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.