Abstract: A reduction of chemical fertilizers and improving fertilizer utilization rate are important for ensuring a balance between plant growth and minimizing the degradation of the black soil. We conducted a 2-year pot experiment with four treatments during 2019 and 2020, including T₀: no fertilizer, T₁: conventional use of chemical fertilizer, T₂: 15% reduction of the chemical fertilizer combined with 400 kg·hm^-2 of humic acid bio-fertilizer (HABF), and T₃: 30% reduction of the chemical fertilizer combined with 600 kg·hm^-2 of HABF, to examine the effect of reduction rates of chemical fertilizers combined with the HABF on soil microbial abundance, enzyme activity and nutrient content in maize cultivation. The results showed that the application of HABF significantly increased the abundance of soil bacteria and fungi, with the number of microbial colonies being positively correlated with the amount of HABF. When measured at the tassel stage of maize growth, T₂ and T₃ treatments significantly increased the activities of urease, sucrase, and catalase in soil by 11.4%-21.6%, 34.9%-46.7%, and 6.5%-13.4%, respectively. The available nitrogen contents in T₂ and T₃ treatments were higher than that in the T₁ treatment by 8.2%-18.1%, which ensured the sufficient nitrogen supply to maize after the tassel stage. Soil available phosphorus and available potassium contents increased by 17.1%-121.0% and 9.6%-57.3%, respectively, compared with T₁ treatment. With the increases of the amount of HABF, the activation effects of soil phosphorus and potassium and dry matter mass per plant increased significantly in T₂ and T₃ treatments compared with T₁ treatment. In conclusion, HABF promoted the proliferation of soil bacteria and fungi, improved activities of catalase, urease, and sucrase, soil nutrient contents, and dry matter mass per plant. The 15% reduction of chemical fertilizer combined with 400 kg·hm^-2 of HABF is the most suitable nutrient management strategy for maize production in black soil.

Key words: humic acid, bio-fertilizer, chemical fertilizer reduction, biological property