Research HighlightsResidue Affects Soil Nitrogen Cycling and Crop Yields
By Laura Temple
In high-yielding crop environments, soybeans fix just 50% of the nitrogen they need, according to Iowa State University research funded by the Iowa Soybean Association. The rest of the N needed to produce the crop must come from the soil.
“In cropping systems that rotate between corn and soybeans, higher corn yields also increase soil N uptake, leaving less residual inorganic N for the following soybean crop,” says Mike Castellano, a professor with the Iowa State University Department of Agronomy. “As soybean yields increase, soybeans become even more reliant on soil N.”
However, he notes that these findings haven’t changed the reality that, when planting corn following soybeans instead of corn, applying roughly 30% less N still boosts corn yield by 10 to 15 bushels per acre.
“Determining the appropriate N rate for corn in different environments is very complex, but the one constant factor is that corn following soybeans requires less N and produces more bushels than corn following corn,” Castellano explains. “In the past, this has been attributed to the ability of soybeans to fix N in the soil, but calculations of grain use of N show a net negative N balance in high-yielding soybean–corn rotation systems. In other words, we harvest more N in soybean grain than the soybean crop fixes. We realized that something else must cause this effect.”
He points out that baling corn residue for past research on cellulosic ethanol caused a similar effect on N rates and corn yield. With less corn residue, lower N rates produced higher yields in corn-on-corn systems.
“These observations sparked a series of questions about the soil N cycle and the influence of crop residue,” he says. “For example, does corn residue impact soybean yield?”
With soy checkoff support from the Iowa Soybean Association, Castellano led a study taking a fresh look at N fertility in high-yielding soybean and corn systems. This included mining years of N application rate data in soybean–corn rotations with a focus on soybean yields and crop residue.
“With hybrid breeding improvements for yield, average corn residue production has been increasing about 100 pounds per acre per year,” Castellano says.
Despite this increase in corn residue production and the negative ways that residue impacts soil N availability, data analysis found that the amount of corn residue has no influence on soybean yield. He explains that higher levels of corn residue reduce available soil N because it promotes microbial immobilization of fertilizer N, converting it into a form plants can’t use. Corn residue also keeps soils cooler and wetter, slowing microbial N mineralization from organic matter and manure, resulting in less available N in the soil for the next crop.
At the same time, Alexandria Logan, a graduate student working with Castellano, conducted an in-field experiment looking at the impact of soybean residue on corn yield. Following soybeans, they compared corn yields on ground where soybean residue was baled and removed, ground where the soybean residue was left as-is, and ground with twice the amount of natural residue.
“We have just one year of data, but it suggests that harvesting soybean residue could increase corn yields,” he reports. “We know crop residue plays a critical role in preventing erosion, but as residue production increases over time, we should understand how residue management could influence future yields and soil fertility.”
In 2023, he plans to expand in-field trials to learn how cover crops interact with crop residue and available N. He also plans to consider how the distribution of soybean residue may play into the initial results.
“The data shows we could be leaving yield on the table,” Castellano says. “We are trying to understand what is happening in the soil with crop residue and N availability so we can account for that impact when adding N fertilizer to the high-yielding soybean-corn cropping system that dominates much of the Midwest.”
He acknowledges that crop genetics, the field environment and management practices all impact the N cycle and availability. Soybean residue may play an unexpected role in that. Figuring out how and to what degree will improve the efficiency of the total cropping system.
Published: Apr 3, 2023
This project was funded by the soybean checkoff. To find research related to this research highlight or to see other checkoff research projects, please visit the National Soybean Checkoff Research Database.