UC Santa Barbara

The transport of organic carbon (C) to deep mineral horizons in soils can lead to long-term C stabilization. In basaltic soils, C associations with short-range-ordered (SRO) minerals often lead to colloid-sized aggregates that can be dispersed and mobilized by changes in soil solution chemistry. In the montane forest region of Hawaii, basaltic soils are exposed to high rainfall and anoxic conditions that facilitate ferric (FeIII) (oxyhydr)oxide reduction. We explored the potential of iron (Fe)-reducing conditions to mobilize C by exposing the surface mineral horizons of three soils from the Island of Hawai'i (aged 0.3, 20, and 350ky) to 21days of anoxic incubation in 1:10 soil slurries. Mobilized C was quantified by fractionating the slurries into three particle-size classes (<430nm,<60nm,<2.3nm≈10kDa). In all three soils, we found Fe reduction (maximum Fe2+ (aq) concentration≈17.7±1.9mmolkg-1 soil) resulted in ~500% and ~700% increase of C in the 2.3-430nm, and <2.3nm size fractions, respectively. In addition, Fe reduction increased solution ionic strength by 127μScm-1 and generated hydroxyl ions sufficient to increase the slurry pH by one unit. We compared this to C mobilized from the slurries during a 2-h oxic incubation across a similar range of pH and ionic strength and found smaller amounts of dissolved (<2.3nm) and colloidal (2.3-430nm) C were mobilized relative to the Fe reduction treatments (p<0.05). In particular, C associated with the largest particles (60-430nm) was dispersed almost exclusively during the Fe reduction experiments, suggesting that it had been bound to Fe-oxide phases. Our experiments suggest that colloidal dispersion during Fe-reducing conditions mobilizes high concentrations of C, which may explain how C migrates to deep mineral horizons in redox dynamic soils. © 2014 Elsevier B.V.