'Sustainable Intensification' on the Farm Reduces Soil Nitrate Losses, Maintains Crop Yields

A nine-year study comparing a typical two-year corn and soybean rotation with a more intensive three-year rotation involving corn, cereal rye, soybean and winter wheat, found that the three-year system can dramatically reduce nitrogen—an important crop nutrient—in farm runoff without compromising yield.

The new findings are detailed in the journal Frontiers in Environmental Science.

"Subterranean drainage pipes called tiles transport nitrogen, in the form of nitrate, from fields to streams, impairing downstream surface waters," the scientists wrote. Nitrate runoff from farms pollutes streams and lakes, some of which supply drinking water for nearby communities. Nitrates also are carried down major rivers like the Mississippi to the Gulf of Mexico, contributing to a vast oxygen-starved "dead zone."

"For maximum crop production we need artificial drainage, in the form of tiles and ditches, across much of Illinois. Unfortunately, nitrate can be lost from the rooting zone with tile water," said Lowell Gentry, a researcher in natural resources and environmental sciences at the University of Illinois Urbana-Champaign who led the new study with Eric Miller, a grower and landowner in Piatt County, Illinois, where the research was conducted. "Our study was designed to see if a more diverse crop rotation could reduce tile nitrate loss and still be competitive with the conventional system of corn and soybean."

From 2015 to 2023, the researchers determined crop yield and monitored nitrate loss from tile-drained fields on a working farm. Their "control treatment" consisted of two conventionally managed fields under a corn and soybean rotation. The more intensive three-year crop-rotation system was employed on an adjacent field. This field was planted with corn, followed by a full season of soybeans, then winter wheat.

A summer harvest of the wheat was followed by a second crop of soybean the same year, or double-crop soybean. Between corn and soybean, a winter cover crop of cereal rye was grown to protect the soil. The cereal rye was terminated with herbicide prior to soybean planting and allowed to decompose on the soil surface, delivering nutrients to the next crop.

A key difference between the rotational systems was the amount of tillage. The control fields were fully tilled in the fall and spring, but the researchers strip-tilled only a narrow swath of the cornfield in the three-year rotation, minimizing the area tilled to one-third of the total field every third year. "By strip-tilling only about a third of the soil at a time, it takes us nine years to fully till the field," Gentry said. This enhances soil stability.

Crops like cereal rye and winter wheat are planted in the fall after corn and soybean crops are harvested. These crops keep the soil intact, helping reduce erosion and nutrient runoff, Gentry said. Tilling the soil and leaving it bare for the fall, winter and spring increases soil erosion and boosts the growth of oxygen-loving microbes that consume soil-organic matter, releasing more nitrate.

Growers, policymakers and scientists have spent decades looking for ways to reduce the loss of nitrate from agricultural lands. Some approaches involve using woodchip bioreactors or installing wetlands to capture the runoff. But those approaches mean growers lose the fertilizing power of the nitrate.

"It's very expensive to make fertilizer, and so I think it's much more strategic to try and conserve the nitrogen, meaning keep it in the field, don't let it leave in the first place," Gentry said. "And that's what the cereal rye and the winter wheat can do. They suck up enough nitrogen during the fall, winter and spring to lower the soil nitrate level. That reduces the tile nitrate level."

The researchers saw a 50% reduction in tile nitrate losses in the three-year rotation when compared with the normal rotation. This was accomplished without compromising yields, the team found.

The long-term experiment allowed the team to learn some important lessons. One year, wet weather prevented early termination of the cereal rye cover crop, allowing it to grow too tall. The added biomass reduced tile nitrate runoff by 90%—a positive outcome—but the excess rye also undermined soybean productivity, lowering yields by 10% that year. Another year, an early killing freeze of the double-crop soybean reduced crop yield and increased tile nitrate loss the next spring.

Gentry also noticed over time that the conventionally managed fields sometimes held standing water after heavy rains, while the experimental fields did not.

"I think that's the result of much less tillage in the experimental field, and the fact that earthworms are now abundant in the diverse crop rotation," he said. "It's interesting to note that both rotations used a conventional herbicide regime, so we know it's not the herbicides that kill the worms; it's the tillage."