standard error. Different letters indicate significant differences (P<0.05). * P<0.05, **P<0.01, *** P<0.001. CK, C, C-N, C-M, C-M-N represent CK, biochar, biochar +N fertilizer, biochar + microbial agent, biochar +N fertilizer + microbial agent (N fertilizer and microbial agent did not refine to levels)