Background levels of DTPA-extractable trace elements in calcareous soils and prediction of trace element availability based on common soil properties

Background levels of trace elements (TEs) are a crucial reference to assess the effects of human activity on soils globally. Traditionally background levels based on total TEs have been determined. However, levels of available TEs, the quantity of TEs that affects soil organisms and plant growth, have much greater relevance for biogeochemical processes. Therefore, here we determined background levels of diethylenetriaminepentaacetic acid (DTPA)-extractable contents of nine TEs in 100 soil samples from a region in Western Iran dominated by calcareous soils that has experienced low levels of human activity. Background levels determined using the iterative 2-δ technique were as follows (with comparable results using the distribution function method): Mn 11.6, Fe 6.9, Pb 1.6, Cu 1.1, Ni 0.6, Zn 0.6, Co 0.4, Cd 0.3, and Cr 0.1 mg kg⁻¹. We further assessed how much of the variance in percentage available of total TE levels (DTPA-extractable fractions) can be explained by common soil properties. Unsurprisingly, pH and calcium carbonate equivalency were crucial parameters and for example explained 34 % of Cr availability. Electric conductivity and clay/sand content were equally important and explained 28 % and 35 % of Cu and Ni availability, respectively. The factor that explained most of the variance across all TEs was the soil organic matter content (explained for example 41 % and 21 % of Mn and Zn availability). Interestingly, the DTPA-extractable fractions of Cu, Mn, and Ni increased 4-fold and Zn 2-fold with an increase in organic matter (OM) content from 0.1 to 2 % in such high-pH soils. Multiple linear regression modelling using common soil properties reliably predicted TE availability (R² values for measured versus predicted availability 84–93 %). These findings could help understand and predict TE behavior in calcareous soils and guide management practices.