## 4 Discussion
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Characteristic**} & \multicolumn{2}{c}{_Bacillus Mucilaginous_} & \multicolumn{2}{c}{_Bacillus Megaterium_} & \multicolumn{2}{c}{_Ochrobactrum_**sp.**} \\  & **AS1153** & \multicolumn{2}{c}{**AS1217**} & \multicolumn{2}{c}{**ACCC10085**} \\ \cline{2-7}  & \(-\)**biochar** & \(+\)**biochar** & \(-\)**biochar** & \(+\)**biochar** & \(-\)**biochar** & \(+\)**biochar** \\ \hline Potassium content & \multirow{2}{*}{\(9.51\pm 0.43\)} & \multirow{2}{*}{\(67.27\pm 0.44\)} & \multirow{2}{*}{\(-\)} & \multirow{2}{*}{\(-\)} & \multirow{2}{*}{\(-\)} & \multirow{2}{*}{\(-\)} \\ (\(\mu\)g:mL\({}^{-1}\)) & & & & & & \\ Phosphate content & \multirow{2}{*}{—} & \multirow{2}{*}{\(28.37\pm 0.65\)} & \multirow{2}{*}{\(98.98\pm 1.13\)} & \multirow{2}{*}{\(86.32\pm 0.89\)} & \multirow{2}{*}{\(107.11\pm 0.71\)} \\ (\(\mu\)g:mL\({}^{-1}\)) & & & & & & \\ \hline \multicolumn{7}{l}{\({}^{*}\)\(-\)\(-\)\({}^{\prime\prime}\)\(+\)\({}^{\prime}\): Bacterial strains were cultured without or with biochar addition. \({}^{*}\)—\({}^{*}\): Not determined.} \\ \end{tabular}
\end{table} 
Table 2: Impact of corncob biochar on potassium-/phosphate-releasing content of bacterial cultures. also spread differentially across the other tested strains (data not shown), suggesting that biochar could be applied as a potential inoculum carrier. The adsorbing capability of biochar towards _K. viridis_ was 25.1% but its growth was negatively influenced by biochar addition. It was presumed that besides the intrinsic structure of biochar, other factors might influence bacterial growth. Water-soluble organic compounds (WSOCs), such as karrikins, carboxylic acids, phenolic and organic N compounds, and polycyclic aromatic hydrocarbons, have been proved to be available in biochar and could positively or negatively influence plant growth by affecting microbial-mediated processes in soil, such as C and N cycling [23; 24; 25; 26]. Therefore, the influence of water-soluble components in biochar on cell growth was investigated. After soluble components were removed from biochar, the growth-promotion degree of most tested strains decreased except for _A. droococcum_ and _B. megaterium_ (ACCC01667). Whereas the negative influence of washed biochar on cell growth was detected in _Pseudomonas_ sp. and _B. cereus_, which is in accordance with the previous observation that the intrinsic properties of biochar could adversely impact bacterial growth [6]. Especially, the growth inhibition of _K. viridis_ disappeared when the corncob biochar was washed (Figure 4). Correspondingly, the growth of _A. droococcum_, _B. megaterium_ (ACCC01667), and _K. viridis_ was blocked by the water extracts of biochar (Figure 4). More recently, compounds inhibiting microbial activities have also been found either in biochar [7] or its water extracts after introduction into soil [27]. Besides, the water-soluble components in biochar promoted the growth of strain AS1217 but inhibited the other strain ACCC01667 belonging to _B. megaterium_ (Figure 4). Therefore, it was unlikely that the impact of WSOCs spread equally across different phylotypes [9]. All these results indicated that water-soluble components in biochar could either stimulate or inhibit bacterial growth depending on individual bacteria, which was in line with the previous findings that some chemicals that commonly adsorbed in the biochar had the possible species-specific effect on a variety of microorganisms [28; 29]. To further understand how biochar improves crop production by enhancing soil fertility, the influence of corncob biochar on potassium-/phosphate-solubilizing activities was also examined in this paper [21]. The K-solubilizing strain _B. mucilaginous_ could convert unavailable potassium to its available form through biological processes, which explained its plant growth promoting activity [12]. The present study showed that biochar could dramatically enhance the potassium-solubilizing activities of _B.