2.1. An Overview of Redox Processes in Soils with Biochar

Redox processes involving the donation and acceptance of electrons play an important part in soils, such as in nutrient cycling (phosphorous and nitrogen), scavenging of free radicals, formation and destruction of ethylene, methane and nitrous oxide [ 6 7 ]. Decomposing organic matter, including biochar, can be regarded as an electron-pump supplying electrons to more oxidised species present in the soil system [ 8 ]. However, unlike organic matter, biochar oxidises at much slower rates and has the potential to store electrons [ 9 ].

n = number of electrons, m = number of protons exchanged: Ox + mH+ + ne− → Red (1) E h = E ° + RT × nF ln [ Red ] [ Ox ] + 2.303 mRT × nF pH (2) As Husson [ 10 ] notes, oxidation and reduction conditions are assessed by measuring the redox potential (Eh;V). Eh is derived by combining the standard free energy change of the generic redox reaction in Equation (1) with the Nerst equation, where F is the Faraday constant, Ox is the oxidized species, Red is the reduced species, R = gas constant, T is temperature in kelvin,= number of electrons,= number of protons exchanged:

The zero point for the Eh scale (E°) is set by the standard hydrogen electrode (SHE). Equation (2) shows the relationship between Eh and pH. As Eh is correlated to pH, electron activity needs to be compared at a given pH, which can be done by correcting Eh to pH 7 using the regression given by Equation (2) [ 11 12 ]. As an example of the importance of this correction, a biochar suspension having an Eh of 500 mV and pH 5 has a higher electron activity than a biochar suspension having an Eh of 400 mV and pH 9. Soil and biochar suspensions can be characterized simultaneously by their Eh and pH using a modified Pourbaix Diagram [ 4 8 ]. In the same way, the Eh-pH of a particular biochar suspension can be superimposed on this diagram. Examples are given and techniques are discussed in a latter section. The issue in measurement of the Eh and pH of these diagrams has been discussed in [ 8 10 ].

Poise is the resistance to change in Eh when a small amount of oxidant removes electrons from a system or, conversely, a small amount of reductant adds electrons. Poise and Eh are expressions that can be compared to buffer capacity and pH in soils. Poise increases with the total concentration of oxidant plus reductant, and, for a fixed total concentration, it reaches a maximum when the ratio of oxidant to reductant is unity [ 13 ]. Since biochars function mainly as reductants, we suggest that, when added to soil, biochar may cause an increase in soil poise [ 10 ].

2 (E° = 1.2 V), NO 3 − (E° = 0.88 V), and Fe3+ (E° = 0.66 V) that can also cause oxidation of organic C [ 2 consumption by both soil organisms and root respiration. Thus, there are two soil redox interfaces; namely (i) the low soil Eh rhizosphere zone, with a high Eh of surrounding well-aerated soil; and (ii) the low Eh rhizosphere region and its interface with the high Eh surfaces of live and growing roots. Similarly, when biochar is added to soil and adsorbs water, there will be internal pores that have no or very low concentrations of O 2 and where the Eh is very low, as well as an area surrounding the biochar where the Eh can be high (in aerated soils [ Molecular oxygen is the most common electron-acceptor species, although, in its absence, there are alternative electron acceptors in soil, such as MnOOH (E° = 1.5 V), MnO(E° = 1.2 V), NO(E° = 0.88 V), and Fe(E° = 0.66 V) that can also cause oxidation of organic C [ 6 ]. Soil moisture status and soil structure are key factors controlling redox reactions. In well-aerated arable soils, there is a mosaic of anaerobic microsites, in particular in the rhizosphere, resulting from Oconsumption by both soil organisms and root respiration. Thus, there are two soil redox interfaces; namely (i) the low soil Eh rhizosphere zone, with a high Eh of surrounding well-aerated soil; and (ii) the low Eh rhizosphere region and its interface with the high Eh surfaces of live and growing roots. Similarly, when biochar is added to soil and adsorbs water, there will be internal pores that have no or very low concentrations of Oand where the Eh is very low, as well as an area surrounding the biochar where the Eh can be high (in aerated soils [ 5 ]).

et al. [ Redox transformations are most apparent at interfaces where there are two unlike environments and, hence, a driving force for reaction. Typically, these are wetlands, flooded fields and the regions between roots and moist soil in the rhizospheres of many plants [ 6 ]. Additions of high concentrations of biochar into the rhizosphere could introduce an environment that contrasts the one that would naturally develop there from the typical soil clays, silt, sand and organic matter components. The redox potential in the immediate area around the biochar particle could change as solutions rich in organic compounds, cations and anions, diffuse in and out of the macro- and meso-pores of the biochar. Joseph 5 ] detailed biotic and abiotic reactions that could take place on the surface and in the pores of the biochar, with regards to both the C matrix and mineral matter.

2+ and Fe3+ oxidation states and both oxidation states can exist in the biochar usually as an iron oxide (Fe 2 O 3 , Fe 3 O 4 , FeOOH). The concentration of different Fe compounds with different oxidation states depends on the pyrolysis conditions (air is often entrained with fuel) as well as complex catalytic processes that take place during pyrolysis between the alkali metals and the organic molecules [ Research has also highlighted the role that iron minerals play in redox processes in soil [ 7 ]. It has been hypothesised that iron minerals in biochar could catalyse a range of redox reactions associated with nutrient cycling, nitrification and denitrification [ 13 ]. Iron exists in all types of biomass both in the Feand Feoxidation states and both oxidation states can exist in the biochar usually as an iron oxide (Fe, Fe, FeOOH). The concentration of different Fe compounds with different oxidation states depends on the pyrolysis conditions (air is often entrained with fuel) as well as complex catalytic processes that take place during pyrolysis between the alkali metals and the organic molecules [ 5 ]. Some of these Fe phases have diameters less than 10 nm and are completely surrounded by C, while others exist on the surfaces of pores and can be oxidised by air. Once biochars are placed in soil, Fe compounds can precipitate out on the surface of the biochar or, if they are in a water saturated environment, they can be reduced [ 4 ].

(1) Electron transfer from organic matter to Fe(III) (hydr) oxides via C oxidation [ 14 ]. (2) Reduction of NO 3 − to NO 2 − with the oxidation of Fe2+ to Fe3+. (3) 4 + [ 4 + to NO 2 − with the consequent reduction of Fe 3+ to Fe2+. Mineralisation of organic N to NH 15 ] and the oxidation of NHto NOwith the consequent reduction of Feto Fe (4) 4 + to NO 2 − with the consequent reduction of Fe3+, formation and oxidation of FeS minerals in the sulphur (S) cycle [ Oxidation of NHto NOwith the consequent reduction of Fe, formation and oxidation of FeS minerals in the sulphur (S) cycle [ 7 ]. (5) Cycling of S from solid to soluble liquid species driven by oxidation or reduction of Fe species [ 7 ]. The redox processes involving Fe may include:

3 − and NO 2 − by lowering the free energy required for the process [ Biochar with a high content of Fe oxide nanoparticles at the surfaces of pores could significantly increase the rate of reduction of NOand NOby lowering the free energy required for the process [ 13 ].