Nitrate input induced AVS and Fe(II) oxidation
Five sampling timepoints divided the incubation into four stages. Each
stage of the nitrate group had distinct physiochemical features. On Day
4, nitrate was consumed by 13.8% (from 7.74 to 6.82 mM, P< 0.001) but little nitrite (< 0.01 mM) was produced
(Figure S1A). Similarly, acid volatile sulfide (AVS) was consumed by
19.3% (from 0.47 to 0.38 mM, P < 0.001) and little
sulfate (from 0.029 to 0.033 mM) accumulated, suggesting an incomplete
AVS oxidation (Figure S1B). Nitrate and sulfate levels of control
sediment were too low to be detected (< 1 μM) while AVS was
around 0.53 mM. On Day 8, nitrate and nitrite levels barely changed but
sulfate doubled from 0.04 to 0.09 mM (P < 0.001), which
substantiated the AVS oxidation. Nitrate sediments on Day 16 featured a
further nitrate reduction and coupled AVS oxidation, with a slight
nitrate consumption (by 8.1%, P < 0.001), a
significant AVS consumption (by 56.96%, P < 0.001) and
a drastic sulfate accumulation (by 221%, P < 0.001).
This trend continued to the final stage (Day 16 - 32), except that
Fe(II) participated in nitrate reduction. By the end of the incubation
AVS was too depleted (~0.022 mM) to sustain nitrate
reduction and instead a drastic consumption of ferrous ion took place
(from 3.403 to 0.417 mM, by 87.75%). Interestingly, the levels of total
acid-soluble iron, i.e. the sum of Fe(II) and Fe(III), changed slightly
(~4.30%) so that most Fe(II) was oxidized to Fe(III)
(Figure S1D). Fe(II) consumption supported AVS depletion since Fe(III)
spontaneously oxidizes AVS and thus Fe(III) accumulation indicated
little AVS remaining. In summary, 0.450 mM AVS and 2.99 mM Fe(II) were
oxidized throughout the incubation, yielding 0.404 mM sulfate and 2.559
mM Fe(III) respectively. Correspondingly, 2.51 mM nitrate was reduced,
yielding merely 0.094 mM nitrite. Note that more nitrate in
stoichiometry was consumed than the sum production of sulfate and
Fe(III) if nitrate was denitrified to nitrogen gas.