Here, we present the protocol for in situ measurement of soil carbon using the neutron-gamma technique for single point measurements (static mode) or field averages (scanning mode). We also describe system construction and elaborate data treatment procedures.
Method Article
Measurements of Soil Carbon by Neutron-Gamma Analysis in Static and Scanning Modes
In This Article
Summary
Abstract
The herein described application of the inelastic neutron scattering (INS) method for soil carbon analysis is based on the registration and analysis of gamma rays created when neutrons interact with soil elements. The main parts of the INS system are a pulsed neutron generator, NaI(Tl) gamma detectors, split electronics to separate gamma spectra due to INS and thermo-neutron capture (TNC) processes, and software for gamma spectra acquisition and data processing. This method has several advantages over other methods in that it is a non-destructive in situ method that measures the average carbon content in large soil volumes, is negligibly impacted by local sharp changes in soil carbon, and can be used in stationary or scanning modes. The result of the INS method is the carbon content from a site with a footprint of ~2.5 - 3 m2 in the stationary regime, or the average carbon content of the traversed area in the scanning regime. The measurement range of the current INS system is >1.5 carbon weight % (standard deviation ± 0.3 w%) in the upper 10 cm soil layer for a 1 hmeasurement.
Introduction
Knowledge of soil carbon content is required for optimization of soil productivity and profitability, understanding the impact of agricultural land use practices on soil resources, and evaluating strategies for carbon sequestration1,2,3,4. Soil carbon is a universal indicator of soil quality5. Several methods have been developed for soil carbon measurements. Dry combustion (DC) has been the most widely used method for years6; this method is based on field sample collection and laboratory proces....
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Protocol
1. Construction of the INS system
- Use the general INS system geometry shown in Figure 1.
Figure 1. INS System Geometry. Please click here to view a larger version of this figure.
- Use the INS system design shown in Figure 2.10
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Results
Soil INS & TNC and TNC gamma spectra
A general view of the measured soil gamma spectra is shown in Figure 4. The spectra consist of a set of peaks on a continuous background. The main peaks of interest have centroids at 4.44 MeV and 1.78 MeV in the INS & TNC spectra. The second peak can be attributed to silicon nuclei contained in soil, and the first peak is an overlapping peak from carbon and silicon nucle.......
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Discussion
Building on the foundation established by previous researchers, the NSDL staff addressed questions critical to the practical and successful use of this technology in real world field settings. Initially, NSDL researchers demonstrated the necessity to account for the INS system background signal when determining net carbon peak areas.11 Another effort showed that the net carbon peak area characterizes the average carbon weight percent in the upper 10 cm soil layer (regardless of carbon depth distri.......
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Disclosures
The authors have nothing to disclose.
Acknowledgements
The authors are indebted to Barry G. Dorman, Robert A. Icenogle, Juan Rodriguez, Morris G. Welch, and Marlin Siegford for technical assistance in experimental measurements, and to Jim Clark and Dexter LaGrand for assistance with computer simulations. We thank XIA LLC for allowing the use of their electronics and detectors in this project. This work was supported by NIFA ALA Research Contract No ALA061-4-15014 "Precision geospatial mapping of soil carbon content for agricultural productivity and lifecycle management".
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Materials
| Name | Company | Catalog Number | Comments |
|---|---|---|---|
| Neutron Generator | Thermo Fisher Scientific, Colorado Springs, CO DNC software | MP320 | |
| Gamma-detector: | na | ||
| - NaI(Tl) crystal | Scionix USA, Orlando, FL | ||
| - Electronics | XIA LLC, Hayward, CA | ||
| - Software | ProSpect | ||
| Battery | Fullriver Battery USA, Camarillo, CA | DC105-12 | |
| Invertor | Nova Electric, Bergenfield, NJ | CGL 600W-series | |
| Charger | PRO Charging Systems, LLC, LaVergne, TN | PS4 | |
| Block of Iron | Any | na | |
| Boric Acid | Any | na | |
| Laptop | Any | na | |
| mu-metal | Magnetic Shield Corp., Bensenville, IL | MU010-12 | |
| Construction sand | Any | na | |
| Coconut shell | General Carbon Corp., Patterson, NJ | GC 8 X 30S | |
| Reference Cs-137 source | Any | na |
References
- Potter, K. N., Daniel, J. A., Altom, W., Torbert, H. A. Stocking rate effect on soil carbon and nitrogen in degraded soils. J. Soil Water Conserv. 56, 233-236 (2001).
- Torbert, H. A., Prior, S. A., Runion, G. B. Impact of the return to cultivation on carbon (C) sequestration. J. Soil Water Conserv. 59 (1), 1-8 (2004).
- Stolbovoy, V., Montanarella, L., Filippi, N., Jones, A., Gallego, J., Grassi, G.
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