Mobilization of colloidal carbon during iron reduction in basaltic soils

- Fe-reduction mobilized > 5 g kg− 1 organic carbon from Hawaiian soils.
- Much of this mobilized organic carbon was colloidal (2.3-430 nm).
- Oxic incubations at similar pH mobilized less carbon than Fe-reducing incubations.
- Larger colloidal carbon (60-430 nm) was dispersed primarily during Fe reduction.

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 350 ky) to 21 days of anoxic incubation in 1:10 soil slurries. Mobilized C was quantified by fractionating the slurries into three particle-size classes (< 430 nm,< 60 nm,< 2.3 nm ≈ 10 kDa). In all three soils, we found Fe reduction (maximum Fe2 + (aq) concentration ≈ 17.7 ± 1.9 mmol kg− 1 soil) resulted in ~ 500% and ~ 700% increase of C in the 2.3-430 nm, and < 2.3 nm size fractions, respectively. In addition, Fe reduction increased solution ionic strength by 127 μS cm− 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 (< 2.3 nm) and colloidal (2.3-430 nm) C were mobilized relative to the Fe reduction treatments (p < 0.05). In particular, C associated with the largest particles (60-430 nm) 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.