“Legacy Phosphorus” and Our Waters

For decades, phosphorous has accumulated in Wisconsin soils. Though farmers have taken steps to reduce the quantity of the agricultural nutrient applied to and running off their fields, a new study reveals that a “legacy” of abundant soil phosphorus has a large, direct and long-lasting impact on water quality.

The study, published in the journal Ecosystems and focused on southern Wisconsin’s Yahara watershed, may be the first to provide quantifiable evidence that eliminating the overabundance of phosphorus will be critical for improving the quality of the state’s lakes and rivers.

For example, the results indicate that a 50 percent reduction in soil phosphorus in the Yahara watershed’s croplands would improve water quality by reducing the summertime concentration of phosphorus in Lake Mendota, the region’s flagship lake, by 25 percent.

“If we continue to apply phosphorus at a greater rate than we remove it, then phosphorus accumulates over time and that’s what’s been happening over many decades in the Yahara watershed,” says Melissa Motew, the study’s lead author. Motew, working with CALS agronomy professor and co-author Christopher Kucharik, is a doctoral candidate at the UW–Madison Nelson Institute for Environmental Studies.

Phosphorus seeps into soils primarily by way of fertilizer and manure, and what crops and other plants don’t use to grow then leaks into waterways with rain and snowmelt runoff. Scientists have long believed that excess soil phosphorus is a culprit behind the murky waters and smelly algal blooms in some of Wisconsin’s lakes and rivers.

Conventional efforts, like no-till farming and cover crops, have tried to address nutrient runoff by slowing its movement from soils to waterways. However, the study shows that simply preventing runoff and erosion does not address the core problem of abundant soil phosphorus, and this overabundance could override conservation efforts.

“Solutions should be focused on stopping phosphorus from going onto the landscape or mining the excess amount that is already built up,” says Kucharik.

Using newly advanced computer models, the study shows the watershed has about four times more phosphorus in its soil than is recommended by UW–Extension, which writes the state’s nutrient management recommendations based on what crops need and a landscape’s potential for nutrient runoff.

Currently, the only method known to draw down soil phosphorus is harvesting crops, but Kucharik explains that plants take up only a small amount of the surplus each year.

“It is unlikely that any cropping system will quickly draw down the excess,” he says.

It will require working with farmers to practice better nutrient accounting and counter the tendency of some to apply more fertilizer, as an insurance measure, than is needed.

Food production need not be compromised by potential solutions, Kucharik says. There is enough excess phosphorus in our soils “to support plant nutrient needs for a long time.”

The research, funded by the National Science Foundation, is part of UW–Madison’s Water Sustainability and Climate project.

Vanishing Waters?

Even before he bought a summer cabin on Long Lake in 2004, Brian Wolf was concerned about the water level. At its deepest point, the Waushara County lake was only about five feet deep, down from historical highs around 10 feet. “I contacted the folks at a state agency asking, ‘Can you tell me what’s up? I’m worried it’s a dying lake,’” says Wolf, a psychologist based in Kenosha. “The response I got back was, ‘There’s no such thing as a dying lake.’”

During his family’s first summer there, Wolf’s worries seemed unfounded. His children caught and released dozens of bass. But by the summer of 2006, the lake had gone almost completely dry—it was a muddy bed littered with dead fish. Now it’s just a grassy field that floods during rainy spells. “Do we still enjoy our cabin? Absolutely. Do we enjoy it as a lake place? No,” says Wolf. Property values dropped so much that “they basically stopped taxing folks on Long Lake as having lake property,” he says.

And Long Lake wasn’t the only place with water problems. Around the same time, a number of other lakes and streams in the Central Sands region, the heart of Wisconsin’s $6 billion potato and vegetable industry, went dry or hit record lows. A landscape once covered in prairie and scrub oak, the region is now a mosaic of circles—160-acre fields of potatoes, sweet corn, peas and other specialty vegetables watered by center-pivot irrigation systems. These systems keep the region’s crops alive between rains and are fed by high-capacity wells that each can pump more than 100,000 gallons per day from the region’s aquifer. Many of those concerned about the area’s disappearing surface waters see the pumps as the obvious culprit, sucking water up and away from local groundwater-fed lakes and streams. Yet the problem may not be that simple.

To help understand and solve the region’s growing water dilemma, CALS has launched a Central Sands Water Initiative with support from the college’s Wisconsin Institute for Sustainable Agriculture. The initiative aims to bring together a broad network of stakeholders—including farmers from the Wisconsin Potato and Vegetable Growers Association, members of local lake and environmental groups, state and federal agency staff, and scientists from UW–Madison and beyond—not only to study the impact of irrigation pumping on the region’s ground and surface waters, but also to find solutions to help refill the dry lakes and streams. At the same time, the initiative’s scientists also plan to explore the possibility that a larger force is altering the overall water balance in the region: climate change.

“Some believe the facts are well established, that irrigation is the only reason for the problem, but we don’t really know that yet,” says Sam Kung, a professor of soil science and the initiative’s coordinator. “It’s also possible that the region’s climate has changed in such a way that the atmosphere is taking more water away from the system.”

Either way, water shortages are expected to increase around the world, where 70 percent of the fresh water people use goes to food production.

“Water is on the cusp of being limited in a substantial way almost everywhere around the globe, and because of water’s importance in agriculture, it’s obvious that solutions will have to have an agricultural focus,” says A.J. Bussan PhD’97, a horticulture professor who leads the initiative’s crop production studies. “Through this initiative, the Central Sands has the potential to serve as a model for other places with multifaceted water demands, showing them how a community can come together to devise a water policy that’s effective.”

But getting the Central Sands community to sit down and talk won’t be easy. Its members are engaged in a contentious debate about the region’s water woes, and with so much at stake—including growers’ livelihoods, residents’ investments and the survival of local lakes and streams—it’s bound to be a difficult conversation.

“This isn’t the typical CALS situation where there’s a problem out there that everybody acknowledges, we marshal CALS science and go solve the problem, and everybody lives happily ever after,” says Peter Nowak, a rural sociologist and emeritus professor at UW’s Nelson Institute for Environmental Studies who last summer agreed to moderate a series of community forums for Central Sands citizens.