Does anoxic processing of dissolved organic matter affect organic–mineral interactions in paddy soils?

• Anoxic processing of DOM caused strong decrease in its adsorption to minerals.
• Effects were most pronounced for hydrous Fe oxides.
• Number and reactivity of minerals’ sorption sites did not change.
• Reduced DOM sorption, thus, seems to be due to compositional changes of DOM.

Adsorption of dissolved organic matter (DOM) in soil relates closely to hydrous Fe oxides, being one of the most important adsorbents. In paddy soils, reductive dissolution of hydrous Fe oxides may reduce the capacity of topsoils to retain organic matter. Dissolved organic matter produced and processed under anoxic conditions in the topsoils, thus, is leached into deeper soil horizons that still have considerable amounts of hydrous Fe oxides. Yet, the effect of the anoxic processing on adsorption of the leached DOM in oxic subsoil horizons is not known. We tested the adsorption of DOM from oxic and anoxic solution to minerals typical for paddy subsoils (goethite, ferrihydrite, ferruginous smectite). Adsorption of organic carbon (OC) to hydrous Fe oxides was up to five times larger for oxic than anoxic solutions. Ferruginous smectite adsorbed less OC than hydrous Fe oxides and differences between oxic and anoxic solutions were less pronounced than for the hydrous Fe oxides. Phosphate contained in the DOM test solution adsorbed strongly to the Fe oxide surfaces, while little to no phosphate was adsorbed by the smectite. Phosphate adsorption to hydrous Fe oxides was not affected by redox conditions. That finding is in line with the largely invariable equilibrium pH, suggesting no changes in number and reactivity of sorption sites at the hydrous oxides' surfaces. Also, we found no indications of reductive dissolution of minerals during the experiments. The reduction in DOM sorption, thus, seems to be due to compositional changes of DOM during anoxic processing.