Burning fossil fuels has resulted in changes in the sulfur cycle. Emissions of sulfur dioxide and reactive sulfur into the atmosphere has caused devastating health and environmental impacts, resulting in calls to decrease sulfur emissions. However, human-driven changes in the sulfur cycle continue. In this perspective piece, the researchers use four case studies from the U.S to show how not only sulfur emissions, but sulfur added as fertilizers in agriculture are a major, yet under-studied, source of disturbances to the sulfur cycle. Long-term, these additions cause similar detrimental effects on soil health and aquatic ecosystems to acid rain. Further research on the sulfur cycle will need studies that examine the intersection of climate, water, and other element cycles in order to fully understand modifications in the sulfur cycle. This research needs to include scientists, farmers, regulating authorities and land managers in order to monitor and manage environmental and human health impacts caused by these changes.
The researchers wanted to understand how the shift in S deposition from atmospheric emissions to agricultural additions impact human health and the environment.
Since the Industrial Revolution, mining and fossil fuel combustion has greatly altered the global sulfur (S) cycle through emissions of sulfur dioxide and sulfate aerosols. These emissions have been produced at a rate that far outpaces the return of emissions into geologic reservoirs, posing a threat to the S cycle.
Since the 1950s, researchers have studied the consequences of these emissions. S emissions have degraded air, soil, and water quality impacting ecosystems and the health of the public. Acid rain caused by S in the air lowered pH, increased nutrient losses, and mobilized trace metals in soil and aquatic ecosystems. This caused changes in the structure and function of aquatic ecosystems, resulting in deaths of forest species and increased aluminum in surface waters, which is toxic.
In the U.S, these studies resulted in the passage of the 1970 Clean Air Act and Title IV Amendments of the 1990 CAA, which resulted in three-fold reductions in sulfur emissions since the 1970s. Atmospheric S emissions has decreased to near pre-industrial levels in the United States, indicating a massive decline. Figure 1 shows the magnitude of decreases in S emissions throughout the U.S from 1989 to 2017.
However, there are still significant changes in the S cycle caused by human activity, especially due to the addition of S in fertilizers, pesticides, and soil conditioners used in agriculture. Although nitrogen and phosphorus addition has received much attention, there has been little directed towards the use of S. With climate change, increased productivity in agriculture, and decreased S emissions, agricultural S usage will most likely increase and create many health, environmental, and ecological consequences. In this perspective piece, the authors explore how human-made changes in the S cycle have shifted from atmospheric emissions to agricultural additions, and suggest how future research should address this shift.
Sulfur additions to croplands
S usage in agriculture makes up over 50% of the annual S produced. In the United States, S usage are much higher than the 1973 peak in acid rain deposition in the northeastern U.S, and in some cases, is even comparable to additions of nitrogen and phosphorus. Estimates of S usage show that agricultural S use is on par with elevated atmospheric S emissions. Figure 2 shows S usage in different US crops relative to the 1973 peak S levels in acid rain.
Sulfur is an important nutrient for all organisms, and important for nitrogen uptake in plants. Reducing S in atmospheric S deposition due to regulation increases the need for S use in agriculture, since less S is being deposited in the soil. This is evident in farmers increasing fertilizer S applications to canola in Germany, soybean and corn in the Midwest, and alfalfa in the U.S.
Sulfur is also a pesticide and fungicide used widely for crops plagued by disease, such as grapes and sugar beets. For example, in California, S is the most widely used pesticide.
Where does S manipulation occur?
In this section, the authors present four case studies from the U.S that highlight differences in atmospheric S deposition and S usage and reasons for usage, highlighting many of the same environmental issues and unknowns in crop systems across the world.
A reference: Wild River, northeastern United States
The Wild River drawings in the White Mountains of New Hampshire and Maine. Agriculture is not a large presence in this area, and the dominant form of S deposition from the atmosphere. This area has followed national trends in decreased emissions of sulfur dioxide.
Observations suggest that S deposition peaked in this area in 1973 and has declined since, following reductions in atmospheric S deposition. There have been fluctuations due to changes in S discharge. Cumulative S retention peaked in the early 2000s, and since then the Wild River has been a S source with around 2 to 3 times higher S export than the atmosphere.
The concentration of S in the wild River reflects these changes. Early in the record, there was a sharp increase in S concentration following the summer growing season, suggesting increases in autumn discharge due to flushing of the soil. Similarly, over time, many watersheds in the United States have shifted from a S sink to an S source, which is expected to continue as atmospheric S deposition decreases.
Corn and soy in the Midwestern United States
The Ohio River crosses 7 states that received the highest historical atmospheric S deposition, but recently it has decreased to pre-industrial levels. Historically, crops in this area received sufficient atmospheric S deposition, but increasingly, farmers are using S fertilizers, especially for corn and soybean.
