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 CO 2 . 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 δ 13 C signature confirmed that the CO 2 originated from DOC rather than from bulk soil organic matter (SOM). Soils with higher Fe(II) (∼35 μM) in clay-rich or metamorphic parent material produced up to ∼25 nM •OH in 24 h and released ∼20–25 % additional CO 2 upon short-term re-oxygenation. Volcanic soils with ∼15 μM Fe(II) generated fewer radicals (∼5–10 nM) and only 5–10 % extra CO 2 . Micropores concentrated Fe(II), intensifying •OH formation and driving an abiotic CO 2 flux that reached 25 % of total soil respiration. We condensed this effect into a single coefficient, ready for implementation in soil carbon models. Consequently, short redox pulses can oxidize 5–20% of DOC via hydroxyl radicals produced by Fe(II) oxidation, adding a non-microbial flux to the total CO 2 released from soil. These results revise the common view that soil CO 2 originates exclusively from microbial and root respiration by revealing a sizeable abiotic contribution under fluctuating redox conditions. • Ferrous iron exceeds DOC as main electron donor in dark Fenton oxidation. • Short anoxic–oxic pulses yield up to ∼25% abiotic CO 2 from DOC oxidation. • Fenton-type oxidation adds a nonmicrobial CO 2 flux to total soil emissions. • Iron availability and DOC aromaticity drive radical-mediated carbon losses.