Applied In-Situ Gamma-Ray Spectrometry for Evaluating Erosion and Fertilizer Accumulation in the Tropical Soil Around Small Isolated Wetland

Abstract

Gamma-ray spectrometry has great potential to enhance the understanding of tropical soil properties, including those under intensive agriculture. We utilized gamma-ray spectrometry to assess erosion and the potential accumulation of NPK fertilizer in soil intensively cultivated for sugarcane monoculture, surrounding a small isolated wetland. Conducted in southeastern Brazil, our study involved measurements at 899 soil surface points using gamma-ray spectrometry to detect emissions from radionuclides such as uranium (U²³⁸), thorium (Th²³²), and potassium (K⁴⁰). Additionally, we measured the hydraulic conductivity (Ksat) at 42 points to evaluate the hydro-physical attributes of the soil surface. The γ-radiation exhibited a close relationship with soil texture, leaching, and prevailing chemical removal. Hydric erosion that affects upslope soil shows high Ksat and very low K. The results indicate that the low K radionuclide contents in the upper soil horizon surrounding the small wetland are attributed to advanced tropical pedogenesis intensified by hydric erosion, with a high correlation with soil texture. The spatial analysis indicates heterogeneity of eU, and to a lesser extent eTh, exhibited levels lower than those found in the parent material, saprolite, and upper soil layers from the weathering profile. This suggests that soluble fertilizers have limited impact on the concentration of the radionuclides in agricultural areas surrounding the small wetland. Significant mobility, particularly of eU, can be associated with pedogenesis. Gamma-spectrometry detected variations in the distribution of the autocorrelated eU and eTh, suggesting a complex bonding with Fe-oxides or clay contents influenced by the small wetland.