Response of Seedling Growth and Nutrient Characteristics to Biochar, Fulvic Acid and Nitrogen Form of Pinus kesiya var. langbianensis

To explore the effects of coffee shell biochar, fulvic acid (FA), and N form on the growth and nutrient status of Pinus kesiya var. langbianensis seedlings, experiments were conducted using an L₉(3⁴) orthogonal design with biochar supplement levels (0, 25, 50 g/L), FA concentrations (0, 0.8, 1.6 g/L), and N forms (no nitrogen, nitrate nitrogen, ammonium nitrogen). A total of 9 treatments were used. The effects of factor levels and their combinations on the growth and nutrient characteristics of P. kesiya var. langbianensis seedlings were analyzed. Coffee shell biochar, FA and N significantly promoted the seedlings growth (P < 0.05). Height, ground diameter, and biomass increased first and then decreased with increasing biochar concentration, and seedling growth was optimal at 25 g/L biochar. Biochar supplementation decreased the N content of seedlings, but increased the P and K content. With the increase in biochar, K content increased in parallel, whereas P content increased first and then decreased. FA application significantly increased the height, biomass, N, and P contents of the seedlings. N addition promoted the growth of seedlings and increased the contents of N and P. The growth and P content of seedlings treated with ammonium nitrogen were significantly higher than those of nitrate nitrogen, and ammonium nitrogen significantly increased the P content. In contrast, nitrate nitrogen had no significant effect on P content. Positive interaction effects between FA and N were observed on height and N content of the seedlings. The range of effect degree of each factor on seedling growth was coffee shell biochar > N > FA. Correlation analysis showed that seedling height, BD, and biomass increased synergistically, and growth parameters were significantly correlated with P content (P < 0.01), indicating P was a limiting element for seedling growth. Coffee shell biochar, FA and N promote seedling growth by enhancing the absorption of N, P and K. The optimal treatment is 25 g/L biochar and 0.8 g/L FA combined with ammonium nitrogen, and it is recommended to supplement P fertilizer appropriately during production.