Iron plays important roles in ecosystem function, including structural roles as part of the soil and colloid matrix; as a sorbent for nutrients, carbon and trace elements; and as a mediator of electron-transfer reactions. Our recent work on soils from the Luquillo Critical Zone Observatory in Puerto Rico explores how fluctuations in redox conditions influence Fe reduction rates, phosphorous mobilization and carbon mineralization. We imposed systematic redox oscillations to soil slurries and examined dynamic behavior of Fe, C and P in the aqueous and dilute acid extractable phases. We constrained these oscillations using three fundamental parameters: (1) The oscillation frequency, measuring the length of time between redox shifts; (2) the amplitude of those fluctuations; and (3) the length of time oxic or anoxic conditions persist — which can also be expressed as the time ratio between the two conditions (τox:τred). First, we show that Fe-reduction rates increase dramatically over 28 - 56 days when high amplitude (0 - 21% O2) redox oscillations are imposed at a set τox:τred ratio of 1:6, across frequencies ranging from 3.5 to 14 days (i.e., τox ranging from 12 h to 48 h). The stimulation of iron reduction coincides with the formation of an iron(III) phase that can be rapidly reduced in microbial Fe reduction assays. Second, in a separate experiment, we imposed 2.9- and 11.6-day redox oscillations on a similar soil that was however first pre-conditioned to oscillating redox conditions and found Fe reduction rates were substantially higher in the shorter frequency treatment, which comprised τox lengths of only 10 h. Evidently, when the length of oxic conditions becomes short enough, the timescale for Fe reduction can change. These results highlight how the dynamics of redox conditions can affect rates of Fe reduction, which in turn influence ecosystem processes—including C mineralization and P availability—that depend on iron behavior.

Timescales of soil redox oscillations and the role of iron in the critical zone

249th National Meeting (2015)

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Timescales </b>of soil redox oscillations and the role of iron in the critical zone