A landmark 12-year study by Iowa State University agronomists has confirmed that digestate, the byproduct from anaerobic digestion of manure and corn stover, can significantly enhance soil organic carbon and potentially reduce the need for synthetic nitrogen fertilizers. Conducted at Sievers Family Farm near Stockton, Iowa, the study highlights the untapped value of digestate not just as a waste product, but as a strategic input in sustainable farming systems.

The ISU team, led by agronomy professor Fernando Miguez and Lisa Schulte Moore, director of ISU's Bioeconomy Institute, tracked digestate application over more than a decade on commercial corn and soybean acres. Their findings revealed that fields treated with digestate saw an average annual gain of 714 pounds of carbon per acre, with an overall 9% increase in soil organic carbon. In fields with initially low carbon levels, that gain reached a remarkable 47% increase over the study period.

"It's remarkable to see soil organic carbon increase that much over 12 years in a corn cropping system," said Schulte Moore, noting how difficult it is to build carbon while actively growing row crops. Digestate offers a rare advantage by delivering more stable carbon than raw manure, making it more effective at sequestering carbon in the soil.

Digestate comes in liquid and solid forms, both of which were applied to the Sievers farm fields. Unlike traditional manure, its composition post-digestion leads to longer carbon retention in soils. The practice may also generate additional phosphorus in soils, with the potential for tailoring digestate properties to match field-specific nutrient needs.

Bryan Sievers, co-owner of the farm, said the research validates his own experience. "It confirms what we've long believed - that digestate improves soil health and yields. Now we can begin assigning economic value to these best practices."

While the research confirms gains in soil carbon, ISU is also studying nitrogen dynamics, which could prove key to replacing synthetic nitrogen fertilizers with digestate-based solutions. This would mean not only lower input costs but reduced environmental impact.

Miguez noted that the team has conducted on-farm nitrogen trials, though data analysis is still pending. Early signs are promising, and if confirmed, farmers using digestate could improve nutrient efficiency, boost crop performance, and even market digestate to neighbors lacking anaerobic systems.

Anaerobic digestion remains relatively novel in Iowa agriculture, but its appeal is rising thanks to its dual role in manure management and renewable natural gas production. According to Miguez, the combined benefits now include soil improvement and nutrient cycling, all of which contribute to more sustainable and resilient U.S. agriculture.

"I hope people recognize the broad potential here," Miguez said. "We're talking about managing manure, producing energy, processing food waste-and now, proving that digestate can enhance soil health and cut our reliance on synthetic nitrogen."