Optimizing Nitrogen Management in Acidic Tea Orchard Soils: The Role of Biochar-Based Fertilizers in Reducing Losses and Enhancing Sequestration

Biochar-based fertilizers have attracted increasing attention as sustainable soil amendments due to their potential to enhance nitrogen (N) retention and mitigate N losses. However, their effects on N dynamics in tea orchard soils remain inadequately understood. This study investigated the impact of biochar-based fertilizer (BF) on N migration and transformation into acidic tea orchard soils through controlled laboratory experiments comprising nine treatments, including sole urea (U) applications and various combinations of BF and U. The results showed that ammonia (NH 3 ) volatilization peaked within seven days after application. Compared with urea-only treatments, the application of BF at 15 t·ha −1 combined with a low U application rate (0.72 t·ha −1 ) significantly reduced NH 3 and total dissolved nitrogen losses by up to 22.33% and 33.56%, respectively, while higher BF rates increased these losses. BF applications markedly improved soil N sequestration, as evidenced by increases in total nitrogen, ammonium nitrogen (NH 4 + -N), nitrate nitrogen (NO 3 − -N), and the NH 4 + -N/NO 3 − -N ratio. Additionally, soil organic carbon, urease activity, and pH were significantly enhanced. Random forest analysis identified soil pH and organic carbon as the primary predictors of NH 3 volatilization and soil N retention. Partial least squares path modeling revealed that the BF-to-urea ratio governed N dynamics by directly influencing N transformation and indirectly modifying soil physicochemical properties. BF applied at ≤15 t·ha −1 with low U inputs exhibited potential for improving N use efficiency and sustainability, pending further field validation.