Illinois is the No. 1 contributor of nitrate to the Gulf of Mexico dead zone. It’s also the warmest heavily drained state in the Corn Belt. That means tile drainage flows 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.

The climate is warming, with tile discharging more days out of the year than the historical average. But the culprit of the eighth-largest Gulf dead zone since data has started being recorded is more complicated than this. Even before rains created gaps between spring fertilizer applications and planting across the Corn Belt — ensuring more fertilizer would be sent downstream — nitrate loads at a sampling point on the Embarras River in Camargo, Ill., were heavy.

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 Nutrient Research and Education Council and the Foundation of Food and Agricultural Research.

“We know that fall N application often loses more nitrate to tiles than spring N; however, this past year, wet field conditions prevented fall application. Not only did we not get much fall applied N this year, we know fertilizer sales numbers just don't show any dramatic increase in use since the 1980s,” Gentry says.

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Nitrate leaching complex

These factors suggest that nitrate leaching is complicated and not simply a matter of excessive N fertilization. In his 25 years of monitoring tile concentrations coming from corn and soybean systems, he has never seen tile nitrate concentrations this low coming from corn plots, which were about half of what came out of the adjacent soybean plots.

“Because tile drainage started early last fall, and spring fertilizer N was delayed due to wet spring weather, we could assess the impact of the previous crop on tile nitrate like never before,” Gentry says.

If soil microbes eat plant residues with a wide carbon-to-nitrogen ratio like corn, then immobilization of soil nitrate can occur. Whereas, when N rich soybean residue is decomposed, then the soil microbes release N in a process called mineralization.

Warmer winters predicted by climate models will contribute to greater N release during the non-growing season. These data suggest that tile nitrate from mineralization is an important source of N to streams and rivers that flow to the Gulf of Mexico.

Gentry says his corn was N deficient throughout the 2018 growing season at the Filson site due to a cold April that decreased mineralization.

“All 18 corn plots showed stalk nitrate deficiency at harvest, which created a very lean corn crop with residue containing a wide C:N ratio. It’s not that tile nitrate concentrations were particularly high after soybean last year, but rather, tile nitrate losses from corn were unusually low,” Gentry says. “We speculate that microbial immobilization of N was at play here.”

Using the replicated tile drainage site at Filson, Gentry’s data shows soybean fields are quick to leak nitrogen after harvest, with tiles discharging 4 parts per million of nitrate in October, 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 soybean leaves “melt” into the soil, with microbes quickly consuming N-rich leaves and nodules within weeks. The microbes soon run out of available carbon, and N is released back into the soil.

“It doesn’t matter if nitrate came from residue or fertilizer, nitrate is nitrate, and it can leach to tile lines,” Gentry says.

He says fields of recently harvested soybeans are carbon-limited and lose as much or more nitrogen to mineralization than cornfields. "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.

While the average winter is warming in the Corn Belt , adding growing degree units to the growing season, there is a downside effect of more mineralization. Particularly with carbon-limited soybean fields, the losses will become more severe.

“We are going to have more mineralization and more nitrate loss than what we would normally see from 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 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 split between 50% in the spring preplant and 50% early sidedress.

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 can be problematic ahead of corn 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 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 up nitrogen produced through mineralization, tying it up for the next 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.