American researchers say they have found that modern farming is the largest human source of sulfur in the environment
Sulfur pollution has reached levels of up to 10 times higher than the peak sulfur loads of the acid rain days of the 1960s and 1970s.
The source appears to be additions of sulfur to agricultural crops.
“For several years, my research group has been studying the fate of high sulfur loads applied to California vineyards as a fungicide,” said Eve-Lyn Hinckley, assistant professor of environmental studies and fellow at the Institute of Arctic and Alpine Research (INSTAAR) at the University of Colorado, Boulder.
“Recently, we started to look at trends in sulfur applications in a broader suite of important crops across the (United States). We saw that these applications are high, often four to 10 times the amount of sulfur that was in acid rain at its peak.”
Hinckley said excess sulfur as sulfate, an oxidized form, can leave crops in runoff and stimulate production of methylmercury in wetlands. Methylmercury is a neurotoxin that accumulates as it moves up the food chain and affects each predator more than the prey it consumes. Sulfate can transform into hydrogen sulfide in wetlands, where it is toxic to many plants.
Sulfur has been used on agricultural lands as a fertilizer and a pesticide to improve production and crop health. The researchers examined trends in sulfur applications across a number of important crops such as corn in the Midwest, sugarcane in Florida, and wine grapes in California. Their models of sulfur in surface waters showed that in areas that are recovering from the impacts of acid rain, the amount of sulfur is again increasing.
Hinckley said in a news release that sulfur is being moved through the environment and ecosystems at a fast rate. In the Florida Everglades, long-term research by the U.S. Geological Survey linked large applications of sulfur on sugarcane to the production of methylmercury in the wetlands preserve.
“The question now is whether purposeful additions of sulfur to agricultural areas have the same effects locally and in downstream areas,” she said.
The nature of the sulfur addition to the environment is different than it was in the past. As a component of acid rain, it was a diffused, chronic input over broad regional areas such as forests downwind of industrial centres. As an agricultural product today, it is a targeted addition to a specific area that is local-to-regional in scale.
Hinkley said that, while many U.S. states have mandatory reporting for sulfur pesticide and fungicide use, the data are not always made public. The U.S. Department of Agriculture surveys sulfur fertilizer use in some crops such as corn, but funding is slim to cover all the monitoring programs.
In 2016, Environment and Climate Change Canada scientists, in collaboration with NASA scientists, began using satellite remote sensing to measure ground level sources of sulfur air emissions.
Sulfur can irritate some people’s airways when inhaled, which can exacerbate asthma and other respiratory ailments.
Hinckley’s research group is working closely with farmers in California to determine the consequences of agricultural sulfur in crops where it is used as a fungicide. She said many of the farmers are interested in spray technologies that would deliver sulfur more precisely where and when it is needed on crops.
“An important limitation of our study is that, in many places, data on agricultural inputs — in this case the use of sulfur products — is not publicly available,” she said.
“While we were able to get data on sulfur inputs from a number of national and state databases, as well as the peer-reviewed literature, as we move forward, more transparency on sulfur use would help to develop more sustainable management plans.”
In addition to documenting the impacts of increased sulfur, Hinckley said increased monitoring and research should include the input of farmers, regulatory agencies, and land managers.
“We have an imperative to understand the impact we are having on the environment,” she said. “We need to work together toward solutions to mitigate those effects.”