## 4 Discussion
### Soil Nutrients Improvement
[57] who stated that low mineralization rates of biochar underlines the biochar stability and its inherent ability to resist microbial degradation. [PERSON] and [PERSON] [58] further concluded that decreasing rates of soil organic matter mineralization may be caused by different factors including aeration and N availability. Application of RSRH biochar increased the soil organic C. The biochar role as a C storage in the soil was further confirmed by [PERSON] et al. [55] and [PERSON] et al. [59]. Biochar is a C rich substrate and resistant to decomposition due to its aromatic structure [60]. The RSRH biochar interacts synergistically with soil to enhance the soil organic matter and soil C. Treatments amended with RSRH biochar also increased the soil EC. Higher soluble salt presence in the biochars might have resulted in the higher EC values. [PERSON] et al. [61] stated that biochar increases soil EC because cations on biochar surface may not bind strongly by electrostatic force, causing it to dissolve as a soluble salt. [PERSON] and [PERSON] [62] and [PERSON] et al. [63] further agree that incorporation of biochar to acidic soil increases soil EC due to the release of weakly bound cations and anions into soil solution which are available for plant uptake. Soil amended with RSRH biochar increased the soil EEC (Table 8). This could be due to the inherent characteristic of biochar which has a high surface area and porous nature [64]. The increase of soil CEC amended with RSRH biochar may also be due to the slow oxidation of biochar material as a result of biotic and abiotic factors which oxygenate the biochar surface functional group and boost the development of organo-mineral complexes [65; 66; 67]. [PERSON] et al. [68] also stated that the charge density of per unit biochar surface was high, so its incorporation could increase the cations' sorption of soils. In the presence of biochar, soil available P increased significantly, and this could be attributed to the addition of biochar, which binds PO\({}_{4}\)\({}^{3-}\) ions. The presence of polar and non-polar surface sites on biochar enabled it to adsorb more nutrients [36]. It was further agreed by [PERSON] et al. [69] and [PERSON] et al. [70] that biochar adsorbed both NH\({}_{4}\)+, NO\({}_{3}\)\({}^{-}\), and PO\({}_{4}\)\({}^{3-}\) onto its exchange sites. The adsorbed PO\({}_{4}\)\({}^{3-}\) onto the biochar surface reduced the P fixation to Al and Fe. Fixation of P with Al and Fe formed insoluble oxides and hydroxides, causing it to become unavailable for plant uptake. However, application of RSRH biochar into the soil successfully adsorbed P. [PERSON] et al. [71] stated that organic amendments (compost and biochar) successfully reduced soil P fixation by Al and Fe. Incorporation of RSRH biochar increased available K and available Ca significantly (Table 9). The increase might be due to the inherent contents of these cations in the treatments amended with RSRH biochar. The increase of available K and available Ca can be associated with higher CEC of RSRH biochar (Table 4). [PERSON] et al. [72] reported that the addition of biochar improved CEC of the soil, thus directly increasing the retention of basic cations in soil. Further, the highest retention of available cations (K and Ca) might be attributed to the presence of ash in biochar which helps in the immediate release of Ca and K [64; 73; 74].