Volcanic ash soils retain the largest and most persistent soil carbon pools of any ecosystem. However, the mechanisms governing soil carbon accumulation and weathering during initial phases of ecosystem development are not well understood. We examined soil organic matter dynamics and soil development across a high-altitude (3,560-3,030 m) 20-kyr climate gradient on Mauna Kea in Hawaii. Four elevation sites were selected (~250-500 mm rainfall), which range from sparsely vegetated to sites that contain a mix of shrubs and grasses. At each site, two or three pits were dug and major diagnostic horizons down to bedrock (intact lava) were sampled. Soils were analyzed for particle size, organic C and N, soil pH, exchangeable cations, base saturation, NaF pH, phosphorous sorption, and major elements. Mass loss and pedogenic metal accumulation (hydroxlamine Fe, Al, and Si extractions) were used to measure extent of weathering, leaching, changes in soil mineralogy and carbon accumulation. Reactive-phase (SRO) minerals show a general trend of increasing abundance with increasing rainfall. However carbon accumulation patterns across the climate gradient are largely decoupled from these trends. The results suggest that after 20 kyr, pedogenic processes have altered the nature and composition of the volcanic ash such that it is capable of retaining soil C even where organic acid influences from plant material and leaching from rainfall are severely limited. Carbon storage comparisons with lower-elevation soils on Mauna Kea and other moist mesic (2,500 mm rainfall) sites on Hawaii suggest that these soils have reached only between 1% and 15% of their capacity to retain carbon. Our results suggest that, after 20 kyr in low rainfall and a cold climate, weathering was decoupled from soil carbon accumulation patterns and the associated influence of vegetation on soil development. Overall, we conclude that the rate of carbon supply to the subsoil (driven by coupling of rainfall above ground plant production) is a governing factor of forms and amount of soil organic matter accumulation, while soil mineralogy remained relatively uniform.

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.