Illuminating the elements under our feet

A vast field of tall, skinny trees sways in a light breeze. In the future, poplar trees in a scene like this could be a source of sustainable fuel to power aircraft or heavy vehicles. They could also help us store more carbon in the soil. Both bioenergy and carbon storage are important strategies for reducing the amount of carbon dioxide in our atmosphere that causes climate change. 

But before that can become a reality, scientists need a better understanding of what is happening both above and below the soil. Researchers at the Department of Energy’s Oak Ridge National Laboratory have led the way in using a unique measurement tool to analyze plants, soil, and other biological samples. It’s part of the work that scientists have done for more than a decade at the Center for Bioenergy Innovation (a DOE Bioenergy Research Center) to improve the growth of bioenergy crops.

Investigating below the surface

Plants and their roots are part of incredibly complex and diverse ecological systems. Two nearby plots of soil or even the same plot at different depths can diverge in their structure, moisture, and microbes. 

In particular, the chemical elements in the soil can have major effects on how plants grow and respond to stress. Nitrogen, phosphorous, calcium, and other elements are essential to plant growth and survival. How plants take up and use these elements depends on their genes and environment. While scientists know that there are relationships between all of these different factors, they don’t have a lot of solid information about those relationships. That lack of data makes it hard to know what farmers could change to affect plant growth and carbon storage. 

Being able to quickly, accurately, and in detail find out what elements are present in plants and soil would greatly increase our understanding of the connections between what happens above and belowground. This information could help us develop biotechnology that helps farmers grow bioenergy crops more sustainably. It could also help climate scientists better understand how plants respond to environmental change, including wildfires. 

A blast of a tool

Laser-induced breakdown spectroscopy (LIBS) offers a powerful tool for investigating the chemical elements in plants and soil. Like detectives use human fingerprints to find out more about a crime scene, LIBS offers scientists a way to create a unique chemical fingerprint of that sample. 

The tool scientists use for LIBS produces a high energy nanosecond laser pulse that creates a spark. That spark is so hot that it removes the top layer of the sample in a way that makes a plume of plasma. (Plasma is a state of matter where electrons separate from atoms, resulting in charged ions.) As the plasma cools, it gives off light – but not just any light. The wavelengths of light are specific to particular elements in the material being studied. The scientists then use a spectrometer that measures wavelengths of light and a detector that records the intensity of the light waves.  

In the past, scientists mainly used LIBS to monitor industrial processes and study materials used to manufacture goods. But Madhavi Martin, a scientist at DOE’s Oak Ridge National Laboratory, pioneered the use of LIBS to study biological materials like soils and plants.

Compared to other analysis methods for biological materials, LIBS is incredibly fast and efficient. If you’ve ever had soil tested in your garden, you had to send it off to a lab. In contrast, scientists can potentially use LIBS right in the field. With little to no preparation of the sample, scientists can get high-resolution results very quickly. LIBS can analyze samples in milliseconds and allow scientists to test more than 100 samples a day. It’s especially useful for detecting lighter elements that are important to biologists, like nitrogen and carbon. LIBS can also detect toxic elements that can reveal environmental contamination, like mercury and lead. 

Growing to understand LIBS’ uses

But scientists didn’t just jump into using LIBS widely. First, they wanted to ensure that it was going to give accurate results in a variety of circumstances. Martin started by applying the technique to air pollution and then soils. Other scientists started using it to test plant leaves, roots, and wood samples as well. 

Switchgrass is a particular area of interest. Native to North America, switchgrass is a versatile plant that grows in places a lot of other crops can’t. It’s a very promising plant for bioenergy, soil conservation, and carbon storage. In one study, Martin and her team used LIBS to analyze the elements in switchgrass samples that had been burnt to ash. Understanding the elements in this ash can help scientists improve methods to turn switchgrass into renewable biofuels. By analyzing samples from 11 different farms, they found that LIBS gave accurate results for this usage. 

Poplar is another promising bioenergy crop. Along with being a common native tree, poplar grows quickly and has a relatively simple genome compared to other trees. As a result, scientists could potentially change its genetics to improve its growth and carbon storage. LIBS can help scientists know where the elements are in poplar trees and how that relates to which genes are associated with which functions.

In one study, Martin and her team tested the limits of how little preparation is necessary to use LIBS. In the past, scientists had dried and ground up plants samples before testing them. That method both took a lot of time and resulted in less detailed data. There were methods that kept the plants whole – like embedding them in wax or freezing them – but those took even more time. 

In contrast, the team did as little preparation with as little equipment as possible, save the LIBS tool. They wanted to mimic what might be used in the field. With a scalpel and blade, they took root samples of greenhouse plants, just as a farmer might do. To sample the soil, they used a spatula. To attach the soil to the microscope slide, they used ordinary double-sided tape. 

The tests revealed what scientists do and don’t need to do to use LIBS effectively. The tool accurately analyzed the samples of the plants’ roots and shoots, revealing a number of major nutrients. The results from the fresh, unprepared samples were almost identical – if not more accurate – to those with more preparation. However, the soil testing didn’t go as well. Even though they removed the excess liquid, the soil didn’t stick well to the slides. It slid around and resulted in a lot of unhelpful “noise” in the data. Once the scientists dried out the soil and pressed it into pellets, the resulting data was much more reliable. Despite the challenge with the unprepared soil, the scientists concluded that testing both prepared and unprepared soil may be useful. Unprepared soil can provide some insights that can help scientists decide the complexity of sample preparation beforehand. 

With these studies and others, scientists are growing in their understanding of what LIBS can and can’t do as a tool. Martin is currently working with fungal biologists to examine fungi’s relationship with plants. She even used LIBS to assist police in a murder case! A professor from the University of Tennessee Knoxville had been called as an expert witness to analyze wood from a crime scene and asked Martin for assistance. By analyzing the samples using LIBS, she found that the wood matched wood from a different site. It was one piece of evidence that led to an eventual conviction. 

While LIBS is a powerful tool on its own, Martin and her team revealed its true potential. Without their creative thinking about its applications, LIBS wouldn’t be available to biologists to use. From bioenergy to ecology, these scientists have opened the door to new insights and discoveries.