## 1 Introduction  
From a circular bioeconomy perspective, the adoption of sustainable management techniques that enhance organic inputs into the soil while minimizing losses offers significant potential for mitigating climate change. This is achieved by reducing the emission of greenhouse gases (CO\({}_{2}\), CH\({}_{4}\), N\({}_{2}\)O) and valorizing by-products traditionally considered as wastes [1; 2; 3]. Anaerobic digestion (AD) plays a crucial role in the circular economy by converting various biomass and waste into renewable biogas, offering a sustainable alternative to manage farmyard manure and/or industrial organic waste by producing green energy [4]. Additionally, AD results in the production of a valuable by-product known as digestate, which is rich in essential nutrients. This digestate can serve as a sustainable resource for enhancing soil quality and fertility in food production systems by being utilized as either soil amendments or fertilizer [4; 5]. Throughout AD, a portion of organic nitrogen (N) is transformed into ammonium (NH4+-N), while phosphorus (P) is partially converted into orthophosphate. Both these nutrient compounds become readily available for plant uptake [6; 7]. Furthermore, digestate contains a significant amount of carbon (C), contributing to the organic C supply in the soil [5] and improving its physical properties [6]. The application of digestate directly to soil has the potential to significantly increase environmental and economic values by reducing the use of mineral fertilizer, saving synthetic fertilizer costs, and restoring soil organic stocks [6; 8]. Biochar, a C-rich by-product, is produced from organic materials like agricultural residues and sewage sludge through processes such as pyrolysis or bio-gasification conducted under low oxygen conditions [9; 10; 11]. Biochar serves as a valuable soil amendment and for long-term C sequestration [1; 10]. It possesses a large specific surface area and porosity, making it an effective biosorbent, and contains trace minerals [9]. Biochar has been observed to reduce nitrous oxide (N2O) and methane (CH4) emissions from soil [12; 13; 14], as well as adsorb NH4+-N through chemisorption and physical adsorption [5]. Moreover, studies suggest that the combination of digestate with high rates of biochar can enhance soil organic carbon (SOC) in the <20 \(\upmu\)m fraction by up to 20% [15] contributing to improve soil structure, particle aggregation, and water retention [16]. Given its agronomic and environmental benefits in water and nutrient retention, as well climate change mitigation, the application of biochar to soil has recently attracted much attention becoming a widely accepted practice [11; 14; 16]. Both digestate and biochar contribute to the circular bioeconomy principles by closing the nutrient loop [4]. However, it is important to note that their application to soil has the potential to alter microbial habitats, thereby impacting the composition and activity of soil bacterial communities [6; 17; 18]. Due to its porous nature, biochar can create an environmental niche conductive to sheltered microbial growth in the soil [19]. The application of digestate and/or biochar has the potential to enhance microbial processes, such as nitrogen mineralization and ammonia (NH4+-N) oxidation [20]. Microorganisms serve as key actors in soil ecological processes, influencing nutrient cycling, plant growth, and disease suppression [18]. Preserving the functionality and diversity of soil microbial community is crucial for fostering sustainable agricultural production [8]. However, predicting the consistent response of the soil microbial community to digestate and/or biochar application is challenging due to the inherent heterogeneity of soil and C amendments across studies. Some studies demonstrated a positive impact of digestate application on soil nutrient availability [8], microbial community composition [8; 21], respiration, and enzyme activities [22], while others reported minor changes or no significant effects [23].