# Carbon Mineralization in a Soil Amended with Sewage Sludge-Derived Biochar
## 3 Results and Discussion
### Nitrogen Availability  
The highest nitrate (N-NO\({}_{3}\)\({}^{-}\)) levels were observed in S + SS, whereas the other treatments did not increase N-NO\({}_{3}\)\({}^{-}\) in comparison to the control (Figure 3). N-NO\({}_{3}\)\({}^{-}\) concentration decreased in the biochars with increasing pyrolysis temperature (Table 2), as a consequence of the volatilization process [2]. Although the application of biochars did not increase N-NO\({}_{3}\)\({}^{-}\) levels (available N form), total nitrogen may have not been fully available for plant absorption due to organic binding in recalcitrant compounds [45]. It is believed that biochars have the potential to render nitrates \begin{table}
\begin{tabular}{c c c} \hline \hline
**Materials** & **CCE (mg of C-CO\({}_{2}\) kg\({}^{-1}\) of Soil)** \\ \hline S & 63.2 & b \\ S + SS & 103.8 & a \\ S + BC300 & 101.6 & a \\ S + BC400 & 101.1 & a \\ S + BC500 & 59.4 & b \\ \hline \hline \end{tabular}  
* S: soil (control); SS: sewage sludge; BC300: SS-derived biochar at 300 \({}^{\circ}\)C; BC400: SS-derived biochar at 400 \({}^{\circ}\)C; SS-derived biochar at 500 \({}^{\circ}\)C. Means followed by the same letters in the column are not significantly different by the Tukey test (\(P<0.05\)).  
\end{table} 
Table 3: Cumulative C-CO\({}_{2}\) efflux (CCE) from soil amended with sewage sludge and biochar. unavailable and decrease its leaching, whereas N fertilizer application to biochars and co-composting of biochars increases N-NO3- availability to plants and microbial activity [46]. Furthermore, [PERSON] et al. [47] suggest that current N-NO3- extraction methods underestimate N-NO3- availability to microbial activity and/or immobilization. The amount of N-NH4+ in soil was found to be higher than N-NO3- contents, for all treatments. Total N-NH4+ contents were higher for the low temperature biochars, following the decreasing order: S + BC300 > S + BC400 > S + SS > S + BC500 > S (Figure 4). Total N-NH4+ increased approximately 450%, 311%, and 150% when BC300, SS, and BC500 were applied to soil compared to the control (S), respectively (Figure 4). These results indicate that, unlike N-NO3-, biochar is able to supply N-NH4+ to the soil over the short term. Biochar produced at a lower temperature (BC300) had the highest capacity to supply N-NH4+ to soil compared to BC500. N is removed in the forms of N-NH4+ and N-NO3- and through volatile matter containing N during the pyrolysis process [48]. However, with increasing pyrolysis temperatures, N forms are gradually transformed into a pyridine-like structure or converted to more stable compounds, like pyrrole-type N [49]. Similarly to our study, [PERSON] et al. [50] found that increasing pyrolysis temperatures from 300 \({}^{\circ}\)C to 700 \({}^{\circ}\)C decreases total N content by 55% due to the volatilization of N during the pyrolysis process. Our results also indicate that the relatively high proportion of N is conserved at lower pyrolysis temperatures; increasing pyrolysis temperature from 300 \({}^{\circ}\)C to 500 \({}^{\circ}\)C decreased N-NH4+ and N-NO3- contents by approximately 64% and 91% when applied to soil, respectively. Contrasting results regarding the effects of biochar N mineralization may be related to the different feedstocks used to produce the biochars and their impacts on soil microbial biomass C and N. Biochars with low amounts of N and with high C/N ratio may limit N use by microorganisms [13], leading to N-NH4+ microbial immobilization and a decrease of available N-NH4+ in soil [51]. On the other hand, N-rich biochars with a low C/N ratio (e.g., maize and manure-wood-derived biochars) limit C use by microorganisms, leading to soil N mineralization and an increase of available N-NH4+ [52; 53]. Biochars produced at lower temperatures may contain significant amounts of labile C, which can be promptly used by soil microorganisms [54]. However, these biochars, when added to soil, may, in a short-term period, result in soil available-N microbial immobilization. As such, [PERSON] et al.