Digging into soil organic matter

AMES, Iowa – A new study from Iowa State University scientists sheds light on the mystery of how soil organic matter forms across a wide range of ecosystems.

The study, published in the academic journal Nature Geoscience, drew on an archive of data on soils from across North America to find patterns in the chemical composition of organic matter. The analysis will help scientists understand the “rules” that govern the formation of soil organic matter and better predict the composition of soils, said Steven Hall, lead author and assistant professor of ecology, evolution and organismal biology.

Soil organic matter is the organic component of soils, mostly composed of residues left by dead plants and microorganisms. Understanding how soils break down or preserve organic matter is important because organic matter plays a central role in the kind of services soils can provide, such as whether they make good agricultural soils or if they can sequester carbon to slow climate change.

“If you pick up soil and it’s dark colored, many people would say that’s because of organic matter,” Hall said. “But what that matter actually is has been a debate for over 100 years. With this study we wanted to address the questions of whether organic matter is chemically similar across environments or if it varies predictably across environments.”

To answer those questions, the researchers turned to the National Ecological Observatory Network, which has collected an archive of various soil types from a huge range of environments in North America. Those samples included soils taken from tundra, tropical rainforests, deserts, prairies and beyond. They supplemented the data with soil samples taken from additional sites. These soils underwent a technique called nuclear magnetic resonance spectroscopy at laboratories at Baylor University. The technique allowed the researchers to analyze the chemistry of carbon atoms in the soil. 

The findings that emerged from the analysis revealed patterns that held true for soils across climates.

“Maybe the most robust finding is there appear to be really consistent tradeoffs in the chemistry of the organic matter,” Hall said. “If you have more of certain compounds, those samples always have less of other compounds.”

For instance, Hall said the researchers looked closely at lignin, a compound in many plants that’s a key component of wood. The study found that soils rich in lignin usually contained less protein, and vice versa. They also found patterns that seem to correspond to climate and mineral content, Hall said.

Understanding these patterns, or rules for how and why organic matter forms and persists in soil, will help scientist predict how soils in various ecosystems store carbon, Hall said. Carbon can contribute to climate change when released from soil into the atmosphere as a greenhouse gas. But an improved understanding of what kinds of soil carbon exist in different environments can paint a clearer picture of how soil carbon may affect climate in the future, and vice versa.

“The idea is to give us a conceptual model for predicting differences in organic matter among ecosystems and understanding why that organic matter might be present,” Hall said.