Biochar with or Without Nitrogen Fertilizer Influences Soil Microbial Carbon Utilization, Bacterial Diversity, and Nitrogen Dynamics in a Eucalyptus Plantation

Biochar amendment has the potential to enhance plant growth by influencing soil microbial activity and increasing nutrient availability. This study investigated the effects of biochar and N fertilizer on soil microbial carbon use, bacterial community diversity, and nitrogen dynamics in a eucalyptus plantation, aiming to enhance plant nitrogen use efficiency and improve soil health through sustainable management practices. We applied no biochar and no fertilizer (referred to as the control), biochar (BC; 1.4 kg plant^− 1), N fertilizer (NF; 8.0 g plant^− 1), and BC + NF (NFB) to the Eutric Fluvisol in a eucalyptus plantation in Guangxi, China. The results showed that NF and NFB significantly increased plant atom% ¹⁵N in soil and eucalyptus, as well as the soil water content (SWC). Significant positive correlations were observed between atom% ¹⁵N, NO₃^−, and the relative abundance of phyla Actinobacteria and Bacteroidetes in soils. Soil atom% ¹⁵N showed a significant negative relationship with the microbial use of phenolic acids and amines. The path analysis model demonstrates that plant atom% ¹⁵N significantly influenced various dependent variables, including the soil microbial ACE index, soil atom% ¹⁵N, as well as nutrient use efficiency (NUE), chlorophyll content, and total nitrogen (TN) in plants. This study highlights the role of biochar in shaping nitrogen dynamics in soils and plants by positively regulating bacterial diversity and microbial carbon utilization. These findings provide valuable insights into the potential of biochar as an effective soil amendment for improving plant growth and nutrients.