Illinois is the No. 1 contributor to the Gulf of Mexico dead zone. It’s also the warmest heavily drained state in the Corn Belt. That means tile drainage mains flow from fields throughout the nongrowing season. Here, 99% of river nitrates in low-population, heavily drained watersheds have been traced back to ever-flowing drainage mains.
Another heavily tile-drained state, Michigan, is warming, with tile discharging more days out of the year than the historical average. However, the culprit of the eighth-largest Gulf dead zone since data has started being recorded is more complicated than this.
Lowell Gentry, a University of Illinois scientist from the Department of Natural Resources and Environmental Sciences, oversees the sampling site and a nearby research field in Filson, Ill., with support from the Foundation of Food and Agricultural Research. He says the source of the dead zone is further complicated when considering that last fall, many Illinois growers were prevented from applying anhydrous ammonia due to wet conditions.
In 25 years of monitoring, tile concentrations coming from corn and recently harvested cornfields have never come in so low, and because of a warm winter, microbial activity was sustained in soybean fields, unlocking more “free” nitrogen from the soil and residue than average over the nongrowing season.
“That free N ended up going down the tile,” Gentry says. “These dead zones are getting bigger, and we’re showing more nitrates year over year. And it’s partly because we have more rain and more flow. But not only did we not get much fall-applied N this [past] year, we know fertilizer sales numbers just don’t show any dramatic increase in use since the 1980s.
“Nitrogen per grain is less than it used to be because it’s diluted by carbohydrate. And so therefore, even though yield has gone up, we’re not removing that much more nitrogen with the grain than we used to.”
He adds that a maximum return to yield calculated rate of N fertilizer at the Filson site is now 180 pounds, with the application split to suit five different fertilizer treatments being investigated on the site’s 36 tile laterals.
Soybeans leading source of N runoff
Gentry’s data shows soybean fields are quick to leak nitrogen after harvest, with the three different spring fertilization trials showing soybeans discharging 4 parts per million of nitrate in October and steadily increasing to 8 ppm just before late May. Cornfields after harvest remained at about 1 ppm all winter and modestly increased to 3 ppm by the end of spring.
Gentry says the soybeans “melt” into soil, with microbes eating much of the stubble within weeks. With 2-year-old no-till corn residue already mostly decomposed, it doesn’t take long for the ratio of carbon and nitrogen in the topsoil to become imbalanced.
Both nitrogen in the soil and fixed by the former soybeans in rhizobia on the roots outweigh what little carbon residue is added by aboveground leaves and stems. So microbes that eat nitrogen increase in population, drawing all the carbon they can get to fuel their feast and depleting long-term soil organic carbon deposits.
“We are carbon-limited, and we can lose as much or more following soybean than following corn. So therefore, a good portion of what goes down in the river is from soil and residue mineralization,” Gentry says, adding soybean fields represented 200% of lost N over the nongrowing season, while normally, it’s 75%.
"The soybean plant actually removes more nitrogen from the soil than it adds from fixation. So why is more nitrogen coming out of a soybean field than a cornfield this year? It's because of the quantity and type of the residue,” he says.
Climate models predict Illinois and the Corn Belt will continue to get warmer. While that might help add growing degree units to the growing season, the downside effect of more mineralization, particularly on carbon-limited soybean fields, will become more severe.
“We are going to have more mineralization and more nitrate loss than what we would normally see in soybean,” Gentry says, “and that’s tough. That is mind-boggling for someone that knows very little about agriculture, like some of our legislators.
“They would love to just say, ‘Use less nitrogen fertilizer and do less tillage.’ And maybe they’re right on both accounts, but that’s such an oversimplification that it does a great injustice to the complexities of what I’m trying to figure out and what a lot of people are working on.”
Overwintering covers key
Cover crops are a key solution in the fight to keep nutrients tied up in the field and out of tile drainage mains. Gentry uses cover crops in a sixth fertilization treatment at the Filson site, and in the past, determined cereal rye can decrease tile nitrate loss after corn by 40% when applying a maximum return to N rate of 50% at planting and 50% sidedressed.
He doesn’t have a similar number for soybeans yet because the choice cover crop following corn and preceding soybeans, annual ryegrass, died over the winter of 2019 in Filson. Cereal ryegrass isn’t an option, because it survives winter so well that it competes with corn for nitrogen the following spring. In a year like 2019, some growers couldn’t get in to spray down or harrow the cereal rye with a comfortable window for planting.
Starting this fall, Gentry and his team will switch to using winter wheat as an easy-to-manage cover crop following soybeans and preceding corn.
“We think the corn puts plenty of carbon in the soil because it’s a bigger plant,” he says. “Soybean could be considered our weak link in sustainability unless we do something about this carbon limitation.” He says planting a cover crop like winter wheat after soybean harvest ensures there are roots taking in nitrogen produced through mineralization, tying it up for the summer growing season.
When wheat residue is added to the soil, it will help raise soil organic carbon. Year over year, this will normalize the seesawing carbon-nitrogen ratio that exists after soybeans and help decrease mineralization’s contributions to the dead zone.