Sulfur, a compound used as a fungicide on grapes, can trigger potentially toxic chemistry in agricultural soils and not just in the wetlands and fields downstream, as previous studies showed. The new research may encourage agricultural users to rethink how they use sulfur. 

“We know sulfur does a lot of bad things in the ecosystem when it’s in excess,” said Laura Rea, a CU Boulder PhD student who presented her findings today at the 2023 AGU conference in San Francisco. “There's a ton of literature around acid rain, which is sulfur pollution, and there's a ton of research around atmospheric sulfur deposition, but there's not very much research on agricultural inputs.” 

Rea and her colleagues in CIRES fellow Eve Hinckley’s Environmental Biogeochemistry Lab are examining the consequences of sulfur in agricultural soils. In Napa Valley, Rea developed a way to measure sulfate reduction rates in soils. Previous studies have found evidence of this process happening in downstream wetlands, but Rea’s research shows sulfate reduction is happening further upstream at its point of entry: soils.

Hinkley, a CIRES fellow and CU Boulder professor of Ecology and Environmental Biology, recently published a paper arguing for increased awareness and considerations around sulfur use, along with the development of sustainable management of agricultural sulfur. 

Sulfate reduction can be harmful because it increases the activity of sulfate-reducing bacteria. These are naturally occurring, but produce methylmercury, a dangerous toxin. Rea’s work found dense patches of soil lacking oxygen, which can support sulfate-reducing activity. These findings were surprising because soils, known for plentiful airflow and oxygen, are typically toxic to sulfate-reducing bacteria.

Rea’s continued work in the lab will determine if these rates in soils are linked to the elevated levels of methylmercury found in the region, specifically in the San Francisco Bay area. 

"We can't say for sure yet whether sulfur inputs from vineyards are directly contributing to mercury methylation,” said Rea. “But we do know that it's certainly time to start thinking about sustainable sulfur use and management."