3.2.1. Nutrient Retention

Retention of soil nutrients has a direct effect to minimize risks of runoff and subsurface contamination of water bodies, highly reducing eutrophication and losses of nutrients [16]. Biochar can be a sustainable solution to latrine soil-bed nutrient leaching, consequently decreasing the nutrient concentrations in runoff and groundwater sources [16]. An increase in the CEC of a soil results in improved nutrient sorption on the colloids of biochar [19,20]. Biochar in soils also have the potential to largely decrease nitrogen losses and carbon dioxide releases [25]. Laird et al. [19] demonstrated an increase in N, organic C, P, K, Mg, and Ca in fine-loamy soil treated with hardwood biochar. A sorghum produced biochar also improved organic C and minimized greater losses of N, P, and K in overflow when combined with the soil [89]. Dissolved NO3-N and PO4-P decreased in wastewater bodies treated with a waste wood biochar-treated soil column [25] and also in soil mixed with an agricultural char (pecan, walnut, and coconut shells and rice hulls) [91]. Other than NO3-N and PO4-P, Beck et al. [92] observed a decrease in total N, total P, and total organic C. It has been seen that an increase in the application rates of biochar can also cause an increase in the nutrient holding capacity of the soil [20,90]. In their study, Huggins et al. [109] illustrated the efficiency of biochar to retain NH4 <sup>+</sup> and PO4 <sup>3</sup><sup>−</sup> from faecal wastewater (Table 4).

**Table 4.** Wastewater treatment and the retention ability of biochar [109].

