Mineralogical and chemical analyses
All the soil samples collected both before and after the incubation were used to identify clay mineral types through X-ray diffraction (Shimadzu 7000, Cu-Kα, Ni, 40.0 kv, 30.0 mA, 0.04 degree) and to quantify clay mineral composition by using the spectral decomposition technique31. After the incubation the silt+clay fraction (< 53 μm) was isolated by wet sieving and shaking with beads and then repeatedly treated with 30% hydrogen peroxide followed by decantation of the supernatant and fresh application of hydrogen peroxide until SOM oxidation was completed, as demonstrated by lack of bubbling with further hydrogen peroxide addition. Hydrogen peroxide treatment was previously shown to have almost no impact on inter-layer structures of clay minerals30 and mineral surface characteristics46. Then the soil samples were rinsed with 60 mL distilled water five times to remove remaining hydrogen peroxide and dried at 60°C. The original soil samples were also used to measure (i) specific surface areas by the Brunauer, Emmett and Teller (BET) N2 adsorption method (Accelerated Surface Area and Porosimetry, 2020 System), (ii) soil and mineral pH (H2O) (volume: soil mass ratio, 2.5:1) using an electronic pH meter, (iii) non-crystalline iron (Feo) extracted using acid ammonium oxalate, sodium pyrophosphate and citrate–dithionite, and (iv) crystalline iron (Fed) extracted using sodium dithionite.