Effects of fertilizer and irrigation on the C, N and P stoichiometry of leaf−root−soil in Populus cathayana × canadasis ‘Xinlin1’

A 10-year-old 'Xinlin1' poplar plantation was selected as the research object. Three fertilization levels were set: 1000 g urea per tree (N), 1000 g compound fertilizer per tree (F),

and 0 g per tree (CK), and three irrigation levels: -20 kPa threshold irrigation (I₂₀), -33 kPa threshold irrigation (I₃₃), and no irrigation (I₀). The carbon and nutrient contents of poplar leaves, fine roots, and rhizosphere soil were measured, and the C, N and P stoichiometry were calculated. The results showed that: Fertilization and irrigation significantly increased the total nitrogen (TN) and total phosphorus (TP) contents in leaves and roots, and decreased C/N and C/P. The I₂₀F treatment had the highest leaf TN (29.8 g·kg⁻¹) and TP content (1.8 g·kg⁻¹), which were significantly higher than those under the I₀CK treatment. The C/P and N/P ratios were the lowest under the I₂₀F (266.4 and 15.7) treatment, which were significantly lower than those under other treatments, with reductions of 26.3% and 5.3% compared to the I₀CK treatment. Fertilization significantly affected soil nutrient contents and stoichiometric ratios, except for TC and C/N. Irrigation only significantly affected the AP and AK contents, and N/P ratio of the rhizosphere soil of 'Xinlin1' poplar. Under the I₂₀F treatment, the rhizosphere soil TP (0.5 g·kg⁻¹), AP (11.4 mg·kg⁻¹), and AK (218.3 mg·kg⁻¹) contents were the highest, which were 28.9%, 44.8%, and 15.6% higher than those under the I₀CK treatment (TP: 0.4 g·kg⁻¹; AP: 7.9 mg·kg⁻¹; AK: 188.9 mg·kg⁻¹), respectively. Correlation analysis results indicated that the annual growth of the diameter at breast height and volume of 'Xinlin1' poplar were significantly positively correlated with the TN and TP contents of leaves and roots, as well as the TP, AP, and AK contents of soil, and significantly negatively correlated with the C/N and C/P ratios of leaves and roots and the N/P ratio of soil. Random forest results showed that the TP contents of leaves and roots and the C/P ratio of leaves were the most important factors affecting tree growth. Fertilization and irrigation significantly increased the nutrient contents of leaves, roots, and rhizosphere soil, and changed the stoichiometric characteristics. The TN and TP contents of leaves and roots and the TP, AP, and AK contents of rhizosphere soil were the highest under the I₂₀F treatment, and the C/P ratios of leaves and roots and the N/P ratio of soil were the lowest. Sufficient water conditions (-20 kPa threshold irrigation) and compound fertilizer application is the best water and fertilizer measures to promote the growth of 'Xinlin1' poplar.