Biochar altered microbial nitrogen transformation and enhanced plant nitrogen utilization efficiency in an alkaline soil

Rhododendron, an important acidophilic plant, often faces challenges in urban soils that typically have an alkaline reaction and high bulk density due to low organic matter. Understanding how biochar alleviates urban soil stresses, promotes plant growth, and enhances nitrogen use efficiency (NUE) is crucial for advancing urban landscaping. A pot experiment was conducted to investigate the effects of different doses of biochar (0%-8% w/w) on the root development and NUE of Rhododendron, with a focus on the underlying micro-ecological mechanisms. Biochar amendment dose-dependently enhanced total fine root length (57.2-85.4%) and plant NUE (94.2-199%), along with increases in the fungal-to-bacterial ratio and mycorrhizal colonization. Illumina HiSeq sequencing revealed that biochar reshaped the soil microbial community by not only suppressing N-competing bacteria (e.g., Lysobacter) and pathogenic fungi (e.g., Dactylonectria) but also enriching beneficial fungi (e.g., Condenascus) and copiotrophic bacteria (e.g., Pseudomonas). These shifts likely influenced soil N transformations. Relevantly, biochar enhanced ammonia-oxidizing bacteria (AOB) abundance but reduced nirS abundance, with concurrent increases in potential nitrification and decreases in denitrification activity. At higher application rates (4%-8%), the decreased (nirS + nirK)/nosZ ratio indicated a potential reduction in N₂O emissions. Partial least squares path modeling confirmed that AOB abundance was associated with soil fertility, while nirS was linked to soil pH, with both pathways ultimately governing NUE. Overall, this study demonstrated that weakly acidic biochar could enhance the growth and NUE of acidophilic plants by selectively reshaping functional microbial communities and steering inorganic N transformation toward more efficient pathways.