Measurements of Soil Carbon by Neutron-Gamma Analysis in Static and Scanning Modes

The overall goal of this procedure is to measure soil carbon in specific sites or across a field. This method is based on the application of inelastic neutron scattering quietness to soil carbon analysis. The main advantage of this technique is that it's a nondestructive in situ method for measuring soil carbon in large volumes of soil.

The analysis of gamma rays is created when neutron interact with soil elements allow us to measure soil carbon in upper 10 centimeter of layers of soil. This method can be used in stationary mode at one point in the field like a traditional soil sample or in scanning mode where the average soil carbon for the whole field is measured. Before working with the INS system, obtain materials for designating a restricted area around the neutron generator.

Verify that the neutron generator emergency interrupt button is unobstructed. Check the power level indicator on the battery charger. Recharge the batteries if fewer than three red lamps are illuminated.

Once the power level is acceptable, turn on the power inverter and the system laptop. Open the data acquisition program. For each detector, check that the DAC high voltage, the DAC offset, and the slow filter energy coefficient match previously defined values.

It is critical for the functionality of the whole INS system that all detectors are berated in the optimal working condition. Then position a Caesium-137 source within five to 15 centimeters of all system detectors. Start acquiring spectra and check the position of the centroid of the 662 kiloelectron volt Caesium-137 peak on each detector.

If the centroids are not at the same channel, adjust the slow filter energy coefficient to bring the centroids to the same channel. The spectra of all three detectors are summarized after the spectra are acquired. So, it is important that the peaks of interest are set on the same channels.

Once the system is ready for calibration, store the Caesium-137 source. To begin calibration using pits with homogenous sand carbon mixtures with known carbon contents, position the INS system over the first calibration pit so that the neutron source projection will be centered on the pit. Clearly mark restricted area with edges no less than four meters from the neutron generator.

Post radiation area hazard signs at the perimeter of this area. Use the key to turn on the neutron generator. Verify that the indicator lamp is green.

Open the neutron generator control software and clear any faults. Set the pulse frequency, duty cycle, delay time, and extension time. Then fill in the neutron beam voltage and current.

Activate the neutron generator and wait for the voltage and current readings to stabilize. Then in the data acquisition software, start acquiring spectra. Collect data for one hour and then stop acquisition.

Save the acquired spectrum for each detector and close both programs. Turn off the neutron generator with the key and verify that the indicator lamp dims. Repeat the process for each calibration pit.

Then import the acquired data into a spreadsheet program. Locate the rows containing output count rates, input count rates, and real time. From these, calculate the lifetimes for all measurements.

Convert the gamma readings to counts per second using the measurement lifetimes. Repeat this process for each calibration pit. Then calculate the net INS spectrum for each pit.

Using analytical software, visualize the first net INS spectrum. Right click on the generated graph and click Show Info to bring up windows with A and B markers. Use the markers to select the 1.78 megaelectron volt peak.

Open the multipeak fitting function and set the baseline as linear and the boundaries as the graph cursors. Use the tool to auto pick the peaks and determine the peak area. Then select the 4.44 megaelectron volt peak and determine the peak area in the same way.

Repeat this process for each net INS spectrum. Calculate the net carbon peak area for each calibration pit from these values. Plot the net carbon peak areas versus the pit carbon.

Apply a linear fit to generate the calibration coefficient for the INS system. To conduct field soil measurements in static mode, apply the same technique used with the calibration pits. To conduct field soil measurements in scanning mode, first design a path for the system to scan the entire area of interest in an hour.

Plant flags at each turning point along the path. Start data acquisition and move the INS system along the predetermined path at the appropriate speed. After data acquisition is complete, turn off the system as previously described.

Analyze the data and calculate the soil carbon for the scanned area using the calibration coefficient. Carbon in the upper soil layer was measured with a traditional dry combustion method and with the INS system in static mode. The results agreed well and carbon distribution maps of the sampled field were very similar for the two methods.

It took about two months to finish the dry combustion measurements for the field site whereas the static mode INS measurements were completed in two days. Scanning mode INS measurements were compared to static mode measurements taken along the scanning path. The scanning mode net INS spectrum matched the static mode spectra within the limits of experimental error.

Further, the scanning mode data were acquired in one hour whereas it took five hours to collect the five sets of static mode data. This INS technique should greatly improve the ability of researchers to measure soil carbon in the field which will help soil scientists invent strategies for carbon sequestration and develop management practices that optimize soil productivity. The significance of an inelastic neutron scattering method versus a traditional soil sampling method is to increase speed of determining the soil carbon and the improved accuracy of analysis on a whole field basis.

Research is ongoing to expand this technique to measuring other soil elements and to measuring soil moisture.