## 3 Results and Discussion
### Influence of Biochar Hydroinfiltrators on Microbial Communities  
Our results confirm that the use of hydroinfiltrators combined with biochar produces significant effects on the structure and diversity of soil microbial communities (Figure 12 and Table 2). This is consistent with previous research pointing out the multi-factorial impact of biochar on soil ecosystems. Biochar is composed of recalcitrant carbon, exhibits a low mineralisation rate and contains essential plant nutrients such as P, K, S, Mg, Ca, and micronutrients [82; 83]. The surface of biochar contains multiple negatively charged sites (OH-, COOH-, phenolic compounds, etc.) that enhance its adsorption capacity at neutral pH, increase electrical conductivity and cation exchange capacity [84]. Its porous structure, high specific surface area, and nutrient retention capacity promote the survival and activity of beneficial soil bacteria, which can lead to improvements in soil fertility and crop productivity [85; 86]. The use of biochar in combination with bacterial inoculation should be considered as a climate change adaptation strategy in agriculture [87]. Metagenomic analysis revealed an increase in diversity and functional shifts associated with the presence of the hydroinfiltrator (Figures 6 and 7), particularly within the charosphere zone, where multiple biogeochemical processes occur [88; 89; 90]. Alterations in key parameters such as pH, porosity and nutrient availability in this zone [91] could explain the expansion of ecological niches for new bacterial genera, as also reported in previous works [92; 93]. The observed bacterial colonisation aligns with the mechanisms described by [PERSON] et al. (2020) [7], in which the quality and dose of biochar, along with the soil type, determine the rate and selectivity of microbial settlement. Our results indicate selective colonisation by genera such as _Sphingomonas_ and _Bacillus_ (Figure 6), both with physiological abilities including active motility, exopolysaccharide (EPS) production, and biofilm formation [53; 94; 95]. In particular, the increase in _Sphingomonas_ in SH40 soil compared to SC40 soil could be related to its motility and adhesion capabilities mediated by structures such as flagella and EPS, as described by [PERSON] (2005) [96] and [PERSON] et al. (2017) [97]. In contrast, the decrease in _Weissella_ in SH40, despite also being an EPS-producing genus [98], suggests that it may have been displaced by other genera with better functional adaptability, such as _Splingomonas_ (Figure 6). This type of competition has been described as a key process in microbial structuring under biochar-modified conditions [96]. Changes in aerobic and anaerobic conditions were also noticeable. Our results revealed a reduction in anaerobic genera (_Cutibacterium_, _Blautia_) and facultative anaerobes (_Weissella_) in soils with a hydroinfiltrator (Figure 6). The anaerobic and facultative anaerobic nature of the three genera mentioned has been described by some authors [98; 99; 100]. The effect of the hydroinfiltrator aligns with the findings of [PERSON] (2015) [4], who point out that biochar improves soil aeration and, therefore, favours the development of aerobic microbial communities. Consequently, the soil environment becomes less favourable for strictly anaerobic microorganisms, being consistent with the results obtained. Regarding the toxicity of biochar, some authors have reported that its alkalinity may reduce or inhibit soil microbial activity in the short term due to its toxic effects [79]. According to our results, biochar does not exert an inhibitory effect across all bacterial populations, but instead, has a select effect for resistant taxa such as _Bacillus_, _Splingomonas_, _Blastococcus_, and _Steroidobacter_ (Figure 6), as documented in previous studies [54; 55; 61; 62]. Other adaptive advantages would be accounted for the tolerance to heavy metals and the ability to degrade toxic compounds, including polycyclic aromatic hydrocarbons (PAHs), for these microorganisms [52; 62].