Longer and more varied crop rotations that use livestock manure for fertilization do offer numerous environmental advantages, but they do not contribute to carbon sequestration, based on a recent study.
Recent research from Iowa State University reveals that longer and more diverse crop rotations utilizing livestock manure provide many ecological benefits, yet they do not help in capturing carbon.
The study’s conclusions, published this month in Nature Sustainability, challenge existing beliefs and may affect various carbon-market practices aimed at alleviating climate change, according to Wenjuan Huang, an assistant professor specializing in ecology, evolution, and organismal biology.
“In a diversified cropping system, we assumed that more carbon would accumulate in the soil due to increased carbon input. However, our 20-year study showed that soil carbon levels remained unchanged, although these regenerative practices are beneficial in other respects,” stated Huang, who is one of the co-authors of the study.
The research utilizes data gathered from a long-term field trial at Marsden Farm, located just east of Boone, Iowa. Since 2001, it has compared a traditional two-year corn-soybean rotation with more complex three- and four-year rotations that incorporate a year or two of alfalfa, clover, or oats, significantly reducing synthetic nitrogen fertilizer for corn by using cattle manure instead.
While these longer rotations increase carbon inputs due to greater root diversity and manure usage, they also enhance microbial activity in the soil. This stimulates decomposition and leads to a rise in carbon dioxide emissions, which may negated the positive impacts of increased carbon. Soil samples from both the surface and deeper layers—over 3 feet down—showed similar organic carbon levels across all test types. Meanwhile, cores from diversified systems released more carbon dioxide during lab incubation over just a year.
By examining stable carbon isotopes in the soil emissions, the researchers determined that the accelerated decomposition in longer rotations was not solely consuming additional carbon inputs. All samples emitted comparable levels of carbon dioxide from corn residues, even though the standard two-year rotation had corn planted more frequently. This discovery indicates that the heightened decomposition in diversified cropping systems partly utilizes older organic matter from past corn crops, as noted by Huang.
The innovative carbon-tracking technique employed in this study, partly funded by a grant from the U.S. Department of Agriculture, could assist researchers and carbon markets in refining their models for forecasting changes in soil carbon levels.
“Isotopes enhance our comprehension of how long carbon stays in the soil. Essentially, we are able to inquire of soil microbes about their food sources,” remarked Steven Hall, a co-author who is now an assistant professor at the University of Wisconsin-Madison. He initiated and led the study while previously at Iowa State.
Despite not accumulating more carbon, diverse cropping systems still hold significant potential for positive climate effects. The speedy decomposition of soil organic matter increases the availability of nitrogen vital for crop growth, especially for corn. Research indicated a 70% higher conversion rate of organic nitrogen into plant-usable inorganic nitrogen in the soils of longer-rotation systems.
The increased nitrogen availability in these diverse cropping systems allowed manure to replace sufficient synthetic fertilizer, thereby cutting nitrous oxide emissions— a powerful greenhouse gas— by an estimated 60-70% in carbon dioxide equivalents. This factor could be important for carbon markets to take into account, according to Huang.
“Understanding the balance between carbon storage and nitrogen supply is crucial,” Huang emphasized.
