Benchmarking the Inelastic Neutron Scattering Soil Carbon Method

Abstract

The herein described inelastic neutron scattering (INS) method of measuring soil carbon was based on a new procedure for extracting the net carbon signal (NCS) from the measured γ spectra and determination of the average carbon weight percent in the upper ∼8‐cm soil layer (AvgCw%8). The NCS extraction utilized the net‐INS spectrum, which was the difference between the INS and thermal neutron capture (TNC) spectra and the net‐INS system background spectrum. The proportionality between NCS and AvgCw%8 for any shape of soil carbon depth distribution was demonstrated theoretically. The theoretical model for NCS calculations accounted for carbon depth distribution and neutron and γ ray propagation laws in our analysis; previous model results were verified by comparison with a Monte Carlo simulation using Geant4. The experimental results confirmed the identified proportionality. The mobile INS system was calibrated using pits filled with synthetic soil; this calibration was used for AvgCw%8 determinations in INS field measurements. The AvgCw%8 was also determined by the dry combustion method. Benchmarking the soil carbon determination by INS demonstrated results that coincided with dry combustion technique (DCT) results (within experimental error limits). Given the agreement between these methods, the described INS measurement system can be recommended as a reliable alternative means for measuring soil carbon.