When hydrology engineer Anita Thompson was growing up in Minnesota, she knew what to expect from winter. The temperatures would drop below freezing around November or December, snow would fall throughout the season, and it would melt in the spring.

But she can’t say the same thing about Midwest winters a few decades later. Snowmelt is much less predictable, and temperatures can often swing from below freezing to well above and back again in a matter of days.

With such increasing variation in weather comes much uncertainty. This includes uncertainty about how excess nutrients from agricultural fields move between the soil and water sources throughout the winter. The unknowns complicate efforts to protect and improve water quality and soil health. That’s why Thompson is leading a multi-institutional research project to better understand what it all means for farming hubs around the Midwest.

“We’ve got periods of freezing and thawing throughout the winter that are increasingly difficult to predict,” says Thompson, professor and chair in the Department of Biological Systems Engineering. “This research will help us better understand how varying winter conditions impact soil freeze-thaw processes and soil and nutrient movement throughout watersheds in
the region.”

This is an especially relevant issue in Wisconsin, and across the Midwest, because many farmers use fertilizers for their crops, and some apply manure. While manure contains many vital nutrients that help plants grow, such as phosphorous and nitrogen, those nutrients can pollute water sources if they don’t get absorbed by the soil. This compromises water quality in streams, rivers, lakes, and groundwater and contributes to harmful algal blooms and waterborne diseases.

“If we want to protect water quality in our region, we need to better understand seasonal variation and, specifically, winter season processes,” Thompson says. “The land management practices that are effective during the warmer growing season might not have the same results during colder weather and may contribute to the decline of soil and watershed health across the Midwest.”

Thompson is collaborating with several other researchers to probe how factors such as air and soil temperature, precipitation, and snow cover affect the transport of nutrients during the winter compared to the warmer growing season. The research team is also evaluating how land management practices influence soil dynamics (the motion and behavior of soil over time) and water quality, all with the aim of assessing which practices are more effective during warm or cold conditions.

The study’s implications are incredibly broad.

“Understanding changes in soil nutrient dynamics and transport in surface runoff and subsurface flow is important for environmental protection, sustainability of water resources, and food security,” Thompson says.

Many Disciplines, Many Regions, Many Scales

The research team launched their project in 2021 with support from the National Institute of Food and Agriculture at the U.S. Department of Agriculture (USDA). They’re an interdisciplinary group — composed of soil scientists, hydrologists, and watershed modeling specialists — hailing from CALS at UW–Madison, Ohio State University, and the Agricultural Research Service at the USDA.

The team’s geographic distribution helps them take a regional approach as they dig into cold weather soil dynamics across a range of weather gradients. They’ll assess data from Wisconsin, Minnesota, and Ohio that will encompass a wide variety of conditions and span many years.

The team is also operating at multiple scales, which means they will draw from research across labs, agricultural plots, and full-scale fields as well as sophisticated watershed modeling. This approach allows researchers to examine real-world conditions and data alongside highly controlled experiments where they can adjust variables to strategically probe their research questions.

“We can take what we’re learning in the field under actual conditions and then use that to inform our lab research,” Thompson says. “It sets up a circular research process across different scales that will offer us valuable insights into what’s actually happening in these watersheds.”

The Not-So-Simple Freezing Point of Soil

The temperature at which soil freezes plays a significant role in how nutrients move through watersheds. But quantifying the exact freezing point of soil has been a challenge — and a misconception — in the scientific community for years.

“We generally think that water freezes at zero degrees Celsius,” says Laxmi Prasad PhD’21, a postdoctoral researcher in the Department of Biological Systems Engineering who’s been involved in the project since its inception. “But that freezing point only applies to pure water; and, due to the solutes, minerals, and pressure in the soil, the freezing point of water in the soil is actually lower.”

Many research studies and hydrological models have incorporated a freezing point of zero degrees Celsius for soil. So, they’ve also incorrectly assumed that an ambient temperature of zero degrees Celsius means no rainfall or snowmelt is infiltrating the soil and, thus, no nutrients are on the move. But that is not what’s actually happening in the ground.

The lab is an ideal venue to investigate how cold temperatures affect key processes within the soil matrix because researchers can create highly controlled and easily replicated conditions. Prasad has developed lab experiments to quantify the actual freezing point of soil while accounting for different variables: the amount of moisture content in the soil, the volume of snow cover, and the various types of soil found around the region.

Prasad is also conducting experiments to examine how freezing and thawing impacts soil characteristics, such as hydraulic properties (how water moves through the soil) and the amount of nutrient transport from soil to water sources. The freezing process also affects the stability of soil because, when moisture in the soil freezes, it can expand and compromise the soil’s structure.

“If we can better understand the soil freezing point, we can improve understanding of how water and nutrients are moving through the soil during winters,” Prasad explains. “We can use this information to develop better models for different climatic scenarios and have a more accurate understanding of how they’re affecting water quality and soil health.”

Prasad has made a lot of progress in the lab and has confirmed that soil freezes at a lower temperature than zero degrees Celsius. He has also discovered distinct connections between moisture content and temperature gradients. He’s excited about how his findings can be applied to this research project and beyond.

