# Carbon Mineralization in a Soil Amended with Sewage Sludge-Derived Biochar
## 3 Results and Discussion
### The Dynamics of C Mineralization  
The C mineralization rate (CMR) was higher at the beginning of the incubation period and gradually decreased. CMR bottomed after day 60 (Figure 1). The higher CMR at the beginning of the incubation period is due to the decomposition of the labile C compounds present in the biochars [19], which can be found in water soluble fractions [36]. When the more labile fraction is degraded, the fractions that are more resistant to microbial attack start to slowly decompose and mineralize at a lower and more constant rate [37].  
Among the samples studied, two distinct groups were observed: one composed of SS, S + BC300, and S + BC400 and another one represented by the S + BC500 and the S (control). The first group showed higher CMR, from 3 to 37 days after the beginning of the incubation period than the second group. From this period on, all samples showed no differences in CMR values (\(P>0.05\)). These results demonstrate that SS biochars produced under pyrolysis temperatures above 400 \({}^{\circ}\)C have a reduced amount of easily oxidizable compounds [2], higher C/N ratio, and higher H/C ratio (Table 1). The H/C ratio is suggested as a stability indicator, which decreases linearly with increasing pyrolysis temperatures, and thus, reflects increased C stability in soil [10]. As the readily oxidizable compounds are depleted, the remaining recalcitrant organic materials are decomposed by a specialized population of fungi and bacteria, which synthesize the enzymes necessary for such decomposition [38]. At this stage, the CMR is slow and more constant. Therefore, the lower CMR found for S + BC500 (due to higher pyrolysis temperature) indicates that this material represents an excellent option for soil C sequestration and to decrease C-CO\({}_{2}\) emissions to the atmosphere.  
Figure 1: Carbon mineralization rate (CMR) of soil and sewage sludge (S + SS), soil and SS-derived biochars obtained at 300 \({}^{\circ}\)C (S + BC300), 400 \({}^{\circ}\)C (S + BC400), and 500 \({}^{\circ}\)C (S + BC500) pyrolysis temperature and soil (S) as the control treatment. \({}^{\star}\) and ns: significative and non-significative using the Tukey test (\(P<0.05\)), respectively.  
Similarly to our results, in a previous study by [PERSON] et al. (2019), the pyrolysis temperatures had a great influence on the biochar's stability, which was indicated by the nature of the C structures. The low CMR was attributed to increasing aromatic C (benzene-like rings), which have extra stability in the soil, to an effective structural change that favors the appearance of turbostatic-C crystallites molecules ([PERSON], 2016), and to the higher silica content (Si-O stretching instead of organic C-OH stretching of carbohydrates) ([PERSON] et al., 2017) with increasing pyrolysis temperature. [PERSON] (2014), studying a range of biochars produced from different feedstocks and at several temperatures, noted that C-CO\({}_{2}\) release from abiotic incubations was 50-90% of that of inoculated (biotic) incubations, and both generally decreased with increasing pyrolysis temperatures. Such findings were similar to our results. Therefore, the less condensed pyrogenic-C, and, thus, the more available oxidizable-C found in the biochars produced at lower pyrolysis temperatures (BC300 and BC400), stimulate soil microbial activity and induce a positive priming effect ([PERSON], 2016).