Radionuclide-based prediction of Soil Organic Carbon: a proximal sensing approach for high-resolution soil mapping

Soil organic carbon (SOC) is a critical indicator of soil health and central to land management initiatives with climate change mitigation co-benefits. Conventional sampling and laboratory analysis however remain labour-intensive and costly, creating significant gaps in spatial and temporal coverage. These limitations hinder the development of explicit, quantitative, and spatially realistic SOC maps needed for environmental modelling, land management, and impact verification. Portable proximal sensing technologies, such as gamma ray spectrometry (GRS), offer a promising solution by enabling rapid, in situ measurements for high-resolution digital soil mapping.

We explore how radionuclide proxy measurements with portable gamma ray spectrometry can provide scalable, spatially explicit SOC estimates that can be integrated into pedometric frameworks. Soil samples were collected from two fields, one arable and one permanent pasture, on an estate in southwest UK. Radionuclide activity concentrations were measured at sample locations, and continuous walking surveys were conducted to generate spatial maps of measured radionuclide activity across both fields.

Correlation analysis and principal component analysis (PCA) were used to explore relationships between radionuclides and SOC. Results show that radionuclide activity concentrations are consistently and negatively associated with SOC, particularly thorium-232 (-0.77) and potassium-40 (-0.69). Elastic net regression and partial least squares regression (PLSR) identified thorium-232, potassium-40/uranium-238, potassium-40, and thorium-232/uranium-238 as consistently important variables (PLSR VIP scores 1.21, 1.12, 1.08, 1.07 respectively), emerging as strong indicators of SOC variation. These findings highlight the potential for radionuclide proxies to explain SOC distribution and offer insight into broader soil health dynamics across contrasting land uses.

The results support the adoption of portable gamma ray spectrometry as a transparent digital soil mapping tool addressing current gaps in spatial SOC representation. To ensure trust and reproducibility, protocols must be validated across a range of soil conditions, and conversion approaches from radionuclide proxies to SOC must be standardized (work currently advancing in the Joint FAO/IAEA Coordinated Research Programme D12015 on “Combining Gamma-Ray Sensing and Digital Technology for Soil Moisture and Soil Property Mapping”). Transparency is essential, from raw proxy measurements through to SOC map products, so that stakeholders can confidently use these data for decision-making. When applied with rigour and data traceability, this approach offers meaningful support for Climate-Smart Agriculture and sustainable land management strategies while reducing uncertainty in soil property mapping.