Quick look
A new study led by Iowa State University ecologists found the base rate of organic carbon decomposition in soil across the U.S. can vary by as much as tenfold, in part due to geochemical and microbial factors often underrepresented in current Earth system models.
AMES, Iowa - Soil stores more carbon than Earth's atmosphere and plants combined, which makes the speed of soil carbon's decomposition an important variable in models used to predict changes to our climate.
A new study by a team that includes four Iowa State University researchers found that even under uniform laboratory conditions, the rate of organic carbon decomposition in soil samples collected across the U.S. differed by up to tenfold, in part due to variations in soil mineral and microbial properties - factors that are often underrepresented in current Earth systems models.
Updating models with an improved understanding of the decomposability of organic carbon in soil - and its subsequent carbon dioxide emissions - could improve the accuracy of soil carbon feedback estimates in models, leading to more refined climate projections, said Chaoqun Lu, associate professor of ecology, evolution and organismal biology.
"For modeling simulations, we've traditionally simplified these variations by assuming carbon in similar soil types or in similar biomes decomposes at the same base rate, if no environmental changes are present. However, our findings show that the base rate actually varied a lot, even within the same soil or biome type. So this will really change a common practice," said Lu, the corresponding author of the study recently published in One Earth.
Leveraging lab data
Scientists who work on Earth systems models - complex simulations that estimate the global effects of intertwined biological, geochemical and physical processes - have long known the model estimates of soil carbon decomposition have large uncertainties.
In hope of better quantifying those variations, Lu's colleagues incubated soil samples from 20 sites in the National Ecological Observatory Network, a federally funded program that monitors ecosystems across the U.S. Over an 18-month period, researchers measured carbon dioxide emissions and key soil properties to inform a soil carbon model that estimated each sample's decay rate (how fast organic matter breaks down) and carbon use efficiency (how much of the decomposed carbon is taken up by microbes).
Machine learning-assisted analysis helped show which of the 26 types of measurements taken from the soil samples were most strongly associated with decomposition variation, said study co-author Bo Yi, a former postdoctoral research associate in Lu's lab and first author of the new study.
Some controlling factors were already well-established, such as soil type and levels of pH and nitrogen. Analyzing the incubation data also revealed a strong connection between decomposition rates and the levels of fungi and certain forms of iron and aluminum. The soil minerals are tightly linked to long-term stability of mineral-associated organic carbon, the portion of soil carbon that can persist in soil for decades or even hundreds of years.
Researchers combined their soil measurements with estimates of the base rates to build AI models that successfully captured the variations in those rates across 156 soil samples. They then applied that model to the continental U.S., creating maps that project carbon use efficiency and decay rates for individual land tracts measuring roughly 2.5 miles on each side. The maps show large regional variations in soil carbon dynamics across the U.S.
Implications for models and incentives
Scientists who work with soil carbon models or Earth systems models to project carbon-climate feedback are likely to use the study's final parameter maps to improve their simulations, Lu said.
"These geochemical and microbial metrics drive a lot of variability, and we haven't included them adequately in previous modeling work," she said.
Lu said the study also shows models should account for how different components of soil carbon decompose, as mineral-associated organic carbon lasts much longer than particulate carbon - mostly plant-derived organic matter in soil that decays in years instead of centuries.
Beyond improved modeling, Lu said the research could also inform conservation and carbon market programs by revealing regional differences in soil carbon vulnerability. In the Southwest, organic carbon in soil tends to decompose more rapidly, and once it is decomposed, a greater proportion of that carbon is released into the atmosphere as carbon dioxide. In the Northwest and the East, soil carbon decomposes more slowly, and a larger share of decomposed carbon ends up being retained in the soil as microbial biomass. Most of the Midwest falls somewhere between the extremes.
Those differences suggest that incentives for increasing soil carbon sequestration should consider soil's carbon retention persistence, she said.
"If carbon remains in the soil longer in certain areas, the same amount of carbon sequestration there could be more valuable than in other areas," she said.
Research team
Co-authors of the paper, "Joint geochemical and microbial controls on soil particulate and mineral-associated organic carbon decomposability across the contiguous US," include:
- Bo Yi, former ISU postdoctoral research associate in ecology, evolution and organismal biology
- Chaoqun Lu, ISU associate professor of ecology, evolution and organismal biology
- Wenjuan Huang, ISU assistant professor of ecology, evolution and organismal biology
- Wenjuan Yu, former ISU Ph.D. student in ecology, evolution and organismal biology
- Adina Howe, ISU professor of agricultural and biosystems engineering
- Samantha Weintraub-Leff, research scientist at National Ecological Observatory Network
- Steven J. Hall, assistant professor at University of Wisconsin-Madison
