Effect of Biochar Using N, P, and K Fertilisers on Growth and Quality of <i>Lithocarpus litseifolius</i>

Objective: In this study, we aimed to investigate the effects of facile combinations of biochar and N, P, and K fertilisers on the growth and physiological characteristics of <i>Lithocarpus litseifolius</i> seedlings, and to optimise the biochar/NPK ratio of <i>Lithocarpus litseifolius</i>. Method: A four-factor three-level orthogonal method was used to conduct a field experiment using 2-year-old <i>Lithocarpus litseifolius.</i> Nine fertiliser treatments and one control treatment (CK, i.e., no fertiliser) were established in a completely randomised block group with six replications of ten treatments. The plants were planted in October 2020 and harvested in November 2021, and fertiliser was applied once in April 2021 and once in July 2021. Results: Rational application could effectively promote the growth of <i>Lithocarpus litseifolius</i>, and T4(C₂N₁P₂K₃) produced the highest increase in plant height growth (17.03 cm), diameter growth (5.47 mm), soluble sugar (94.60 mg/g), soluble protein (4.59 mg/g), and phlorizin (old leaf: 16.00%; tender leaf: 15.13%); T8(C₃N₂P₁K₃) resulted in the highest chlorophyll a content (1.46 mg/g), chlorophyll b content (0.62 mg/g), and total chlorophyll content (2.08 mg/g), and T1(C₁N₁P₁K₁) resulted in the highest contents of starch (11.60 mg/g) and trilobatin (old leaf: 0.29%; tender leaf: 2.28%). The indicators corresponding to the above three treatments were significantly higher than those under the other treatments (<i>p</i> < 0.05). The results as analysed by the affiliation function method show that the highest mean value of the affiliation function was 0.645 after T4(C₂N₁P₂K₃) treatment. Conclusions: The combination of biochar and nitrogen, phosphorus, and potassium fertilisers can effectively increase the biomass and active components of <i>Lithocarpus litseifolius</i> while reducing the amount of chemical fertiliser applied. A comprehensive analysis of the results showed that the T4 treatment (biochar: 20 g/plant; urea: 10 g/plant; superphosphate: 9 g/plant; potassium chloride: 12 g/plant) resulted in the highest comprehensive score, with the highest increase in plant height growth, ground diameter growth, root–crown ratio, soluble sugar, soluble protein, and phlorizin, as well as other indicators.