Abstract: 【Objective】This study aimed to reveal the effects of biochar application on soil microbial biomass C, N and P and its stoichiometry characteristics under a poplar plantation. In addition, we aimed to thoroughly understand the effects of biochar application on the structure and function of the soil microbial community and to systematically elucidate the soil microbiological mechanism behind biochar regulation of the biogeochemical cycle of C, N and P in poplar plantations. Ultimately, results of this study will provide important information for safely using biochar application as a carbon sequestration technology in plantation ecosystems. 【Method】Soil microbial biomass C, N, P(i.e. SMBC, SMBN and SMBP)and physicochemical properties(such as soil total organic carbon, total nitrogen, soluble organic carbon, nitrate nitrogen, ammonium nitrogen, available phosphorus, soil density, pH and moisture content)were examined in a poplar plantation in the Dongtai Forest Farm of Jiangsu Province. Four different treatments were used in this study: low biochar addition(T1, 40 t/hm²), middle biochar addition(T2, 80 t/hm²), high biochar addition(T3, 120 t/hm²), and no biochar addition(Control). 【Result】The SMBC, SMBN and SMBP all increased in the three biochar addition treatments relative to the control. SMBC increased by 687.2, 727.0 and 752.6 mg/kg in T1, T2 and T3 treatments, respectively, which was 1.08, 1.15 and 1.19 times higher than that in the control. SMBN increased by 44.1, 48.7 and 52.9 mg/kg in T1, T2 and T3 treatments, respectively, which was 1.12, 1.24, and 1.34 times greater than that in the control. SMBP increased by 8.2, 9.3 and 10.4 mg/kg in T1, T2 and T3 treatments, respectively, which was 1.11, 1.26 and 1.41 times greater than that in the control. SMBC/SMBN, SMBC/SMBP and SMBN/SMBP ratios in T1 were 15.7, 85.0 and 5.4, respectively. SMBC/SMBN, SMBC/SMBP and SMBN/SMBP ratios in T2 were 15.1, 78.6 and 5.2, respectively. SMBC/SMBN, SMBC/SMBP and SMBN/SMBP ratios in T3 were 14.3, 73.3 and 5.1, respectively. Both SMBC/SMBN and SMBC/SMBP ratios were significantly reduced in the three biochar addition treatments and significant differences were observed among treatments(P<0.05), but SMBN/SMBP ratio was only significantly reduced in T2 and T3(P<0.05). SMBC, SMBN, SMBP showed significant seasonal variation. C, N and P levels were higher in plant dormancy season(December 2014 and March 2015)and lower in the vigorous growing season(July 2015 and October 2015). However, seasonal variation of SMBC/SMBN, SMBC/SMBP and SMBN/SMBP ratios showed opposite trends. Biochar addition increased the contents of soluble organic carbon(DOC), nitrate nitrogen(NO^-₃-N), available phosphorus(AP), total nitrogen(TN), and total organic carbon(TOC). Biochar addition also increased soil pH and soil moisture content(SMC)and reduced soil density and ammonium nitrogen(NH⁺₄-N). Correlation analysis showed that SMBC had a highly significant positive correlation with soil DOC, pH, SMC, NO^-₃-N and AP, while it had a highly significant negative correlation with NH⁺₄-N(P<0.01). No significant correlation was observed for soil density, TN or TOC(P≥0.05). SMBN and SMBP had highly significant positive correlations with DOC, pH, SMC, NO^-₃-N, AP and TN and highly significant negative correlations with NH⁺₄-N(P<0.01). No significant correlations were observed for soil density or TOC(P≥0.05). 【Conclusion】Our results provide preliminary evidence that biochar application is useful for promoting soil N and P and supporting poplar growth due to the increase in SMBC, SMBN, SMBP and its optimized stoichiometry characteristics. Growth may be limited by N and P in poplar plantations in this region.