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

Mechanisms of carbon dioxide (CO) release from soil in the absence of oxygen were studied considering the Fenton process, which encompasses the reaction of HO with Fe(II) yielding a hydroxyl radical (OH), 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 CO 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 CO release even under oxygen limitation. We hypothesised that the Fenton reaction (FR) greatly contributes to the CO released from SOM mineralised under anaerobic conditions especially in the presence of ligninolytic enzymes. We used a novel technique that combines labelled HO and Fe(II) to induce the FR and measured COO, Fe(II) solubilisation, and peroxide consumption in a closed gas circulation system for 6 h. Maximal CO amount was released when the FR was induced in combination with LiP addition. The CO 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 from peroxide to CO release, ii) peroxide consumption, and iii) Fe(II) solubilisation by FR. The amount of consumed peroxide was closely correlated with the COO derived from soil 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.