ELECTRONIC SUPPLEMENTARY MATERIAL
Fig. S1 NO3-, NH4+ and S concentrations in the soil solution as a function of time in the TZ soil (a, c and e) and the
FY soil (b, d and f) (+1 cm refers to 1 cm above interface; 0 cm equals the depth of interface; -2 cm and -5 cm refer
to 2 and 5 cm below the interface, respectively)
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When soil is further reduced, SO42- follows as an electron acceptor. Figure S1e and S1f shows total dissolved S
concentration in soil solution, which is the sum of SO42- and other S species. Because the solubility of sulfide is very
low, most of the S in solution will have been SO42-.
During the first flooding period, the trend of measured S concentrations in both soils was similar to the trend of
DOC (Fig. 2e and 2f), nitrate (Fig. S1a and S1b) and Mn concentrations (Fig. 4a): the initial concentrations of S in
the two top layers were higher than in the two bottom layers; additionally, S concentrations decreased in the two top
layers, while S concentrations increased in the two bottom layers except the sampling occasion at day 23 in the FY
soil. Here also, the initial higher S concentration in the top layers might be caused by the upward displacement of
SO42- during the flooding of the columns from the bottom to the top. And the first sampling occasion was only after
the whole column was flooded. The followed decrease of S concentrations at two top layers during the flooding
period might be because of re-adsorption on Fe (III) (hydr)oxides under aerobic conditions (Fig. 2a). The increase of
S concentrations at two bottom layers during the flooding period might be attributed to the reductive Fe dissolution
(Fig. 4c and 4d) releasing SO42- that was complex by Fe (III) (hydr)oxides.
After the first flooding period, S concentrations continuously decreased to around the detection limit in the TZ soil.
In the FY soil, S concentrations in the bottom two layers peaked at approximately 10 days and then decreased. In the
first drainage period, S concentrations again increased, then slowly decreased in the second flooding period and
steeply decreased in the second drainage period. The decrease of S concentrations could be due to re-adsorption on
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newly-formed Fe (III) (hydr)oxides during the drainage periods and the formation of FeS and other metal sulfides
during the flooding periods as the model predicted. The model predicts, in the TZ soil, that FeS is present at the last
sampling occasion at 2 cm below interface during the first flooding period, and it remains at the first sampling
occasion at this layer in the first drainage period, and later on FeS is present at two bottom layers during the second
flooding period, and remains at 5 cm below interface during the second drainage period. These model predictions
are consistent with the measured low S concentrations in the TZ soil. The model predicts, in the FY soil, that FeS is
formed during second flooding period, which is consistent with the observed low S concentrations at same period.
The observed increase of S concentrations during the first drainage period might be due to oxidation of FeS as
model predicted, and less complexed SO42- with Fe (III) (hydr)oxides in the FY soil than in TZ soil because of the
higher pH in the FY soil.
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