Faster redox fluctuations can lead to higher iron reduction rates in humid forest soils

Iron (Fe) minerals play an important role in carbon (C) and nutrient dynamics in redox fluctuating soils. We explored how the frequency of redox oscillations influence Fe reduction rates and C content in Puerto Rican soils. We hypothesized that iron reduction rates would be faster during short oscillation periods than in longer oscillation periods. Surface soils from an upland valley in a humid tropical forest were exposed to systematic redox oscillations over 49 days. The oxidation events were triggered by the introduction of air (21% O-2), maintaining the time ratio under oxic or anoxic conditions at 1:6 (tau(ox)/tau(anox)). After pre-conditioning the soil to fluctuating redox conditions for 1 month, we imposed 280- and 70-h (or 11.67- and 2.5-day) redox oscillations, measuring Fe-II every few days. We found that by the end of the experiment, Fe reduction rates were higher in the short oscillation period (tau(ox) = 10 h, tau(anox) = 60 h) than in the long oscillation period (tau(ox) = 40 h, tau(anox) = 240 h). Carbon and nitrogen loss however was similar for both treatments. These results suggest the characteristics of redox fluctuations can alter rates of Fe reduction and potentially influence ecosystem processes that depend on iron behavior.