication, fish kills, and harmful algal blooms downstream of agricultural areas.
Hinckley and Driscoll say it is time for the research community to apply lessons obtained from investigations into the effects of nitrogen and phosphorus fertilizers to studies involving the implications of high sulfur use in agriculture. Further inquiry, the researchers continue, must seek not only to document its environmental and human health effects but also to collaborate with farmers to investigate how to optimize sulfur use.
“Sulfur in agriculture is not going away,” said Hinckley, “Yet there is an opportunity to bring science and practice together to create viable solutions that protect long-term environmental, economic, and human health goals.”