Contribution of the Fenton reaction and ligninolytic enzymes to soil organic matter mineralisation under anoxic conditions

Mechanisms of carbon dioxide (CO2) release from soil in the absence of oxygen were studied considering the Fenton process, which encompasses the reaction of H2O2 with Fe(II) yielding a hydroxyl radical ((OH)-O-center dot), in combination with manganese peroxidase (MnP) and lignin peroxidase (LiP). This study aimed to explain the high rate of soil organic matter (SOM) mineralisation and CO2 release from humid temperate rainforest soils under oxygen-limited conditions. The investigated mechanisms challenge the traditional view that SOM mineralisation in rainforest is slow due to anaerobic (micro) environments under high precipitation and explain intensive CO2 release even under oxygen limitation. We hypothesised that the Fenton reaction (FR) greatly contributes to the CO2 released fromSOMmineralised under anaerobic conditions especially in the presence of ligninolytic enzymes. Weused a novel technique that combines labelled (H2O2)-O-18 and Fe(II) to induce the FR and measured (COO)-O-18, Fe(II) solubilisation, and peroxide consumption in a closed gas circulation system for 6 h. Maximal CO2 amount was releasedwhen the FR was induced in combination with LiP addition. The CO2 efflux with LiP was 10-fold that of abiotic FR reactions without enzymes, or in soils amended with MnP. This was consistent with i) the contribution of O-18 from peroxide to CO2 release, ii) peroxide consumption, and iii) Fe(II) solubilisation by FR. The amount of consumed peroxide was closely correlated with the (COO)-O-18 derived fromsoil without enzyme addition or with LiP addition. Concluding, abiotic Fenton Reaction coupled with oxidative enzymes, such as LiP, are crucial for SOM oxidation under anaerobic conditions, e.g. in temperate rainforest soils. (C) 2020 Elsevier B.V. All rights reserved.

Automated summary

The study explores the mechanisms of CO2 release from soil under oxygen-limited conditions, focusing on the Fenton process and oxidative enzymes like LiP. The results indicate that abiotic Fenton Reaction combined with LiP play a crucial role in SOM oxidation and intensive CO2 release in temperate rainforest soils.