Fig. 7: Relationships of the concentrations of selenium (Se) or sulfur (S) in grass leaves and mixed roots with the water-soluble Se or S speciation in Kohala topsoils (0–10 cm), when excluding the site S3.

Panel a shows the correlations between the concentrations of Se in plants and the concentrations of Se oxyanions in ultrapure water extracts. Similarly, panel b shows the correlations between the concentrations of S in plants and the concentrations of sulfate or of larger and aliphatic organic S in ultrapure water extracts. Panel c shows the correlations between the concentrations of Se in plants and the concentrations of smaller, less negatively charged aromatic Se and small hydrophilic Se in ultrapure water extracts. Similarly, panel d shows the correlations between the concentrations of S in plants and the concentrations of less negatively charged aromatic S and small hydrophilic S in ultrapure water extracts. The correlation coefficient (r²) and associated p values were determined using 2-tailed Spearman correlation. Note that the correlations between the concentrations of total Se in grass leaves and mixed roots and the concentrations of water-soluble Se associated to (organo)mineral (nano)particles is given in Supplementary Fig. 19. For all presented data, the error bars represent standard deviations, which consider i) the standard deviation of plant Se, As and S concentrations that were determined in triplicate; and ii) the standard deviation obtained during the SEC peak deconvolution.