Biochar-nitrogen synergism regulates soil pH and nutrient stoichiometry, enhancing growth and physiological performance of Ochroma lagopus swartz.

This study investigated the synergistic effects of biochar and nitrogen (N) fertilization on soil physical properties and leaf nutrient allocation in Ochroma lagopus Swartz plantations. Seven treatment groups were established: three nitrogen fertilizer levels; three biochar-nitrogen fertilizer combinations; and a control group (CK). The key conclusions as follows: Biochar-N co-application significantly improved soil quality in the short term, mainly reflected in reduced bulk density, increased porosity, mitigation of N-induced soil acidification (pH standardized at 6.8–7.2), and increased soil total N and P contents. At the leaf level, co-application significantly enhanced chlorophyll synthesis and NSCs accumulation, improved N use efficiency (higher leaf N content and lower C/N ratio in optimal treatments), and modulated carbohydrate partitioning (reduced soluble sugar/starch ratio, favouring long-term C storage). RDA showed soil P and pH were key regulators of leaf C: N:P stoichiometry: biochar elevated soil pH to improve P availability. Within the 150-day observation period, BN2 was identified as the most effective regime for maximizing O. lagopus growth (plant height and diameter at breast height) while maintaining soil C: N ratios conducive to nutrient cycling, thus achieving a balance between plantation productivity and soil sustainability during the seedling establishment stage.