An iron-dependent burst of hydroxyl radicals stimulates straw decomposition and CO2 emission from soil hotspots: Consequences of Fenton or Fenton-like reactions

Plant residues in soil create temporal and spatial hotspots of extremely high microbial activities leading to very intensive greenhouse gas (GHG) fluxes that challenge our mechanistic understanding and predictive power. Using a series of well-controlled soil microcosm experiments, we examine how abiotic processes (e.g., iron reduction-oxidation cycling) at residue/soil interfaces contribute to hotspot dynamics. We quantify for the first time the contributions of microbially-initiated Fenton reactions, which produce strongly oxidizing hydroxyl radicals (HO ), to organic matter solubilization and mineralization in hotspots 0–3 mm from the litter surface. The concentrations of ferrous iron (Fe2+), hydrogen peroxide (H2O2) and HO were 2.1–3.0, 3.0–9.0 and 2.6–2.8 times higher, respectively, at the straw-soil interface than in the bulk soil. Thus, iron minerals, especially in concert with microorganisms, produce a burst of hydroxyl radicals that explain extremely high GHG fluxes from soil hotspots. Our findings highlight how Fe minerals and microorganisms synergistically influence global carbon cycling. These synergistic effects should be accounted for mechanistic understanding of processes in soil hotspots.