It may take some time, but cover crops combined with 4R nitrogen (N) fertilizer management can reduce nitrate loads in water and improve water quality, according to research from Purdue and Illinois State Universities.

The 4Rs represent the right source, right time, right rate and right place when applying nutrients. 

Shalamar Armstrong, associate professor of agronomy at Purdue University, and Michael Ruffatti, a crop and soil science lecturer at Illinois State University, studied the effectiveness of cover crops and 4R N management in reducing nitrate loss in a tile-drained system on both research plot and watershed levels. The research analyzed the impact of N application timing and cover crops on water quality as it relates to nitrate loss and nitrous oxide emissions from fertilizer application.

“Nitrous oxide is about 300 times more potent than carbon dioxide,” Armstrong says, “so it is important to look at how your conservation system is playing a part in either reducing or adding nitrous oxide.”

Plot Nitrate Research

The plot research began in 2014. Researchers worked with corn and soybean rotations in mollisol soils with a minimum 3.4% soil organic matter, evaluating the impact of the following on nitrate loss and nitrous oxide emissions:

  1. Moving N application from fall to spring
  2. Moving N application from fall to spring and adding cover crops
  3. Applying N in the fall by strip-tilling into living cover crops

In the third scenario, the farmer had been strip-tilling before corn and no-tilling soybeans for about 10 years. Cover crops were terminated 2 weeks before planting and averaged 1,200 pounds per acre of biomass. 

The data collected showed that while there was no reduction in nitrate loss when moving N application from fall to spring, there were significant reductions when cover crops were added to the system. Armstrong says cover crops affected the nitrate loads in both surface and subsurface drainage.

“When you add cover crops, you get a 46-49% reduction, drastically different than just 4Rs by itself,” Armstrong says. “You need both if you want to reduce nitrate loss via tile drainage. Just doing 4R alone does not move the needle as it relates to N loss reduction in a tile drain landscape. You’ve got to do something else that taps into the legacy N that’s in your soil.”

The cover-cropped field produced a result that surprised Armstrong in 2018. The cover crop produced 2,000 pounds of biomass in 2017.

"It one of our greatest biomass years," Armstrong says. "In 2017, the cover crops significantly reduced the total inorganic N loss, so that's both in nitrate and N2O relative to the non-cover crop and the zero control. The majority of the inorganic N loss that left that system was nitrate and not N2O in this particular year."

The field rotated to soybeans in 2018 and suddenly produced more N2O loss in the cover crop system than the other systems.

"We concluded that we didn't get a lot of breakdown of the residue in that first year, but by the time we got into that second year, even though we didn't add N, we had a lot of N2O loss, so this is coming from possibly the decomposition of that high residue and that high C:N ratio residue in 2017," Armstrong says. "This gives an honest look at some of the dynamics that are at play when it comes to reducing nitrate loss and N2O."

Armstrong says over a 2-year period in a corn-soybean rotation across all three systems, 86% of the total inorganic N loss occurred in the form of nitrate. Cover crops reduced total inorganic N loss — both nitrate and N2O — by more than 50% over a 2-year period, according to the plot research. 

Watershed-Scale Study

The next step was to replicate the research on the watershed scale to see if cover crops could make a similar impact. 

"If we really want to make an impact on the Great Lakes and the Gulf of Mexico, we can't be doing this on a plot or field scale — it has to be done on a watershed scale," Armstrong says. "Now the question is if we get partial adoption on the watershed scale — because you're never going to get 100% adoption on a watershed scale — what's the impact in an uncontrolled environment?

For the watershed-scale study, the Money Creek Watershed in central Illinois was divided into two sub-watersheds. The smaller 770-acre sub-watershed was used as a control, and researchers applied new practices on the larger 1,100-acre sub-watershed. Cover crops were aerial applied on 50% of the larger watershed acres, and water quality at outlet points was measured. Armstrong says cover crops have been flown on for 8 years with good stands some years and poor stands others. The farmers in the watershed asked for radish, oats or other species with low C:N ratios before corn and cereal rye or other species with higher C:N ratios before soybeans.

“Early on, we saw no difference among the watersheds, the cover crop and the controlled water share,” Armstrong says. “But then, as we consecutively cover cropped the same acres over and over, we built a cumulative effect of nitrate reduction.”

In 2023, Armstrong says the larger watershed showed a reduction in nitrate losses of 33-48%, compared to the smaller control watershed of 770 acres, even though there was 24% more total discharge in the 1,100-acre watershed due to slope in the area.

To achieve reductions in nitrate loads and improved water quality, Armstrong says flexibility is key.

“Adaptive management is management that you adapt in field — whether you're changing your cover crop species or the rate that you plant them or how you plant them,” Armstrong says.

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