The Effect of Combined Application of Calcium, Magnesium, Phosphorus and Monoammonium Phosphate on Growth and Nutrient Uptake of Eucalyptus grandis × E. urophylla Seedlings

Taking February-born tissue-cultured seedlings of Eucalyptus grandis × E. urophylla as the research object, under pot experiment conditions, a single-factor randomized block design was adopted. Five different proportional combinations of CMP and MAP were set as treatments. The effects of different phosphorus fertilizer ratios on seedling growth, nutrient distribution and accumulation in various organs, soil fertility, nutrient uptake, and substrate fertility were comprehensively evaluated using the membership function analysis method. The results showed that different phosphorus fertilizer treatments significantly improved seedling height growth rate, ground diameter growth rate, and biomass of various organs (roots, stems, leaves), all of which were highest under T₄. The growth patterns of Eucalyptus grandis × E. urophylla seedlings across treatments were generally consistent, exhibiting a "slow-fast-slow" trend. With the increase in MAP proportion, the absorption and accumulation of nitrogen, phosphorus, and potassium in various organs increased to varying degrees, all significantly higher than CK. Among them, T1 showed the highest potassium content in various organs, T₃ showed the highest nitrogen content and nitrogen accumulation in most organs, and T₄ showed the highest phosphorus content, phosphorus accumulation, and potassium accumulation in various organs. Substrate total phosphorus and available phosphorus content were highest under T₄, ammonium nitrogen and nitrate nitrogen content were highest under T₃, and available potassium content was highest under T₁. Mantel test results indicated that, except for substrate nitrate nitrogen, the growth of tissue-cultured seedlings under different phosphorus fertilizer ratios was significantly correlated with nutrient uptake and substrate fertility. Correlation analysis revealed that, except for root nitrogen, substrate total phosphorus and available phosphorus content were significantly positively correlated with the nitrogen, phosphorus, and potassium content in various organs. Membership function analysis ranked T4 first overall, indicating that this treatment had the best comprehensive promotion effect on seedling growth, nutrient uptake, and substrate fertility. In the early growth stage of seedlings, under equivalent fertilization rates of N, P, and K, the T₄ (m_CMP:m_MAP = 0:4) fertilization treatment is more conducive to the growth and development of tissue-cultured Eucalyptus grandis × E. urophylla seedlings, nutrient absorption and accumulation in various organs, and the increase in substrate nutrients.