Combining N fertilization with biochar affects root-shoot growth, rhizosphere soil properties and bacterial communities under sugarcane monocropping

High-dose nitrogen (N) fertilization has been widely adopted to achieve higher yield in intensive sugarcane monocropping system in Southern China, while long-term excessive N fertilization causes high N losses and negative environmental impacts. Biochar (BC) has been considered a soil amendment to potentially improve soil fertility and crop productivity. However, little is known about the effects of N fertilization combined with BC on sugarcane growth, rhizosphere soil characteristics and microbial community composition. In this study, a greenhouse-pot experiment with six treatments in triplicates was carried out to examine the responses of root-shoot growth, rhizosphere soil properties and bacterial communities to different BC and N fertilization treatments in sugarcane monocropping soil (duration over 20 years). The results demonstrated that N fertilization combined with BC promoted the aboveground growth of sugarcane, while different dose of BC exhibited a significant effect on switching root and shoot growth responses. With the increase in the BC amendment rate, the rhizosphere soil pH, TC and C/N ratio increased correspondingly. As compared with the N-only treatment, N fertilization combined with moderate dose of BC significantly decreased soil NO3--N and nitrification potential (SNP), whereas N treatment with higher-dose BC resulted in a sharp increase in SNP. DESeq2 analysis revealed that N fertilization coupled with higher BC-amendment enriched Mesorhizobium, Porphyrobacter, Rhizobacter and Allorhizobium-Neorhizobium-Pararhizobium-Rhizobium. Redundancy analysis indicated that soil pH, NO3--N and C/N were key edaphic factors that shifted the soil bacterial community composition, especially for nitrifying flora and N-fixing flora. The above findings suggest that the combination of N fertilization with moderate-dose BC might be an alternative strategy to promote crop growth and soil sustainability under sugarcane monocropping system.