Oscillating redox conditions are a common feature of humid tropical forest
soils, driven by an ample supply and dynamics of
reductants, high moisture, microbial oxygen consumption, and finely textured
clays that limit diffusion. However, the net result of variable soil redox regimes on
iron (Fe)
mineral dynamics and associated
carbon (C) forms and fluxes is poorly understood in tropical
soils. Using a 44-day redox incubation experiment with humid tropical forest
soils from Puerto Rico, we examined patterns in Fe and C transformations under four redox regimes: static anoxic, "flux 4-day" (4d oxic, 4d anoxic), "flux 8-day" (8d oxic, 4d anoxic) and static oxic. Prolonged
anoxia promoted reductive dissolution of Fe-
oxides, and led to an increase in soluble Fe(II) and amorphous Fe
oxide pools. Preferential dissolution of the less-crystalline Fe pool was evident immediately following a shift in bulk redox status (oxic to anoxic), and coincided with increased dissolved organic C, presumably due to acidification or direct release of organic matter (OM) from dissolving Fe(III)
mineral phases. The average nominal oxidation state of water-soluble C was lowest under persistent anoxic conditions, suggesting that more reduced organic compounds were metabolically unavailable for microbial consumption under reducing conditions. Anoxic soil compounds had high H/C values (and were similar to
lignin-like compounds) whereas oxic soil compounds had higher O/C values, akin to
tannin- and
cellulose-like components. Cumulative respiration derived from native soil organic C was highest in static oxic
soils. These results show how Fe minerals and Fe-OM interactions in tropical
soils are highly sensitive to variable redox effects. Shifting soil
oxygen availability rapidly impacted exchanges between
mineral-sorbed and aqueous C pools, increased the dissolved organic C pool under anoxic conditions implying that the periodicity of low-redox events may control the fate of C in wet tropical
soils.