Currently, S additions to fertilizer for corn are minimal, but these estimates may be low because S is added to fields where S deficiency is observed. However, these additions are expected to rise especially if atmospheric S deposition continues to decrease.
S has decreased in the Ohio River Watershed since the late 1980s, even though S additions were tenfold greater than atmospheric S deposition in 2017. This suggests that the trends in S levels are dominated by decreases in atmospheric S deposition.
Wine-growing in California
Powdery mildew is a major threat to wine grapes, such that grape growers use S extensively as preventive measures.
The Napa River is located in California, where atmospheric S deposition is low compared to the northeastern U.S. S additions dwarf atmospheric S deposition, such that from 1989 to 2016, S inputs have nearly doubled. These increases coincide with a 30% increase in acres of farmland growing grapes.
The Napa River has become an S source. However, like the Wild River, it sees fluctuations in S levels. Recent drought has caused decreases in S export, while S additions have remained constant.
Sugarcane in Florida
The Everglades Agricultural Area (EAA) is located in Florida, and much of it is dedicated to sugarcane production. S is added to increase acidity and improve phosphorus availability in these areas. Estimations of S usage vary across sources.
Like California, atmospheric S deposition in the EAA is relatively low. In contrast, the S outflow from canals that drain the EAA shows increased S loss that varies by precipitation.
Potential consequences of high S applications
The four case studies above show that in areas that were previously impacted by acid rain and in croplands receiving S additions, a large amount of reactive S is retained in the source area and transported to neighboring, S-limited ecosystems. This manipulation creates the potential for effects on S cycling, ecosystem functioning, and human health.
Both historic atmospheric S deposition and agricultural S addition increase concentrations of sulfate in soil and water. High soil sulfate may provide nutrients to crops, but can also cause acidification of soil which can increase water use in plants and evaporation/depletion of soil water, impacting the water balance in these ecosystems.
The degree to which this acidification occurs will depend on climate change, soil types, how much S is added, and soil management. Acidification can be combated by liming, tillage, or other fertilizer additions. However, more studies are necessary to determine the effects of long-term S addition to crop trials as well as how to mitigate them. For example, how much S additions have degraded soil in California vineyards is unknown.
Methylmercury (MeHg) is an important consequence of S pollution in aquatic ecosystems downstream of agricultural areas using S addition. MeHg reduces environments where sulfate-reducing bacteria and archea are active, causing it to quickly accumulate in food webs and in top predators. This accumulation leads to fish mercury contamination, to the extent that all 50 U.S states have fish-consumption advisories. Agricultural S addition may be an important driver of fish mercury contamination, which has been observed in the Everglades, California vineyards, and the Midwestern United States.
Too much sulfide
In ponds, interactions between S and iron cycling can influence the supply of phosphorus, which are important contributors to eutrophication. With elevated S levels, binding between sulfide with iron limits phosphorus retention, such that sulfate pollution exacerbates the eutrophication of lakes and wetlands.
Another effect of sulfate pollution in water is sulfide toxicity. In plants, high levels of hydrogen sulfide can inhibit energy production and decrease nutrient uptake, decreasingoverall plant growth.
In crops where S is used as pesticide, there may be impacts on human health. Exposure to S may lead to respiratory issues in local residents due to pesticide drift. The long-term effects of such exposure in major agricultural areas are not well-documented.
Future research directions
Future research must focus on quantifying agricultural S addition and determine where S deficiencies are emerging, as well as what S products are best for meeting these deficiencies. In addition, whether or not the S demands of plants are met by atmospheric S deposition is unclear. Research addressing the storage of atmospheric S, how and for how long until it is released, and the degree it must be supplemented by additional S are important next steps.
It is also important to evaluate the long-term effects of S addition on soil health. Long-term addition of S, which oxidizes to sulfate and increases acidity, may post a risk to soil health. The authors suggest periodic surveys and mandatory reporting of S levels by the USDA to assess the long-term impact of S addition.
Finally, it is important to determine the consequences of agricultural S usage more broadly including investigating microbial communities in high S soils and how the S cycle interacts with other cycles such as the nitrogen or carbon cycles.
More research is necessary to understand the consequences of S deposition shifting from atmospheric emissions to agricultural additions, especially in countries like China and India where atmospheric emissions are still high but agricultural production is intensifying. With changes in the climate and water cycle, the behavior of S cycling will vary over space and time. This calls for multidisciplinary collaboration between the fields of environmental science, agriculture, public health and social science. In doing so, scientists can guide actions to address human manipulations to the S cycle.