## 4 Discussion
Current evidence suggests that, over time, the soil microbial community tends to recover and restore itself to levels comparable to the control group after digestate application [36; 58]. This insight is valuable in comprehending the long-term impacts of digestate on soil microbiota. The addition of biochar led to a significant increase in N\({}_{2}\)O soil emissions, particularly at a 10% dose, contradicting the documented lower emissions and increased reduction to N\({}_{2}\) observed in various field and laboratory experiments [12; 13; 14]. This contrasts with the trend observed in CO\({}_{2}\) soil emissions. Nitrification is considered a crucial pathway for N\({}_{2}\)O production in soil [69], is typically associated with an increase in CO\({}_{2}\) emissions, as well as N-mineralization process. Therefore, our findings suggest that, at high concentrations, the biochar used in this study inhibits both N mineralization and nitrification processes, while impacting the activity of denitfying bacteria and leading to incomplete denitrification process with increased N\({}_{2}\)O production. Furthermore, the addition of digestate to biochar enhances the maximum production of N\({}_{2}\)O. Several studies have reported conflicting effects of biochar on nitrification and N-mineralization [10]. The extensive variability in factors such as biochar type, application rate, and soil type can account for the diverse literature reports on how biochar affects nitrification, N-mineralization, and denitrification processes influencing N\({}_{2}\)O loss from biochar-amended soils [12]. For instance, variations in water-holding capacity and surface charge density among biochars could affect N-cycling processes in both negative and positive ways [5; 70; 71; 72; 73]. Biochar in soil directly influences the activity of ammonia oxidizers and nitrifiers by altering NH\({}_{4}\)+ availability. In fact, biochar can limit NH\({}_{4}\)+ availability to ammonium oxidizers by adsorbing NH\({}_{4}\)+, while favoring nitrification in soils receiving inorganic and organic N fertilizers or having higher organic C content. On the other hand, denitrification process is more significant in waterlogged soils [73]. N\({}_{2}\)O reduction to N\({}_{2}\) is more prominent in fine-textured soils, where soil micropores play a role in its retention. Biochar-amended soils show a general trend of lower N\({}_{2}\)O emissions and enhanced N\({}_{2}\)O reduction to N\({}_{2}\), possibly due to the colonization of biochar micropores by denittriiers or micropores acting as traps for N\({}_{2}\)O [73]. Our biochar may not have the characteristics to favor denittriiers' colonization or N\({}_{2}\)O entrapment. Previously, it has been reported that sewage sludge-based chars differ in porosity, which is lower in sludge chars (<0.2 \(\upmu\)m) compared to willow-based biochars (<0.6 mm) [74; 75]. Our unexpected results could be attributed to the chemical composition of the biochar used, characterized by a high N content and low C content compared to other biochars reported in the literature [9; 67; 76]. Thus, the low C content is characteristic for sewage sludge-based biochars [77]. As a consequence, the soil treated with 10% biochar exhibited a significantly higher TN content. Additionally, the application of higher rate of biochar might have contributed to the creation of aerobic microsites favoring coupled nitrification-denitrification pathway leading to increased N\({}_{2}\)O emissions. The reduction of N\({}_{2}\)O to N\({}_{2}\) is highly influenced by oxygen exposure, particularly affecting the _nosZ_ enzyme responsible for this reaction [78]. The properties of biochar undergo significant variations depending on its source materials (e.g., manure, sludge, or wood) and the conditions under which it is produced. The diverse production temperatures contribute to differences in the type and abundance of nitrogen forms present in biochar.