Analysing soil microbial communities is often hampered by DNA adsorption on soil organic compounds such as humic acids. However the role of humic acids in DNA adsorption and stability in soils remains controversial. To characterize DNA-humic acid interactions, we studied DNA adsorption on two commercially available humic acids and a soil humic acid extracted from an Andosol. Desorption of the adsorbed DNA using 4 different solutions-distilled water, 0.1 M NaCl, 0.1 M sodium phosphate buffer (pH 6.0), and 1% sodium dodecyl sulfate solution-was also studied to understand the mechanism of DNA adsorption on humic acids. Here, we show that humic acids play an important role in DNA adsorption to soils. DNA molecules were adsorbed on the humic acids, with adsorption increasing proportionally with the DNA concentrations in the solution. The adsorption on all humic acid samples was fitted with Freundlich equation, and the parameters obtained from the equation indicated a high affinity between the humic acids and DNA molecules. The total amount of DNA desorbed by the 4 solutions was less than 2% of the total DNA adsorbed on all the humic acids. The results demonstrate that DNA molecules are able to bind strongly to humic acids by ligand binding, hydrophobic interaction, aggregation, or precipitation.

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