# Carbon Mineralization in a Soil Amended with Sewage Sludge-Derived Biochar
## 1 Introduction  
Advanced thermal treatments employing pyrolysis are being more frequently used in an effort to find safer solutions for the disposal and utilization of wastewater sludge. The pyrolysis of sewage sludge (SS) results in an SS-derived biochar, which is free of pathogens and of other hazardous organic compounds [1; 2], enabling its use for agricultural purposes. Biochar produced from essentially domestic SS could present a heavy metal concentration below the maximum limits allowed in several countries [3]. One of the main drawbacks of biochars could be the presence of polycyclic aromatic hydrocarbons (PAHs), which have been increasingly studied in several laboratories across the world [4]. Due to the high concentration of several plant nutrients, SS-derived biochars have been increasingly studied [5], and their benefits include nutrient supply (mainly P, N, and Ca) for short-cycle crops, such as radish [6], and grain crops, such as maize [2], in highly weathered soils. The application of 15 Mg ha\({}^{-1}\) of SS-derived biochar to an Osisol was able to fully replace mineral fertilization for maize production [7]. In addition to the nutritional benefits of SS biochar for crop production, it may also reduce soil greenhouse gas emissions [8; 9]. According to [PERSON] et al. [10], the use of biochar can potentially reduce soil annual net emissions by a 1.8 Pg of C-CO\({}_{2}\) equivalent per year, which represents 12% of current anthropogenic C-CO\({}_{2}\) equivalent emissions. The carbon (C) sequestration potential of biochar is related to its recalcitrant pyrogenic-C, which is resistant to degradation [11]. This recalcitrant fraction of biochar is basically influenced by the feedstock and formation conditions under which biochar is produced [12; 13]. Depending on biochars' chemical and physical characteristics, their degradation rates may vary enormously [13; 14]. Finding ways to obtain biochars that are both capable of sequestering C and releasing nutrients in the soil for food security and production is still a challenge for science. An appropriate combination of feedstock and production temperature is a strategy that could allow for the development of \"designed\" biochars for specific environmental and/or agricultural purposes. The land application of either SS or SS-derived biochar causes changes to soil structure and to the functioning of agroecosystems, with the soil microbial community being the most sensitive component [15]. Applying these products to soil may either increase microbial biomass activity due to increases in soil C and nutrients or inhibit it due to the presence of heavy metals and other toxic compounds [16]. Therefore, microbial biomass will respond positively or negatively depending on the quality and quantity of the amendment. For example, the application of biochar with low available nitrogen (N) to soil may increase the half-life of C in soil due to a decrease in C mineralization rates [17]. Moreover, if the objective of applying biochar to soil is to increase soil fertility, then low pyrolysis temperature biochar may be more suitable because of its higher rates of C and nutrient mineralization in soil [17; 18]. To better understand the behavior of labile and recalcitrant fractions of biochars produced by different pyrolysis temperatures, soil incubation tests are extremely useful [19; 20]. The information obtained from incubation tests enables the inference of the relationship between the more labile fraction of biochar, its pyrolysis temperature, and the feedstock used for production [14; 21]. In the last two decades, several studies have found that biochars produced at higher pyrolysis temperatures are highly resistant to decomposition and have the potential to store C in soil [13; 22]. However, because of their high stability, these same biochars have low capacity to release nutrients in a short-term period.