The Pattern of Temporal Redox Shifts Can Determine If Anaerobic FeII or
CH4 Production Dominates
Abstract
Temporal redox fluctuations alter the pools of reducible FeIII and
greenhouse gas emissions in humid upland soils. However, it is less
clear how the characteristics of these fluctuations (length, frequency,
amplitude) impact biogeochemical rates. We hypothesized that anaerobic
rates of FeIII reduction and CH4 emissions are sensitive to the length
of soil oxygen deprivation. To test this hypothesis, we exposed a
surface soil from the Luquillo Experimental Forest 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 FeIII reduction, while longer oxic
intervals stimulated higher anaerobic CH4 emissions (CO2 fluxes did not
change). We propose that short O2 pulses stimulate Fe reduction by
resupplying the FeIII 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.