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.