Soil organic matter
Gamma-ray spectrometry (GRS) is increasingly recognized as a valuable tool for assessing the content of total organic carbon (TOC) in soils. Naturally occurring radioactive elements—such as 40K, 238U and 232Th correlate with soil organic carbon content and provide indirect proxies for TOC content. This method enables high-resolution, non-destructive mapping of soil carbon, which is essential for monitoring land degradation, optimizing agricultural practices, and informing climate change mitigation strategies.
Indirect proxies
The key isotopes measured in GRS respond to variations in mineral content, texture, moisture, and organic matter. Various studies have shown how 40K is often positively correlated with TOC in dry mineral soils, possibly due to the association of potassium-bearing minerals with stable organic complexes. 232Th tends to show a negative correlation with TOC, especially in acidic conditions, where organic acids enhance thorium mobility and removal from the soil profile. 238U exhibits mixed behaviour depending on redox potential and organic matter chemistry.
The correlations are not always direct but emerge through shared dependencies on mineralogy, pH, and moisture content. These relationships allow GRS to serve as an effective predictor of spatial TOC variability when supported by ground truthing and statistical modelling.
Applications and Case Studies
A predictive model combining gamma-ray spectrometry with visible–near infrared (Vis–NIR) spectroscopy achieved high-resolution carbon stock maps with an R² of 0.77 and RMSE of 0.67 kg·m⁻². This approach, tested by Costantini et al. (2016) in Mediterranean arable fields, required only one soil sample per hectare and proved effective for field-scale soil carbon assessment.
In a study of dryland soils in Western Australia, Wong and Harper (1999) observed a strong positive correlation (r² = 0.89) between 40K and total organic carbon, indicating the suitability of potassium gamma signals as a proxy for TOC under arid conditions.
Their findings of Dierke and Werban (2013) highlighted how soil acidity enhances thorium mobility, making it a potentially useful inverse indicator for organic carbon levels. They included the pH of soil in their analysis and found a negative correlation between 232Th and TOC in soils with pH below 7.
The general accuracy and utility of commercially available GRS-based soil maps were assessed by Rhymes et al. (2023) in UK grassland systems, with a special focus on TOC analyses. While absolute values of TOC varied, the study confirmed the value of GRS data in capturing spatial differences and supporting site-specific nutrient and grazing management decisions.
Karjalainen et al. (2024) compared ground-based and airborne GRS systems over boreal forest soils. Their results showed that ground-based measurements offered predictive performance for TOC-related attributes.
Summary
These field trials have confirmed the method’s effectiveness across diverse landscapes. Airborne and ground-based gamma spectrometry have been used to detect spatial variations in TOC, offering a cost-effective, non-destructive alternative to traditional soil sampling. By integrating gamma-ray data with other sensing technologies, researchers can achieve highly detailed carbon assessments, essential for sustainable land management and climate change mitigation efforts.
References
Dierke, C., Werban, U., 2013. Relationships between gamma-ray data and soil properties at an agricultural test site. Geoderma 199, 90–98. https://doi.org/10.1016/j.geoderma.2012.10.017
Costantini, E.A.C., et al. (2016). Field‐Scale Mapping of Soil Carbon Stock with Limited Sampling by Coupling Gamma‐Ray and Vis‐NIR Spectroscopy. Soil Science Society of America Journal, 80(4), 954–964. https://doi.org/10.2136/sssaj2016.01.0018
Priori, S., Fantappiè, M., Bianconi, N., Ferrigno, G., Pellegrini, S., Costantini, E.A.C., 2016. Field‐Scale Mapping of Soil Carbon Stock with Limited Sampling by Coupling Gamma‐Ray and Vis‐NIR Spectroscopy. Soil Science Society of America Journal 80, 954–964. https://doi.org/10.2136/sssaj2016.01.0018
Wong, M., Harper, R., 1999. Use of on-ground gamma-ray spectrometry to measure plant-available potassium and other topsoil attributes. Soil Research 37, 267–278. https://doi.org/10.1071/S98038
Rhymes, J., et al. (2023). Evaluating the accuracy and usefulness of commercially-available proximal soil mapping services for grassland nutrient management. Precision Agriculture, 24, 898–920. https://doi.org/10.1007/s11119-022-09979-z
Karjalainen, V., Malinen, J., & Tokola, T. (2024). Comparison of two gamma-ray datasets measured with different methods and assessment of their performance to predict soil properties. International Journal of Forest Engineering, 36(1), 29–42. https://doi.org/10.1080/14942119.2024.2398947