## 4 Discussion
### Soil Nutrients Improvement  
The higher retention of soil total N, exchangeable NH4+, and NO3- were possible because of the high CEC of RS (75.6) and RH (66.6 cmol kg-1) biochar (Table 4). Further, the use of biochar as an organic amendment increased the retention of N due to the deceleration of N mineralization. This was possible due to the high organic matter of biochar. This was in agreement with [PERSON] et al. [42] and [PERSON] et al. [43]. Moreover, the higher soil total N, exchangeable NH4+ and NO3- in T2 and T3 suggests that the use of RSRH biochar was effective in reducing N, NH4+, and NO3- loss in soils, which is consistent with the findings by [PERSON] et al. [44]. Nitrogen ion retention by RSRH biochar is due to its nature of having numerous pores and a larger surface area which adsorbs NH4+ and NO3- onto its surface (Figures 1 and 2). [PERSON] et al. [45] stated that biochar has a special porous structure and surface functional group that adsorbs nutrient ions effectively. In addition, the ions' adsorption capacity of biochar has been proven in a study conducted by [PERSON] et al. [35]. The amorphous crystalline structure with numerous cracks on the surface area of biochar creates a large volume of empty pores that can be filled with nutrients [46]. The cracks lead to the formation of micro and macro pores during the heating time and the cracks formed are too numerous to be sealed off. This increases the porosity of biochar, where the porosity aids the adsorption capacity of the biochar. Additionally, the biochar has dual adsorption capacity where it adheres both negative and positive charged ions on its surface [47]. This strengthens the reasoning underlying the capacity of biochar to retain more N, NH4+, and NO3- in soil. The presence of biochar regardless of RSRH biochar or commercial biochar increases the pH of the soil. The increase in soil pH was because of the rapid proton (H+) exchange between soil and organic amendments (biochar) [48; 49]. Further, the increase in soil pH was also correlated with the further decomposition of biochar, where it enhanced the formation of organic anions, which consumed the protons available in the soil and solubilized the inherent K, Ca, Mg, and Na contents of biochar. Eventually, this increased the soil pHwhich was amended with biochars. This was in agreements to [PERSON] et al. [50] who stated that soil pH increases with the addition of organic amendments. Increase in the soil pH under T1 could be due to the dissolution of Ca and Mg contained in CIRP. Further, reduction of soil exchangeable acidity in treatments amended with RSRH biochar was partly related to the increase in soil pH. It has been reported by previous researchers that soil pH increases as the exchangeable Al decreases [51; 52]. This is due to the precipitation of exchangeable and soluble Fe as insoluble Al and Fe hydroxides [53]. However, this is in contrast with the findings of this study, whereby the increase in soil pH was not affected by the presence of exchangeable Al and Fe. Despite the highest level of the exchangeable Al and Fe in T4 and T5, the soil pH was increased significantly. This might be due to the alkaline nature of the biochars. Biochars are alkaline due to their ash content which releases base cations; hence, biochar addition neutralizes soil acidity [54]. Biochars have abundant soluble and exchangeable basic cations (Ca\({}^{2+}\), Mg\({}^{2+}\), K\({}^{+}\), and Na\({}^{+}\)) which can be released into the acid soil easily. This suggests that the presence of biochar increases soil pH and reduces exchangeable acidity, although biochar does not significantly reduce the exchangeable Al and Fe. The application of biochar increases the availability of soil organic matter. This finding is consistent with that of [PERSON] et al. [55]. The RSRH biochar increased the soil organic matter significantly, which might be due to slow mineralization and degradation by microbes. This was supported by studies of [PERSON] et al. [56] and [PERSON] et al.