2. Review of Literature
2.1. An Overview of Redox Processes in Soils with Biochar
- Electron transfer from organic matter to Fe(III) (hydr) oxides via C oxidation .
- Reduction of NO3− to NO2− with the oxidation of Fe2+ to Fe3+.
- Mineralisation of organic N to NH4+  and the oxidation of NH4+ to NO2− with the consequent reduction of Fe 3+ to Fe2+.
- Oxidation of NH4+ to NO2− with the consequent reduction of Fe3+, formation and oxidation of FeS minerals in the sulphur (S) cycle .
- Cycling of S from solid to soluble liquid species driven by oxidation or reduction of Fe species .
2.2. The Electrochemical Properties of Biochars, Summary of Literature
2.3. Measurement of the Electrochemical Properties of Fresh Biochars and Soil/Biochar Systems
- High variability of soil Eh in space and time: Eh is largely influenced by hydric conditions (water activity), temperature, microbial activity and respiration of living organisms [27,28]. As a consequence, it is difficult to obtain stable measurements, especially in soils with low poising ability (that is, soils with low organic matter and clay content).
- Irreversibility of redox reactions at the surface of the electrodes, which makes it difficult to conduct Eh measurements over long time periods .
- Chemical disequilibrium in soils .
- Polarisation of and/or leakage from electrodes.
- The influence of electromagnetic fields on water and living organisms, which can greatly perturb Eh measurements in soil samples through an induced current in the electrode .
2.4. Electrochemical Properties of Biochar as Measured Using Solid State Cyclic Voltammetry (SSCV), SEM and TEM
- A wood biochar (Jarrah) produced at 600 °C in a vertical retort . This biochar had a high surface area, high fixed C, high concentration of stable aromatic C and low concentration of functional groups and low ash.
- Acacia saligna biochar produced at 380 °C in a rotating drum kiln. This biochar has high labile C content and a relative low surface area compared with the Jarrah biochar .
- Chicken litter high mineral ash biochar produced at 400 °C in a rotating drum kiln .
2.5. Changes in Eh and pH When Biochars are Added to Soil Using the In-Situ Measurement Technique
3. Conclusions and Future Directions
- Are the redox properties of biochars responsible for some or any of the different effects that biochars have been reported to have in the integrated soil/plant/rhizosphere microbiome system?
- What are the mutual redox interactions of different biochars in soils of different types?
- Do biochars increase the poising capacity of soils and why?
- Are some biochars more effective than others in altering Eh fluctuations, especially in systems (e.g., rice) where flooding and drying cycles occur?
- Does the penetration of root hairs into the pores of biochar and/or the attachment of roots to the biochar surface change the potential across the plant cell wall and change the take up of specific nutrients? If so, why does this occur?
- Do Fe and Mn/Oxide particles with diameters less than 20 nm redox active particles on the surfaces of biochar assist in the breakdown of organic matter, increase P availability and reduce the production of greenhouse gases?
Supplementary FilesSupplementary File 1
Conflicts of Interest
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