Are there any effects of the agricultural use of chemical fertiliser on elements detected by airborne gamma-spectrometric surveys?

Mineral phosphorus fertilisers contain trace amounts of the radioactive isotopes of uranium (U) and thorium (Th) that may accumulate in the soil. This study investigated whether possible increases in gamma-radioactive elements in arable soils could be detected in an airborne gamma-spectrometric survey over a fragmented landscape in central France. Airborne gamma-spectrometric measurements of U, Th and potassium (K) over cultivated and forested areas were compared with respect to either lithological classes or soil units. The results were also compared to U, Th and K concentrations in 20 pairs of soil samples from the same area and ground gamma-spectrometric measurements of the soil sampling sites. There was significantly more U (estimated average increase of ~8%) in the arable soil samples compared to the forest samples. No significant differences were observed in the Th and K contents in the soil samples. However, the gamma-spectrometric surveys showed significantly higher concentrations for all three elements in arable as compared to forest areas, both for airborne (U: +18%; Th: +18%; K: +27% on average) and ground measurements (U: +21%; Th: +19%; K: +17% on average). The differences between the gamma-spectrometric measurements and the soil samples suggest that although there was an increase in the U content in the arable soils, the addition of fertiliser could not explain the differences in the Th and K contents that were observed in the gamma-spectrometric data. The consistency between the airborne and the ground gamma-spectrometric surveys further ruled out attenuation by the forest vegetation as an explanation. The results suggest that part of the effects of land use observed in the gamma data are related to differences in the soil properties; because the soil texture effects were ruled out in this study, the differences in the soil bulk density or moisture remain the two most probable effects to explain the gamma variations over forest/arable areas.