In order to explore the biological effect of monosodium glutamate wastewater on forest soil and determine the appropriate application level of monosodium glutamate wastewater and inorganic fertilizer for improving poplar growth, a pot experiment including five treatments, i.e. CK (neither urea nor monosodium glutamate wastewater was applied), N100 (100% of nitrogen was provided by urea), M10N90 (10% and 90% of nitrogen were provided by monosodium glutamate wastewater and urea, respectively), M30N70 (30% and 70% of nitrogen were provided by monosodium glutamate wastewater and urea, respectively) and M50N50 (50% and 50% of nitrogen were provided by monosodium glutamate wastewater and urea, respectively) was performed. The present study was conducted to determine the effects of different treatments on active organic carbon, carbon pool management index (CPMI), microbial respiration and metabolic quotient in soil, as well as growth of poplar seedlings. Results showed that in comparison with N100 treatment, monosodium glutamate wastewater application significantly increased the contents of active organic carbon, mid-active organic carbon and highly active organic carbon. The active organic carbon content and CPMI in M30N70 treatment were obviously higher than other treatments, increasing 34.78% and 42.96 compared with N100 treatment, respectively. The M30N70 treatment also significantly increased the contents of microbial biomass carbon and microbial biomass nitrogen. In the meantime, the microbial respiration was evidently enhanced by the use of M30N70 treatment, while the metabolic quotient was decreased, increasing 81.13%, 35.21%, 17.07% and 5.49% in microbial respiration over the treatments of CK, N100, M10N90 and M50N50, respectively. And the metabolic quotient was reduced by 9.16%, 10.37%, 6.98% and 5.80%, respectively. Additionally, the ground diameter and plant height in M30N70 treatment also reached the maximum and had significant differences with other treatments. However, in M10N90 and M50N50 treatments, less effect on micro-domain environment characteristics and growth was observed than that in M30N70 treatment. Correlation analysis revealed that ground diameter and plant height were significantly or extremely significantly correlated with different degree of soil active organic carbon, carbon pool management index (CPMI) and microbial activity, indicating close inner link among every indicator. These results suggested that monosodium glutamate wastewater combined with inorganic fertilizer, especially M30N70 treatment, was beneficial to increase the active organic carbon content, enhance the microbial activity in soil and promote growth of poplar seedlings.