The Pattern of Temporal Redox Shifts Can Determine If Anaerobic FeII or CH4 Production Dominates

Temporal redox fluctuations may alter the biogeochemical dynamics of soil organic matter, iron (Fe), and greenhouse gas emissions across different pedoclimatic environments. However, it is less clear how the characteristics of these fluctuations (length, frequency, amplitude) impact biogeochemical rates. We hypothesized that anaerobic rates of Fe^III reduction and CH₄ production are sensitive to the length of soil oxygen deprivation. To test this hypothesis, we exposed a surface soil from the Luquillo Experimental Forest (Puerto Rico) to three lengths of O₂ perturbation during repeated redox oscillations: an anoxic interval of 6 d with oxic intervals of 8, 24, or 72 h. We found that shorter oxic intervals resulted in more anaerobic Fe^III reduction, while longer oxic intervals stimulated higher anaerobic CH₄ emissions (CO₂ fluxes did not change). Across the treatments, Fe reduction rates increased from 0.12 ± 0.02 to 0.26 ± 0.05 mmol kg^-1 h^-1, whereas the cumulative CH₄ decreased from 44.0 ± 4.7 to 12.7 ± 4.6 μmol kg^-1, by shortening the O₂ exposure from 72 to 8 h. We propose that short O₂ pulses stimulate Fe reduction by resupplying the Fe^III electron acceptor, but do not last long enough to inhibit microbial Fe reducers; conversely long O₂ pulses suppress microbial iron reducers to a greater extent than methanogens leading to enhanced CH₄ emissions. Thus, the length of periodic oxidant exposure selectively enhances less thermodynamically favorable anaerobic processes by modulating the competitiveness of dominate anaerobic bacteria, which is important for regulating greenhouse gas emissions in redox dynamic soils.