Laser-induced breakdown spectroscopy measures carbon in soils in a flash

November 26—With increasing international concern about greenhouse gases and global warming, scientists have sought better and more cost-effective approaches for measuring changes in the amount of land-based carbon, much of which is located in soils. But because the amount of carbon varies considerably from one spot to the next, measuring significant changes in land-based carbon in fields, ranch lands and forests is difficult.

The newly developed Los Alamos technology will aid scientists as they try to determine how soil-based carbon is released into the air through natural or manmade causes, or whether atmospheric carbon is being absorbed into soils.

"Carbon truly is the currency of life," said Los Alamos ecologist David Breshears, a co-author of the paper. "For years, scientists have sought to improve accounting of Earth's carbon budget. This instrument will help us improve that accounting and our understanding of how soil carbon responds to different types of land management."

Using a technology called Laser-Induced Breakdown Spectroscopy—known simply as LIBS—scientists can now point a flashlight-sized laser device at a soil sample in the field or taken from the ground and determine how much carbon the sample contains. LIBS works by firing a brief, very intense pulse of laser light at a surface. The laser beam vaporizes a spot on the target sample that's roughly the size of a pencil point. A small spotting scope mounted near the laser source captures light emitted from the vaporized area and directs it to a spectral analyzer.

This analyzer, part of the LIBS device, looks at the signature of the light to determine what elements are present. Each element creates its own spectral fingerprint.

In field trials conducted on agricultural soil, the research team, led by Los Alamos physicist David Cremers and soil scientist Michael Ebinger, configured the spectral analyzer to measure the carbon signature coming from the soil. Subsequent field trials on woodland soils with Carbon-LIBS, or C-LIBS for short, have shown that the instrument can be used reliably to determine the carbon content of different soils.

The Los Alamos researchers plan to take the instrument to other areas to determine how C-LIBS can be used to quantify carbon in soils throughout the world.

"With C-LIBS, we now have a rapid way to collect many measurements of soil carbon and come up with better estimates of how much carbon is in a field plot or across a landscape, and how that amount of carbon is changing over time," Ebinger said.

Added Breshears, "Appropriate land management could increase soil carbon retention. Conversely, disturbances like fires could result in rapid losses of soil carbon. We plan to apply C-LIBS to help scientists get a better handle on these tradeoffs."

Laser-Induced Breakdown Spectroscopy technology itself is not new. Earth and space scientists have proposed using the technology to determine the composition of the surfaces of other planets, asteroids, moons and comets. The device is small, rugged and portable, and is easy to use and maintain. In other Earth-bound applications, LIBS can analyze samples from as far as 50 feet away, or can be used to analyze areas inside tight nooks and crannies that might not be convenient to sample by conventional methods.

The C-LIBS team includes Breshears, Cremers, Ebinger, Los Alamos biologist Pat Unkefer, and Joel Brown of the U.S. Department of Agriculture.