# Duality of Biochar and Organic Manure Co-Compositing on Soil Heavy Metals and Enzymes Activity
## 3 Results and Discussion
### Soil Enzyme Activity  
Regardless of soil treatments, Figure 4A illustrates the activities of soil urease and invertase collected at sampling dates during the green pepper growing season. As described earlier, urease is the enzyme that breaks down urea N fertilizers and produces CO\({}_{2}\) and NH\({}_{4}\)+ in soil. Following the application of soil amendments, in early June, the increased activity of urease was followed by an increase in NH\({}_{4}\)+ concentration in the soil. Since NH\({}_{4}\)+ is a weak base, one of the effects of this hydrolysis is raising the soil pH (the hydrogenionic potential). Accordingly, the increase in urease activity after manure application was followed by a decrease in invertase activity, as shown by the trend line (the dashed line in Figure 4A). Investigators [40] stated that the optimal invertase activity in soil was witnessed at pH 5.0. Figure 4A also reveals that, at a higher urease activity, the activity of invertase tends to decrease, indicating that the pH value plays a significant role in the activity of soil invertase. A high pH can alter the invertase structure and reduce its efficiency, and consequently, the rate of invertase hydrolysis can decrease, impacting the availability of simple sugars for microbial and plant uptake [41]. Figure 4B presents the activity of phosphatase in soil, regardless of soil treatments at different time intervals during the pepper growing season. Soil acid phosphatase activity was much greater compared to alkaline phosphatase activity. Soil pH is not the only parameter that controls phosphatase activity. Several factors can also impact the activity of phosphatases. The reduced activity (inhibition) of alkaline phosphatase shown in Figure 4B could be due to the presence of Cd\({}^{2+}\) in the soil, which reduces the enzyme's affinity to its substrates and reduces reaction velocity [42]. In addition, according to [PERSON] et al. [43], Al\({}^{3+}\), Fe\({}^{2+}\), and Ca\({}^{2+}\) ions (not determined in this study) can inhibit alkaline phosphatase activities, and excess S\({}^{2-}\) ions can also reduce alkaline phosphatase activity by removing Zn\({}^{2+}\) from the active sites of the enzyme [43]. The effects of adding biochar to soil on urease and invertase activities are shown in Figure 5A,B, respectively. Overall, the addition of 10% \(w/w\) biochar slightly decreased the activities of urease, as shown by the trend line in Figure 5A. Biochar, a carbon-rich substance created during the pyrolysis and thermochemical breakdown of biomass, has total organic carbon 88%, total inorganic carbon 0.34%, a surface area of 366 m\({}^{2}\) g\({}^{-1}\) dry, and 54% moisture. The organic content in biomass breaks down during the pyrolysis process, leaving behind biochar's undegradable heavy metals. According to a 2024 study [19], biochar significantly affects soil-hydrolzing enzymes and their secretions. This could be due to the influences of heavy metals in biochar that impact urease, invertase, and phosphatase, which have been determined to be positive [44] or negative [45], and this might be also due to the different compositions of heavy metals in biochar and variations in the levels of absorbing and retaining water molecules that impact soil microbial secretions and soil enzymes' activity. However, Figure 5A,B does not show significant differences