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

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.