Integrating proximal gamma ray spectrometry and apparent electrical conductivity to monitor SOC content at field-scale

Proximal soil sensing methods have been largely used as cost-effective ways for monitoring SOC content, despite numerous drawbacks can limit its effectiveness. In this context the aim of the present work was to evaluate the combination of two proximal sensors, an electromagnetic conductivity (ECa) meter and gamma-ray detector –it quantifies the natural soil radioactivity from Thorium, Uranium and Potassium isotopes– to obtain high detailed maps of SOC content. Field surveys were conducted in 2019 on five agricultural areas of northeastern Italy, whose texture ranged from silty clay to sandy loam, and SOC from 0.8% to 9.4%. A total of 712 undisturbed soil samples were collected at different depths down to a maximum of 45 cm, and analyzed for texture, bulk density, and SOC content. Proximal sensing data were collected simultaneously using ECa (CMD-Mini Explorer, GF Instruments) and gamma-ray (MS-2000 Agri Detector, Medusa) detector connected to a DGPS. Soil moisture was also recorded on the field for gamma-ray spectrum correction. Spatial dependence between ECa, natural soil radioactivity, physical and chemical soil properties was explored with factorial kriging analysis (FKA). Results showed that gamma-ray detector was generally correlated with soil texture, in particular clay content, in mineral soils. In contrast, SOC and soil moisture were predictors of gamma-ray sensed spatial variability in the peaty soil. ECa conductivity meter and gamma-ray detector proved to be effective for capturing the spatial and temporal variability of SOC, suggesting their use in the framework of carbon farming policies.