Early growth and C-N-P stoichiometric responses of Ochroma lagopus to combined application of nitrogen fertilizer and biochar.

Ochroma lagopus, the world's lightest commercially available timber species, is generally nitrogen-limited during growth. While nitrogen application is a common practice to enhance productivity in plantation forests, low nitrogen fertilizer use efficiency often exacerbates phosphorus limitation and poses environmental risks. Biochar, as a soil amendment, can improve nitrogen use efficiency. This study investigated the effects of nitrogen fertilization and biochar application on the growth and biochar (C), nitrogen (N), and phosphorus (P) stoichiometric characteristics of one-year-old O. lagopus trees. Seven treatments were established: no fertilization (CK), and single N application at low (N1), medium (N2), and high (N3) levels, along with corresponding biochar combined application treatments (BN1, BN2, BN3). Results indicate that although N fertilizer alone promotes tree height and diameter at breast height growth (N3 increased by 35.06% and 29.02% compared to CK), high N levels did not yield significant additional benefits. The combined nitrogen-charcoal application BN2 (1000 g biochar + 540 g urea per tree) yielded the optimal effect, with tree height and diameter increments significantly exceeding CK by 51.62% and 46.88%, respectively. Applying N fertilizer alone resulted in decreased leaf P content and elevated N/P ratios (>16), indicating P became the limiting factor. Combined application of biochar restored P content across organs and normalized N/P ratios (BN1 treatment maintained N/P between 14 and 16), effectively alleviating P limitation. O. lagopus optimizes nutrient allocation strategies by enhancing root C, N, and P content and phenotypic plasticity, thereby improving overall utilization efficiency. Principal component analysis indicates that the BN2 treatment received the highest overall evaluation, demonstrating the most optimal fertilization effect. Therefore, it is recommended to apply biochar at medium to high N levels to alleviate P limitation, maintain elemental balance, and enhance productivity in O. lagopus plantations.