libs: a NEW rapid method
for measuring soil carbon
As global warming becomes a more important issue on the domestic and world stage, analyzing greenhouse gases with consistency and accuracy becomes critical to designing policies and programs for mitigation and adaptation. 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.
Laser-Induced Breakdown Spectroscopy (LIBS) is a device that is being tested to measure soil carbon. If used in a carbon credit program, it could reduce the cost of measuring soil carbon levels.
Currently, the most common method for measuring soil carbon is the dry-combustion method. This involves taking a soil sample from the field and processing it in the lab. The sample is air-dried, sieved (to separate out the large organic particles), ground, weighted, and dry-combusted to measure the carbon.
Dry combustion is time-consuming and expensive, so scientists have been formulating other, more efficient ways to measure soil carbon. In addition to LIBS, researchers are examining inelastic neutron scattering (INS) and diffuse reflectance IR spectroscopy in the near-infrared (NIR) and mid-infrared (MIR) wavelengths.
The current LIBS machine was created by researchers at the Los Alamos National Laboratory in 2001, but the underlying technology is not new. Scientists have considered using the same technology to study the surfaces of planets and moons.
The most beneficial aspect of LIBS is its time- and money-saving potential. LIBS can issue an analysis of a soil sample in less than one minute, according to Cesar Izaurralde, of the Joint Global Change Research Institute and Pacific Northwest National Labs. Because labor and time are costly, LIBS would reduce the cost of soil carbon assessment. If used in a carbon credit program, it would in turn reduce the cost of monitoring soil carbon levels, potentially leaving more money for the landowner or producer. In addition, this method can be done directly in the field.
How it works: LIBS comes in portable or lab versions. Scientists take a soil sample, dry it, then use a compressor to pack it tightly into a “dime,” or a slim cylinder about the size of a half-dollar coin. The dimes are then placed on a track connected to the machine. A laser is shot at the sample, leaving a tiny dotted line. This is done five times for each sample, and an average reading is taken.
The heat from the laser excites the chemical bonds in the soil. Every molecular bond breaks, and the matter is reduced to its component elements, such as carbon, oxygen, and potassium. The emitted light from each element is sent into a spectrometer, and creates a wavelength graph. Each element has a unique wavelength that does not vary, so by analyzing the graph, scientists can determine the type and quantity of each element in the sample.
LIBS is currently being tested at Kansas State University because of the campus’ unique grass- and cropland sites. Post-doctoral research associate Autumn Wang is trying to find ways to make LIBS as accurate as the dry-combustion method. Though the speedy analysis makes the machine attractive, it has a precision of 4-5 percent and an accuracy of only 3-14 percent, according to Izaurralde.
To get precise carbon measurements from LIBS readings, Dr. Wang uses site-specific calibration curves based on data derived from the dry combustion process. The lab data from a specific soil are correlated with measurements from LIBS. This helps determine the accuracy of the new technology.
Several factors affect carbon measurements taken by LIBS, such as moisture, mineralogy, the particle size and density of the soil, and organic matter levels in the soil.
One of the dilemmas for LIBS is how to account for undecomposed organic matter, such as pieces of crop residue. In dry combustion, plant material is sieved and finely ground before analysis. In LIBS, it is left heterogeneously in the sample. Wang has created a homogeneous, artificial soil core that contains no visible plant residue and places it on a track next to a natural soil that contains visible plant matter. She shoots the laser straight down the track, first at the homogenous soil, then at the natural soil, to see what effect the undecomposed plant residue has on the measurements.
Also, Dr. Wang has discovered that the specific type of carbon compounds in the soil make a difference, and affect the LIBS analysis differently. This is because of the types of bonds the compounds are composed of. For example, organic carbon’s molecular structure has more complex bonds than inorganic carbon. LIBS reads them differently. This occurs with all elements, and is not exclusive to carbon.
The next step is to develop a standard calibration for LIBS, Dr. Wang says. Woodland soil from New Mexico is very different than agricultural soil from Colorado. Because soil type and structure change from area to area, LIBS needs to be calibrated every time it takes measurements from different soils to account for variations in the soil.
By including all of the chemical elements in the calibration curve, Dr. Wang says a standard calibration can be achieved. That way, factors such as moisture, soil type, soil density, and organic matter wouldn’t interfere.
Another benefit of using LIBS, besides offering quick measuring time, is that it measures other elements in the soil at the same time it measures carbon. This can assist scientists in determining overall soil fertility.
-- Katie Starzec, CASMGS Communications, Kansas State University
The portable LIBS being evaluated at Los Alamos National Laboratory.
Conference information, presentations, and papers are available at http://www.oznet.ksu.edu/ctec/Fall_Forum.htm
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