“The insights we gain from this project are not only going to improve our understanding of soil dynamics and water health,” Prasad says. “They’re also going to help inform climate models, carbon emission modeling, and other environmental research projects.”

The Bridge Between Lab and Field

It’s one thing to make a discovery in the lab. It’s quite another to understand how soil dynamics and management practices are playing out in the real world under a wide variety of conditions.

When a farmer applies liquid manure to a cornfield, the air temperature could be 27 degrees or 60 degrees Fahrenheit, it could be raining or sleeting, and the ground could be dry or covered in 7 inches of snow. To understand how the phosphorous and nitrogen in that manure will move through the landscape, researchers need ways to gauge the process when there are more variations and less predictable conditions.

To do this — and to bridge the gap between the lab and what’s actually happening in the watersheds — the research team is using small plots at UW’s Arlington Agricultural Research Station. This manageable scale helps them add complexity and variability to their experiments while still allowing for some degree of control and isolation. With this setup, they can also home in on the most significant factors that control hydrological and nutrient processes as well as the most influential field conditions and treatments.

The plots are part of a multiyear study examining the effects of weather and management on soil freezing and thawing, water movement, and nutrient transport and runoff. They’re looking at a range of standard agricultural treatments on the plots, including conventional tillage versus no tillage and cover crops (both with and without liquid manure fertilization) versus no cover crops with liquid manure fertilization.

CALS soil scientist Francisco Arriaga PhD’00 is leading the effort to compare the effects of these different agricultural treatments on nutrient and water transport over a range of temperatures, precipitation, and ground cover. He’s been involved in earlier iterations of this research, and he’s found that reality often does not match expectations. For example, research shows that when liquid manure is applied over snow, it can change the energy dynamics of the melting process, even after succeeding snowfall, causing the snow to melt faster — and more phosphorous to move into water sources.

“We’ve seen that what’s actually happening in these plots varies from conventional wisdom about what’s happening in the fields,” says Arriaga, an associate professor and extension specialist in the Department of Soil Science. “We’ve found that some management practices that are meant to benefit soil health might not always be the best choice for water quality.”

The research group is also partnering with Discovery Farms, a UW–Madison Division of Extension program that collaborates closely with farmers to collect data across the Midwest and gather information from actual working farms and nearby water sources. This partnership gives researchers access to years of data about the volume and nutrient content of runoff, weather conditions, and management practices from privately owned farms throughout the region.

Because Ohio, Minnesota, and Wisconsin grow similar crops — namely, corn and soybean — they share many management practices in their respective agricultural communities. But weather variation throughout these states allows the research team to compare the impact of these practices under a range of conditions.

“We’re trying to understand how different weather gradients with similar management tactics are impacting the water quality,” Thompson says. “[These plot and field studies] should help us understand if there are ways we need to adjust our management for different weather conditions so that we can protect our water sources and prevent harmful nutrients from getting into the water.”

Toward More Powerful Computer Models

When biological systems engineer Margaret Kalcic compares field data to her current computer modeling tools, she sees many inconsistencies between reality and model predictions. One of these inconsistencies is the flawed assumption Prasad has been looking to eradicate —that if the air temperature is freezing, there’s no water moving through the soil and, there- fore, no pollutants traveling to water sources.

“But when we look at the field data, we see water is still draining through the soil, and, in some instances, even increasing when the air temperature is freezing,” explains Kalcic, an associate professor of biological systems engineering. “And drainage carries nutrients that pollute water resources.”

Kalcic specializes in watershed modeling, and she’s working to improve the computer models so they can better predict how different weather conditions and management practices will influence nutrient movement, soil dynamics, and water quality. This will be an especially helpful tool as climate change continues to shift weather patterns across the region and increases the need for understanding how climatic variability connects to water quality.

“We need to have tools that can predict water quality and help regions understand the effectiveness of different practices in agricultural lands,” Kalcic says.

The path toward such a tool includes incorporating research findings from labs, plots, and fields into the computer models to enhance their predictive power. Kalcic and her collaborators have already improved the model by integrating an algorithm that better reflects the freezing point of soil and integrates Prasad’s findings about soil freezing and thawing temperatures from the lab experiments.

“The lab results are informing promising improvements to the model so that we can better capture the infiltration of water into the soil, and the movement of nutrients, during winter conditions,” Kalcic explains.

The team will compare their modeling results to data from the Discovery Farms and agricultural research sites to gauge how well they’re forecasting the flow of water and the movement of nutrients, especially during the critical winter months.

The hope is that the combined strength of all four sources of data — labs, plots, fields, and models — can inform more effective agricultural practices that strike the right balance between crop health and environmental protection.

Thompson knows the steady and reliable Midwestern winters of her youth are not likely to return, which underscores the importance of the work she is doing today.

“We want to supply the crops we need for a growing population and ensure food security,” Thompson says. “And we also need to be good stewards of the land and manage agricultural areas in a way that’s protecting water quality and the environment.”