Effects of biochar on sugarcane growth and rhizosphere microecosystem under reduced nitrogen fertilization: a 5‑year field experiment study

Abstract Previous studies have shown that reduced nitrogen application combined with biochar amendment can promote sugarcane growth and improve soil quality; however, their long-term effects on the rhizosphere microenvironment and microbial community in continuously cropped sugarcane remain unclear. A five-year field experiment was conducted with two treatments: basal fertilizer (BF) and basal fertilizer combined with biochar (BF-BC). After fertilization in the first year, and with no additional fertilizers applied in the subsequent years, relevant indicators at the end of the 5-year period were measured. The results showed that, compared with the control BF treatment, the BF-BC treatment significantly increased sugarcane plant height, stem diameter, and leaf nitrogen balance index (NBI) by 10.81%, 25.79%, and 33.90%, respectively, and resulted in significant reductions in total root volume and average root diameter by 31.06% and 21.53% ( P < 0.05). Simultaneously, the rhizosphere soil pH and total potassium (TK) content increased significantly by 17.74% and 79.21%, whereas soil organic matter (SOM), organic carbon (SOC), total phosphorus (TP), available potassium (AK), and exchangeable calcium ions (E. Ca 2+ ) decreased significantly by 37.67%, 39.64%, 21.20%, 47.29%, and 12.11%, respectively ( P < 0.05). Despite receiving no additional fertilization following the initial application, the BF-BC treatment still exhibited significant advantages in promoting sugarcane fine root growth, enhancing rhizosphere soil carbon sequestration, and improving fertilizer use efficiency. Additionally, the BF-BC treatment significantly increased the abundance of beneficial rhizosphere bacteria such as Leptospirillum , Terrimonas , Actinobacteriota , Sphingobacteriia , Chitinophaga , Cyanobacteriia , and Lechevalieria ( P < 0.05). Furthermore, the differentially expressed metabolites in the sugarcane rhizosphere were significantly enriched in major metabolic pathways, including steroids and steroid derivatives, fatty acyl groups, purine nucleotides, imidazole pyrimidines, sphingolipids, organic oxygen compounds, indoles and their derivatives, carboxylic acids and derivatives, and benzodioxoles. Importantly, the BF-BC treatment effectively reduced CO₂ emissions from the soil. In conclusion, the sugarcane root system, surrounding soil, and microorganisms form a complex, interconnected symbiotic ecological network. Thus, even after five years without fertilization, reduced nitrogen combined with biochar application still positively influenced sugarcane root and aboveground biomass growth. This finding suggests that biochar co-application enhances long-term soil fertility. This study provides a reference for fertilization practices and soil improvement in the cultivation of sugarcane and other crops. Graphical Abstract