## 3 Physicochemical Properties of Soils Affected by Biochar  
When mixed with soil, biochar can change the soil's texture, pore size distribution, and bulk density, positively affecting the aeration and the water-holding capacity [29; 30; 31]. The final soil characteristics following biochar addition are highly dependent on the initial quality of the soil, and its type and composition (porosity) in general. The application of biochar can serve first to improve the soil physical status, which may through several indirect effects affect other soil chemical and biological characteristics. In clayey soils, for instance, a larger pore size can be suggested (larger than the main pores of the clay, as 10 nm in diameter) in order to increase the aeration of these rather compacted soils. The opposite is possible in soils with high sand contents, where biochar can be used to improve the water-holding capacity and nutrient adsorption, not aeration. The variability of the porous structure of biochar, therefore, can increase the amount of water stored and the size and distribution of minerals as soil formers [32]. Biochar can have a surface area of up to 800-5000 mm\({}^{2}\) per gram, and this porous, airy structure indirectly contributes to a generally high microbial activity and surviving ability. This beneficial property of biochar is also exploited in contaminated soils where environmental (stress) conditions (air limitations, lack of nutrients, critically low temperatures) would otherwise impede the remediation processes, such as the degradation of antifreeze propylene glycol [33]. Meanwhile, in the case of other forms of pollution, the positive effect of biochar on soil porosity is highlighted. Still, microorganism habitats and the protective surface (for biofilm production) provided for microorganisms are also a key issue in many soils, especially colloid-poor sandy soils [33; 34]. Biochar has a limiting effect [35] and increases soil cation exchange capacity by raising the pH, influencing the availability of nutrients, and thus preventing their leaching [36; 37]. For example, the availability of phosphate, iron, boron, zinc, and manganese has been shown to decrease at high pH [37]. Increasing pH also increases microbial nitrification, which results in losses of nitrate and limited availability of ammonium, the preferred nitrogen source for plants [38]. In this way, in some cases, the pH-raising effect of biochar can create unfavorable conditions for plants, particularly in calcareous soils; this can result in yield losses [38; 39]. Biochar has its own nutrient content (Table 3); depending on the raw material used and the pyrolysis temperature, it may contain large amounts of macro- and micronutrients with different levels of availability [40]. Therefore, biochar can be directly involved in the nutrient replenishment of plants [41]; however, aging, oxidation, and microbial degradation can alter the surface functional groups and chemical structures of biochar, with the result that the sorption capacity of biochar generally decreases with time. The effect of the aging, oxidation, or microbial degradation of biochar-derived dissolved organic matter on pollutants and the soil environment over time is still unclear [42]. In biochar, nutrients can be present in three forms, depending on the production conditions and the rate of utilization, as mentioned above. The nutrients in the ash fraction can be