Low-dose biochar and nitrogen fertilizer reduces nitrogen loss in acidic tea soils

Excessive nitrogen fertilizer in red-soil tea plantations reduces quality and yield in southern China and causes environmental issues. This study employed 15 N isotope tracing to evaluate the effects of co-applying biochar (0%, 1%, 2%, and 4%) with nitrogen fertilizer on tea growth, nitrogen use efficiency (NUE), and soil microbial communities in acidic red-soil plantations. The application of 1–2% biochar showed positive outcomes, including increased tea biomass, reduced fertilizer residue and loss, and enhanced soil microbial diversity. In contrast, a high dose of biochar (4%) decreased NUE in whole plants. This reduction was accompanied by an 82.1% increase in urease activity and a 49.5% decrease in nitrate reductase activity. Our findings suggest that co-applying 1–2% biochar with nitrogen represents a promising strategy to optimize root nitrogen uptake, improve tea biomass, and regulate soil microbial communities, thereby supporting more sustainable nitrogen management in tea plantations.