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Diversified cropping systems boost nitrogen supply but not soil carbon, ongoing field trial discovers

Longer, more diverse rotations of crops fertilized with livestock manure have many environmental benefits, but carbon sequestration isn't one of them, according to a new study led by Iowa State University researchers.

The findings, this month in Nature Sustainability, counter long-standing assumptions and could have implications for various carbon-market initiatives designed to help mitigate climate change, said Wenjuan Huang, assistant professor of ecology, evolution and organismal biology.

"In a diversified cropping system, there's more carbon input. So we have figured there would be more carbon stored in the soil. But actually, carbon levels in the soil didn't change over 20 years, though these regenerative management practices are still valuable in other ways," said Huang, one of the study's lead co-authors.

The study is based on data collected from the ongoing field trial at Iowa State's Marsden Farm just east of Boone, which since 2001 has compared a traditional two-year corn-soybean rotation to three- and four-year systems that mix in a year or two of alfalfa, clover or oats and replace most of the synthetic nitrogen fertilizer for corn with cattle manure.

A greater variety of roots and the addition of manure increase carbon input in the three- and four-year rotations. But putting more organic matter in the soil also stimulates microbial activity, which boosts decomposition and causes an uptick in carbon dioxide emissions that can counteract the increased carbon input.

Samples of both topsoil and cores a little more than 3 feet deep had similar soil organic carbon levels in all three types of test plots, while the soil cores from diversified cropping systems produced more carbon dioxide when incubated in the lab for a little more than a year.

By analyzing stable carbon isotopes in the soil core emissions, researchers found the intensified decay in longer rotations wasn't just using up the additional carbon inputs. All samples gave off similar levels of carbon dioxide from corn-plant residue, even though corn was grown more frequently in the standard two-year rotation. That shows the amped-up decomposition in diversified cropping systems feeds in part on older organic matter from previous corn plants, Huang said.

The novel carbon-chasing method used in the study could help researchers—and carbon markets—improve their models for predicting carbon change in soil.

"Isotopes improve our understanding of how long carbon can remain in soil. In a sense, we can ask the soil microbes what they had for dinner," said study co-author Steven Hall, now an assistant professor at the University of Wisconsin-Madison, who initiated and led the study during his previous position at Iowa State.

Even without sequestering more carbon, diversified cropping systems can have a positive climate impact. Soil organic matter breaking down faster also produces more of the type of nitrogen crops need to thrive, especially corn. Organic nitrogen converted into plant-feeding inorganic nitrogen at a rate about 70% higher in the longer-rotation soil samples, researchers found.

Heightened nitrogen availability in the diversified cropping systems helped manure supplant enough synthetic fertilizer to reduce emissions of nitrous oxide, a potent heat-trapping gas, by an estimated carbon dioxide equivalent of 60–70%. That also could be a relevant factor for carbon markets to consider, Huang said.

"The trade-off between carbon accrual and nitrogen supply is important," Huang said.