## 3 Results
### The Correlation between Adsorption Ability of Biochar and Bacterial Growth  
Considering that the maximal promotion on cell growth by biochar was observed on _B. mucilaginousus_ (Figure 1), the growth activity of _B. mucilaginousus_ was examined after its inoculum was treated by corncob biochar. To confirm the cell adsorption, the BET surface area and SEM image of corncob biochar with or without bacteria adsorption were determined. As can be seen from the SEM image in Figure 2, the surface of untreated corncob biochar was entirely different from that of cell-adsorbed biochar. Many holes were regularly distributed in untreated biochar; however, after incubation with _B. mucilaginousus_ culture, the aggregation of many particles on the surface of biochar made it rugged. BET surface area of cell-adsorbed biochar dramatically decreased to 7.9153 m\({}^{2}\)-g\({}^{-1}\). All the results indicated that _B. mucilaginousus_ cells existed on the surface of corncob biochar. After _B. mucilaginousus_ was adsorbed with biochar during the seed culture process, biochar-adsorbed cells were incubated in fresh nutrient broth without biochar and the cell growth curve was detected. As shown in Figure 3, significantly different growth profiles were observed in _B. mucilaginousus_ fermentation cultures incubated with biochar-treated seed and untreated seed. Compared to the culture incubated with untreated seed, _B. mucilaginousus_ grew more rapidly with the biochar-adsorbed cells as the inoculum,  
\begin{table}
\begin{tabular}{c c c c} \hline
**Property** & **Rice Husk Biochar** & **Corncob Biochar** & **Bamboo Biochar** \\ \hline pH & 10.01 & 9.64 & 9.27 \\ TN (g\(\cdot\)kg\({}^{-1}\)) & 6.68 & 12.69 & 9.17 \\ TOC (g\(\cdot\)kg\({}^{-1}\)) & 460.50 & 541.00 & 859.80 \\ Volatile matter (\%) & 16.45 & 21.94 & 13.90 \\ Ash content (\%) & 42.75 & 18.85 & 0.66 \\ Surface area (m\({}^{2}\)-g\({}^{-1}\)) & 92.52 & 58.38 & 0.11 \\ Average pore size (nm) & 2.72 & 3.34 & — \\ \hline \end{tabular}
\end{table}
Table 1: The physico-chemical properties of biochars from different raw materials.  
Figure 1: Impact of biochars derived from different feedstock on bacterial growth in the artificial culturing medium.  
and the biomass was increased by 48.3% when _B. mucilaginous_ was fermented for 18 h. The result indicated that cells released from biochar gained better growth activity than free cells did. The similar promotion in growth activity was also observed differentially on the other tested bacteria (data not shown) [19].  
It has been suggested that the large surface area and porous structure of biochar could supply a favorable habitat for microbial colonization [6; 20]. Thus, the correlation between the bacterial-adsorption ability of biochar and the promotion of cell growth was examined. As shown in Figure 4, the corncob biochar showed the strongest adsorption on _B. thuringiensis_, and great adsorption on _B. cereus_, _B. megaterium_ (AS1217), _B. mucilaginous_, and _Orthobactrum_ sp., but little adsorption on _A. chroococcum_. However, the variation trend in bacterial growth was not in accordance with that observed in bacterial adsorption. As for the strain _K. viridis_, the strong adsorption by biochar could also be observed, however, its growth was dramatically suppressed by the addition of biochar. Thus, the adsorption of corncob biochar towards bacteria probably is not the sole factor affecting bacterial growth. Other factors, such as chemical compounds and nutrients in biochar and water retention potential of biochar, etc., might also influence bacterial growth.  
Figure 3: The growth curve of _Bacillus mucilaginous_ in the nutrient broth with biochar-adsorbed cells (\(\blacktriangle\)) or free cells (\(\blacktriangledown\)) as inoculum.  
Figure 2: Scanning electron microscope (SEM) images of corncob biochar. (**A**) SEM image of untreated biochar (control); (**B**) SEM image of biochar adsorbed with _Bacillus mucilaginous_ cells.