Quick look
The slurry left behind when using anaerobic digestion to turn manure and agricultural residue into fuel significantly improves a key aspect of soil health when applied to fields, according to an analysis of 12 years of data from a commercial farm in southeast Iowa.
AMES, Iowa - Making renewable natural gas from manure and agricultural residue could make farmers more money, farming more sustainable and - based on a new long-term, on-farm study by Iowa State University researchers - farmland soil richer in carbon.
Digestate, the byproduct of using anaerobic digestion to turn biological matter such as corn stalks and cattle manure into fuel, can boost soil organic carbon levels when applied to fields, according to an analysis of 12 years of data collected on a farm in southeast Iowa where digestate has replaced nearly all of the synthetic nitrogen fertilizer on corn fields.
Researchers studying anaerobic digestion's potential benefits have expected digestate to be a valuable fertilizer and soil amendment, but there are few studies of how the slurry-like product affects soil organic carbon on a commercial-sized farm, said agronomy professor Fernando Miguez, co-author of the study recently published in the Journal of Agriculture and Food Research.
"Smaller studies don't necessarily translate to the commercial level," Miguez said. "But now we've shown that this practice can be successful at a scale that matters."
Data analyzed in the study came from Sievers Family Farms near Stockton, Iowa, where Bryan and Lisa Sievers raise beef cattle, corn and soybeans. The Sievers, both Iowa State graduates, are co-authors of the study, which helped quantify the benefits of the regenerative system driven by their on-farm, fuel-producing anaerobic digester tanks.
"It validates what we have long believed - that we're seeing enhancements in the health of our soil which has translated into improved yields," Bryan Sievers said. "Now we can start to assign a monetary value to these best management practices. And that's what I've been seeking all my life in farming."
'Remarkable' restoration
Soil organic carbon, derived from decaying plant and animal matter, is vital for productive and resilient farmland. It improves water retention, reduces erosion and contamination, and feeds the microbes that provide crops needed nutrients. Numerous studies have shown that farming can degrade soil organic carbon over time, causing losses that are difficult to reverse.
"It has taken a long time to deplete carbon from soil, so restoring it isn't easy," Miguez said.
On the Sievers farm, digestate appears to have helped. Based on 421 samples gathered in 14 of their fields between 2011 and 2022, the soil gained about 714 pounds of carbon per acre per year. For fields that already had relatively high levels of carbon, that amounted to an average increase of about 9% over the time of the study. But in the fields where carbon levels were the lowest, the average increase was 47%, the researchers found.
Those are impressive results, said Lisa Schulte Moore, Charles F. Curtiss Distinguished Professor in Agricultural and Life Sciences and the director of Iowa State's Bioeconomy Institute. Most common management practices that benefit soil organic carbon levels in corn fields, such as minimizing tilling and planting cover crops, aim more for retention than restoration.
"It's remarkable to see soil organic carbon increase that much over 12 years in a corn cropping system. There are practices you can use to maintain it, but it's very difficult to build carbon levels in fields under row-crop production," said Schulte Moore, a co-author of the study.
Using digestate as a soil amendment also can lead to an excessive accumulation of phosphorus, the Sievers study and prior research have found. Numerous researchers are working on methods for filtering out phosphorus, which could be used as a fertilizer component in other applications, said study co-author Sebastian Villarino, a postdoctoral researcher in Miguez's lab.
"Before you invest in extracting phosphorus, it's important to establish the usefulness of digestate," Villarino said.
Digesters on upswing
Anaerobic digestion happens when bacteria break down organic matter without oxygen, releasing a methane-rich gas. When the Sievers family began using anaerobic digesters, there were only two others in use on Iowa farms. Now, there are about 20 on-farm anaerobic digesters in Iowa, said Dan Andersen, an associate professor of agricultural and biosystems engineering whose research focuses on manure management and treatment.
That count doesn't include the industrial digesters making renewable natural gas at the Verbio plant near Nevada, Iowa. The biorefinery's digestate was the subject of another recent study by a team of ISU researchers, including Miguez and Villarino, which also found in a lab-based incubation experiment that digestate can lead to accrual of soil organic carbon.
Understanding the impacts of anaerobic digesters in agriculture is one of the goals of the Grass2Gas project managed by the Consortium for Cultivating Human And Naturally reGenerative Enterprises (C-CHANGE), an Iowa State-led research group in partnership with Pennsylvania State University and Roeslein Renewables, a St. Louis-based company. Integrating anaerobic digesters on farms could create a market for winter crops and perennial grasses as digester feedstock, driving adoption of conservation practices that reduce nutrient runoff, erosion and pollution.
"We can use something that we think of as a waste to produce energy, improve soil health and fight climate change," Villarino said.
Anaerobic digesters are more common in Europe and a handful of U.S. states, but for Iowa farmers considering the soil health benefits of similar systems, knowing how it has worked over many years on an in-state farm is bound to be influential, said Schulte Moore, director of C-CHANGE.
"Having Iowa data will speak more strongly to farmers here than data from Europe, California or Pennsylvania," she said.
