March 28, 2014 鈥� Carbon dioxide in the atmosphere may get the lion鈥檚 share of attention in climate change discussions, but the biggest repository of carbon is actually underfoot: soils store an estimated 2.5 trillion tons of carbon in the form of organic matter.
鈥淭he combined amount of carbon in vegetation and the atmosphere is only half of the carbon stored in soils,鈥� said Melanie Mayes of ORNL鈥檚 Environmental Sciences Division. 鈥淗ow quickly that carbon moves in and out of soils is one of the big uncertainties in modeling the carbon cycle.鈥�
With an eye on the big picture, Mayes and her colleagues are taking a closer look at soil carbon by studying nanoscale interactions between organic matter and minerals in soil. The team鈥檚 novel combination of neutron analysis and supercomputer simulations is providing experimental and theoretical data that challenge long-held assumptions in soil science.
鈥淚n environmental science, we tend to think about interactions between one organic compound and one soil mineral,鈥� Mayes said. 鈥淎 dissolved organic compound can form a chemical bond with a soil mineral, and that鈥檚 it.鈥�
In recent decades scientists have theorized that organic compounds might instead make bonds with soil minerals and other organic compounds, resulting in layers of organics on soil minerals. But the new conceptual model has lacked direct experimental verification.
鈥淥ur experiments are some of the first to interrogate the structure of layered organic matter鈭抦ineral interfaces, in part because there are only a limited number of techniques capable of studying these systems at a nanoscale resolution,鈥� said ORNL鈥檚 Loukas Petridis.
The researchers first used neutron reflectometry at ORNL鈥檚 Spallation Neutron Source to analyze a representative soil system. Since this neutron technique is sensitive enough to detect nanoscale differences in organic composition, the researchers were able to verify the formation of layers.
The team then simulated the system using molecular dynamics, which revealed the fundamental driving forces behind the molecules鈥� layered formation. Using neutron reflectometry and supercomputer simulations in tandem was key to the project鈥檚 success, said Petridis.
鈥淭hose two techniques haven鈥檛 been used together very much, if at all,鈥� he said. 鈥淲e鈥檙e excited about successfully melding these two techniques to get consistent results.鈥�
Gaining insight into soil systems at such a fundamental level not only has implications for carbon cycle models, it could also affect scientists鈥� understanding of related ecosystems. Most models that scientists use to understand soil contamination or fertility, for instance, do not consider the possibility of layered organics residing on soil minerals.
鈥淚t changes the whole way we think about how carbon, nutrients and contaminants interact with soils, which therefore affects fertility, water quality and the terrestrial carbon cycle,鈥� Petridis said.
The researchers hope to expand upon their initial research by further developing their experimental techniques and applying their analysis to a variety of soil systems.
鈥淲e have unique capabilities at ORNL to interrogate soil carbon and mineral interactions,鈥� Mayes said. 鈥淥ur latest research is an introduction to using these techniques to study a system at a very small scale. Now that we know this method works, we can begin to understand the fundamental principles governing organic carbon storage and reactions in soils.鈥�
The results of the team鈥檚 latest study are published in . Coauthors are ORNL鈥檚 Loukas Petridis, Haile Ambaye, Sindhu Jagadamma, S. Michael Kilbey, Bradley Lokitz, Valeria Lauter, and Melanie Mayes. The researchers were supported through ORNL鈥檚 Laboratory Directed Research and Development program.
- Morgan McCorkle, 865.574.7308, March 28, 2014