The dark side of the soil carbon cycle: Hydroxyl radicals and abiotic CO2 production

Abstract

<p>Fenton-type reactions without light (Dark-Fenton) in some forest soils generate hydroxyl radicals (•OH) from ferrous iron [Fe(II)] and dissolved organic carbon (DOC) under fluctuating anoxic–oxic conditions. We hypothesized that Fe(II) concentrated in micropores (<10 μm) raises radical production in soil, exceeding electron donation solely by DOC, and that radical-mediated abiotic oxidation releases CO2. Four undisturbed humid forest soils, ranging from sandy loam to silty clay loam with contrasting parent materials, were incubated anoxically (∼14 days) and then exposed to oxygen for 24 h in the dark. We introduced hydrogen peroxide (5–300 μM), and the δ13C signature confirmed that the CO2 originated from DOC rather than from bulk soil organic matter (SOM). Soils with higher Fe(II) (∼35 μM) in clay-rich or metamorphic parent materials produced up to ∼25 nM •OH in 24 h and released ∼20–25 % additional CO2 upon short-term re-oxygenation. Volcanic soils with ∼15 μM Fe(II) generated fewer radicals (∼5–10 nM) and only 5–10 % extra CO2. Micropores concentrated Fe(II), intensifying •OH formation and drove an abiotic CO2 flux that reached 25 % of total soil respiration. We condensed this effect into a single coefficient, ready for implementation in soil carbon models. Concluding, short redox pulses can oxidize 5–20 % of DOC via hydroxyl radicals produced by Fe(II) oxidation, adding a non-microbial (abiotic) flux to the total CO2 released from soil. These results revise the common view that soil CO2 originates exclusively from microbial and root respiration by revealing a sizeable abiotic contribution under fluctuating redox conditions.</p>