# Carbon Mineralization in a Soil Amended with Sewage Sludge-Derived Biochar
## 3 Results and Discussion
### Kinetics of C-CO\({}_{2}\) Efflux  
The results from C-CO\({}_{2}\) efflux from the different biochars, SS, and the control (S) are presented in Figure 2. The E-first order model presented high R\({}^{2}\) values indicating adequate adjustment (model fit) to describe the dynamics of C mineralization from the biochars. Additionally, the kinetics of C-CO\({}_{2}\) evolution from soil amended with BC300 was well fit to this model with a determination coefficient of 0.74 (Figure 2). Microbial activity was higher in the first 20 to 25 days after the beginning of the incubation period and decreased over time. The higher mineralization at the beginning may be attributed to the available readily oxidizable organic C, found in the biochars produced at temperatures lower than 500 \({}^{\circ}\)C ([PERSON], 2016); C-CO\({}_{2}\) effluxes are positively correlated to SS labile fractions ([PERSON], 2016). Once this fraction is depleted, C-CO\({}_{2}\) effluxes are maintained constant. In general, C-CO\({}_{2}\) effluxes were higher when the soil was mixed with biochars produced at lower temperatures (BC300 and BC400) and with SS, where there was a stimulus to C mineralization, particularly during the initial incubation phase. Differently, the biochar produced at a higher temperature (BC500) produced a similar C-CO\({}_{2}\) efflux from the control, showing the low capacity of degradation of this biochar. Figure 2: Kinetics of the E-first order model describing C-CO\({}_{2}\) efflux (C mineralization) from soil and sewage sludge (S + SS), soil and sewage sludge-derived biochars produced under different pyrolysis temperatures of 300 \({}^{\circ}\)C (S + BC300), 400 \({}^{\circ}\)C (S + BC400), and 500 \({}^{\circ}\)C (S + BC500) and soil (S) as the control. In a study by [PERSON] et al. [42], the C-CO\({}_{2}\) effluxes were also similar between the control (S) and the soil mixed with the biochar (hardwood-derived) produced at a higher temperature (500 \({}^{\circ}\)C). Therefore, our results show that the SS biochar (produced at 500 \({}^{\circ}\)C) application to soil and agricultural systems is a strategy to mitigate C-CO\({}_{2}\) emissions from soil, in contrast with the direct application of SS or low-temperature-derived biochars. Although BC500 did not alter the microbial activity, changes in the microbial composition may occur, as suggested by [PERSON] and [PERSON] [43]. Microorganisms that colonize fresh biochars, which contain more volatile post-pyrolysis compounds on their surfaces, differ from those microorganisms that colonize it afterward when these compounds have been metabolized [43]. Therefore, future studies should focus on evaluating these different microorganisms involved in the degradation of biochar with time. Such studies are important to achieve a better understanding of the effects of adding different feedstock-biochars to various soil types and their interactions with plant growth. The kinetic model shows two types of behavior: one containing a labile phase and the other a more stable phase. The chemical character of the two sets are the following: (1) an aliphatic portion found mainly in the outer surfaces of particles and that occurs in higher amounts in biochars produced under lower pyrolysis temperatures, and (2) another portion composed of highly condensed or condensed aromatic structures within protective inner pores, which are more abundant in the biochars produced at higher temperatures [14; 39]. These results suggest that, in the long term, the interaction of biochar-soil increases soil C storage through the sorption processes of organic matter and biochar, in addition to physical protection. The C-CO\({}_{2}\) effluxes measured in this study are very similar to those found in previous studies, although they differ in several aspects, such as the type of feedstocks and temperatures used to produce the biochars, the incubation time, and the measurement (technique) used to determine the C-CO\({}_{2}\) evolution.