Mechanisms of biochar-based organic fertilizers enhancing maize yield on a Chinese Chernozem: Root traits, soil quality and soil microorganisms

The Chinese Chernozems face extreme challenges of soil degradation and crop productivity decline. Biochar-based organic fertilizer (BOF) is effectively applied to improve crop yield and soil quality. Nonetheless, the mechanisms involved in yield enhancement in interaction with root traits, soil quality, and microorganisms after BOF application remain unclear. Herein, a two-year field experiment with three BOF rates [3.0 Mg ha⁻¹ (BOF3), 6.0 Mg ha⁻¹ (BOF6), 9.0 Mg ha⁻¹ (BOF9)] was conducted to evaluate the impact of BOF on root traits, soil quality, and soil microorganisms, with no BOF applied as control treatment (CK). Compared to CK, BOF increased maize yield by 9.1 ∼ 32.0 % in the second year of application, and the highest yield (16.1 Mg ha⁻¹) was obtained with BOF6. BOF increased the plant growth index by 29.7 ∼ 98.9 %, mainly attributed to the increase in root volume (2.4 ∼ 57.4 %), aboveground biomass (31.4 ∼ 44.0 %), and nitrogen (N) uptake (123.2 ∼ 216.0 %). The soil quality index was increased not only in the topsoil (27.7 ∼ 31.1 %) but also in the subsoil (22.7 ∼ 25.1 %) with BOF, particularly with the two highest rates (BOF6 and BOF9). This was mainly ascribed to the notable increase in microbial biomass carbon, total N, NH₄⁺-N, and N-acquiring enzyme activity (normalized by leucine aminopeptidase and alkaline phosphatase). Moreover, BOF increased the abundance of Actinobacteria, Mortierellomycota at 0–40 cm, Chloroflexi at 0–20 cm, Acidobacteriota, and Ascomycota at 20–40 cm, which confirmed their key roles in activating soil nutrient and subsequent enhancing soil quality and stimulating plant growth. Overall, biochar-based organic fertilizer with a rate of 6.0 Mg ha⁻¹ is a promising fertilizer regime to enhance soil quality by regulating soil keystone taxa, thereby boosting maize growth and yield on the Chernozem soil.