The Dark Side of the Soil Carbon Cycle: Hydroxyl Radicals and Abiotic Co2 Production

Abstract

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) 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 material 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. The δ13C signature verified that DOC, not SOM, accounted for most of the abiotic CO₂. Consequently, short redox pulses can oxidize 5–20% of DOC via hydroxyl radicals produced by Fe(II) oxidation, adding a non‐microbial flux to total CO2 released from soil. Concluding, the common concept that all CO2 released from soils originates from microorganisms and roots needs to be adapted by abiotic oxidation triggered by reactive oxygen species (e.g. •OH). Such abiotic oxidation is especially important in soils with fluctuating redox conditions.

Keywords: Dark Fenton‐type reactions, Hydroxyl radicals, Ferrous iron, dissolved organic carbon, Redox fluctuations.