While the rhizosphere is crucial to the overall global carbon cycle, scientists still view its exact role as an open and debated question. A team of researchers at UC Santa Cruz and Stanford University are leading a new project in which they aim to use dynamic imaging and modeling of carbon in the rhizosphere to answer these longstanding questions. What they find will inform efforts to capture and store carbon from the atmosphere in order to combat climate change.
Associate Professor of Electrical and Computer Engineering Shiva Abbaszadeh will lead the new, multidisciplinary project, which was funded by a three-year, $3.7 million grant from the Department of Energy’s Energy Earthshot Initiative. UCSC Professor of Environmental Studies Weixin Cheng and Stanford Professors of Radiology Craig Levin and Adam Wang are co-investigators on the project.
“With the radiologists' work, we will be able to have a convincing, dynamic story of how the young, fresh root carbon moves and changes things in the soil,” Cheng said.
The project is focused on understanding two key pathways for organic carbon stabilization and destabilization in the rhizosphere.The first pathway is the soil matrix: how carbon is moved by roots into or out of the soil, which is a mix of minerals and different sized particles made more complicated by drying and rewetting cycles in the earth. The second pathway is exudate-driven microbial turnover, which refers to how living and dead microbes contribute to the long-term stable carbon in the soil.
Rather than testing a specific hypothesis, the team is approaching this research with a mindset of discovery around the dynamic movement of carbon through these pathways. Although the two pathways are ultimately linked in nature, the researchers are approaching them separately for the purpose of the project, but hope to bring them together in the long term.
The team aims to understand these pathways by using next-generation tomography technology to create three-dimensional imagery and other measurements of carbon in root soil mixtures. Tomography is imaging by sections or sectioning that uses any kind of penetrating wave and has a wide range of applications, from computed tomography (CT) and positron emission tomography (PET). Using these techniques for this project allows the researchers to get a picture of what is happening under the soil, just as an Ultrasound allows doctors to see what is going on under the skin.
Abbaszadeh is an expert in imaging and tomography, and specializes in developing new instrumentation, computational approaches, and measurement techniques to advance the technology for applications such as medical imaging and high energy-physics. This project takes advantage of her expertise to address the unique challenges of imaging carbon processes beneath the soil surface on both short and long time scales.
The researchers will use PET and CT, techniques which will allow them to see carbon deep in the soil as compared to traditional optical methods. In the past, other researchers have used methods to mimic the rhizosphere with artificial soil, but these techniques make it impossible to study the microbiome and other natural factors that are crucial to carbon processes.
Their specialized PET and CT methods will enable the team to look two to ten centimeters into the soil and get a clearer, continuous, three-dimensional picture of roots, fungi, microbes and soil structure. The collaborators developed these methods together with the support of a previous DOE grant.
“We are utilizing novel approaches to enable accurate material decomposition and enhance image quality in the high attenuating soil environment,” Abbaszadeh said.
Cheng and his lab will develop a specialized plant, soil, and root system so that the team can capture images of an intact system, rather than pulling samples that may disrupt the flow of carbon flow through the soil matrix. Then, the researchers will use various carbon isotopes to study how carbon moves through these soil systems at different time scales — from a temporal resolution of just a few minutes to thousands of years.
The research team will create CZT and PET image
of a specialized plant, soil, and root model to better
understand how carbon flows through an intact system.
They are using a novel approach to study carbon at such a large temporal scale. Carbon 11, a radioactive isotope with a short half life, will be used to measure carbon that gets pushed out by roots in a matter of minutes. Carbon 13, a stable isotope, will help them understand at the scale of thousands of years. Carbon 14, another radioactive carbon with a very long half life, will bridge the gap between the two.
The experiments with the radioactive isotopes carbon 11 and 14 will be conducted at Stanford using their Cyclotron and Radiochemistry Facility (CRF), which has space and radiotracer infrastructure dedicated to this project and can logistically accommodate the challenges of using carbon 11 given its short half life. Data from these experiments will be modeled and analyzed back at UCSC’s Baskin School of Engineering in Abbaszadeh’s Radiological Instrumentation Lab.
In the long term, Cheng hopes this will move the field toward a holistic, location-specific approach to managing the plant and soil systems, rather than approaching plant management and soil management separately. Ultimately, he hopes this project will help resolve long-standing questions in the field and inform the future of carbon sequestration research and development.
“If our study is successful, we'll be able to address and connect the dilemma between young carbon and old carbon,” Cheng said. “The old approach in carbon studies has created a lot of arguments and counterarguments. We’re hoping that with this data people will see that active root carbon can change the old carbon in mass quantities. If we get there, that has major implications for the global carbon cycle, and what we should do with carbon sequestration.”
The Department of Energy launched the Energy Earthshot Initiative to spur the country’s decarbonization efforts. The Initiative provides $264 million in funding for 29 different projects, with a range of focuses including industrial decarbonization, carbon storage, and offshore wind. The UCSC/Stanford project fits under the “Carbon Negative Shot,” which is aimed at removing and storing carbon dioxide from the atmosphere.
