PNNL

Abstract

Oscillating redox conditions are the norm in tropical soils; driven by an ample supply of reductants, high moisture, microbial oxygen consumption, and finely textured clays that limit diffusion. Yet the net result of variable soil redox regimes--their effect on biogeochemical fluxes, and susceptibility to environmental change--is poorly understood in tropical soils. Using a 44-day redox incubation experiment with humid tropical soils from Puerto Rico, we examined patterns of 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 anoxic. Prolonged anoxia promoted reductive dissolution of Fe-oxides, and lead 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 carbon, presumably due to acidification or direct release of OM from dissolving Fe(III) mineral phases. The average nominal oxidation state of water-soluble carbon 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 carbon was highest in static oxic soils. These results highlight the volatility of mineral-OM interactions in tropical soils. They suggest that shifting soil O2 availability has rapid impacts on exchanges between mineral-sorbed and aqueous C pools, accelerates the turnover of carbon, and changes available OM composition, implying that the periodicity of low-redox moments may control the fate of C in wet tropical soils.