How carbon contents affect the pXRF results of soil samples with different textures from Cerrado?

Elemental contents detected by portable X-ray fluorescence (pXRF) spectrometry may be affected by the carbon (C) presence, especially in large quantities. However, rare studies evaluated such effects on very weathered soils from the Brazilian Cerrado with variable texture. This study assessed how different C contents, texture, and sampling depth affect pXRF results of several elements. 106 soil samples were collected (0-5; 15-20 cm), with varying clay (0.4-71.4%) and C (0.3-3%) contents, conditions compatible with most Brazilian agricultural soils. PXRF results for Al, Ca, Cr, Cu, Fe, P, Si, Ti, V, Zn, and Zr were obtained from soil samples before and after oxidation of C with hydrogen peroxide; in parallel, C contents were determined by dry combustion. These results were used to determine the effects of C on pXRF results, taking into account C content, texture, and sampling depth. The C effect on pXRF results varied depending on the element. The largest differences were observed for Al and Si at both depths, and for Cr only at 0-5 cm. C contents between 1-2% most affected Al and Cr results (0-5 cm), increasing their values. In contrast, Si contents decreased when C<2% (0-5 cm) and C<1% (15-20 cm); Al values (15-20 cm) reduced when C>2%. Although P and Cu contents showed small variation ​​before and after oxidation, there were clear differences caused by texture at both depths, unlike other elements. Even low C contents can affect pXRF results for very weathered soils, especially for Si, Al, and Cr.