Abstract

Fenton-type reactions without light (Dark-Fenton) in some forest soils generate hydroxyl radicals (center dot 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 mu m) 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 (similar to 14 days) and then exposed to oxygen for 24 h in the dark. We introduced hydrogen peroxide (5-300 mu M), and the delta C-13 signature confirmed that the CO2 originated from DOC rather than from bulk soil organic matter (SOM). Soils with higher Fe(II) (similar to 35 mu M) in clay-rich or metamorphic parent materials produced up to similar to 25 nM center dot OH in 24 h and released similar to 20-25 % additional CO2 upon short-term re-oxygenation. Volcanic soils with similar to 15 mu M Fe(II) generated fewer radicals (similar to 5-10 nM) and only 5-10 % extra CO2. Micropores concentrated Fe(II), intensifying center dot 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.