Fenton chemistry and reactive oxygen species in soil: Abiotic mechanisms of biotic processes, controls and consequences for carbon and nutrient cycling

Although most organic matter (OM) in soil is mineralized by microorganisms, the nonmicrobial processes, e.g., Fenton reactions and photo-degradation, strongly contribute to OM decomposition and CO2 emission and are frequently the chemical background of biotic transformations. Fenton oxidation is a catalytic reaction chain of hydrogen peroxide (H2O2) with ferrous iron (Fe(II)) and Fe (oxyhydr)oxides that generates highly reactive hydroxyl radicals (HO•) oxidizing OM to CO2. Globally, reactive Fe (oxyhydr)oxides store at least one quarter (~600 Gt) of organic C in soil, which may be subjected to Fenton reactions, in which nano-sized Fe (oxyhydr)oxides act as nanocatalysts. The Fenton mechanisms depend on the sources and pathways of reactive oxygen species (ROS): O2•−, H2O2 and HO•. Given that microorganisms continuously produce ROS, biotic Fenton chemistry is ubiquitous in all soils (including subsoil), especially in those with strong UV radiation, fluctuating O2 concentrations and redox conditions, microbial hotspots such as rhizosphere and detritusphere, and high contents of amorphous or short-range ordered Fe (oxyhydr)oxides. Charcoal and biochar mediate heterogeneous catalysis and ROS formation in soil directly – as an electron shuttle – or indirectly by electron transfer from the valence band to the conduction band in the minerals under UV irradiation. Despite the extremely short lifetime (from nanoseconds to a few days), ROS are continuously produced and sustain the ubiquity of chelators and Fe(III) reduction. For the first time, we calculated the fundamental Eh-pH diagrams for ROS species and showed their relevance for Fenton reactions under specific soil conditions. Based on its extremely high reactivity (Eo = 2.8 V), HO• is one of the most powerful oxidants and may provide the most efficient energy release from Fenton reactions in soil. Even though the direct contribution of Fenton reactions to OM oxidation and CO2 emission is less than 0.5% on the global level, in some soils and ecosystems (e.g., hot deserts, red soils in the tropics and wet subtropics) it can reach 30% and even exceed 50% of total CO2 emissions. Fenton reactions are omnipresent and play a dual role for soil C cycling: they stimulate OM mineralization (including the most stable C pools such as charred C) and facilitate long-term C stabilization due to the increased recalcitrance of remaining OM and organo-mineral complex formation. Agricultural management positively affects Fenton reactions, accelerating C cycling and nutrient acquisition by plants. Accordingly, Fenton reactions and their effects on OM decomposition and formation are an emerging research field that explains the chemical background of many oxidative enzymatic processes. This may crucially change our views on C, energy and nutrient cycling in soils, especially in a changing world.