Abstract:

Objective: The aim of this study is to investigate how different stand types affect soil phosphorus availability and to identify the critical inorganic phosphorus fractions that regulate this availability. This research seeks to provide a theoretical foundation for maintaining soil fertility and implementing sustainable management practices in the different plantation soils of Baishanzu National Park. Method: This study took the four types of plantations as objectives, including Chinese fir pure forest, masson pine pure forest, Chinese fir-masson pine mixed forests, Chinese fir-moso bamboo mixed forests. We determined the basic properties, inorganic phosphorus fractions and available phosphorus content of the 0−10, 10−20, and 20−40 cm soil layers, and analyzed the differences of total phosphorus content, available phosphorus content, and inorganic phosphorus fractions among stand types to reveal the key driving factors for the variations in soil phosphorus availability. Result: The total phosphorus and available phosphorus content in the 0−40 cm soil layer across the four stand types were 0.14−0.24 g·kg⁻¹ and 0.73−1.72 mg·kg^?1, respectively, with average values of 0.19 g·kg^?1 and 1.11 mg·kg^?1, respectively. The phosphorus activation coefficient in the 0−40 cm soil layer across the four stand types was 0.42%−0.72%, with an average value of 0.59%. Notably, these values reached their peak in the Chinese fir-moso bamboo mixed forests. The content of each inorganic phosphorus fraction in the 0−40 cm soil layer across the four stand types showed differences, with the Chinese fir-moso bamboo mixed forests having higher values than the other three stand types. Except for occluded phosphate content, the contents of aluminum phosphate, iron phosphate, calcium phosphate, and soluble phosphorus were all significantly positively correlated with available phosphorus content (P<0.01). Standardized major axis analysis revealed that the allometric index for the relationship between aluminum phosphate content and available phosphorus content is significantly below 1.00 (P<0.01). In contrast, the allometric index for iron phosphate content, calcium phosphate content, and soluble phosphorus content in relation to available phosphorus content are all significantly above 1.00 (P<0.01). The random forest analysis identified aluminum phosphate content, hydrolyzable nitrogen content, and total nitrogen content as the most significant regulatory factors influencing available phosphorus content. The partial least squares structural equation model indicated that stand type indirectly enhanced available phosphorus levels by influencing basic soil properties such as total nitrogen content and hydrolyzable nitrogen content, stoichiometric ratios including organic carbon to total nitrogen and total nitrogen to total phosphorus, as well as inorganic phosphorus fractions such as aluminum phosphate content and iron phosphate content. Conclusion: In this study area, while stand type does not have a significant direct impact on available phosphorus, it significantly influences available phosphorus indirectly by modulating nitrogen levels, nutrient ratios, and inorganic phosphorus fractions. Aluminum phosphate emerges as the most critical regulatory factor affecting soil phosphorus availability. Overall, developing mixed coniferous and broad-leaved forests will be a vital strategy for enhancing soil phosphorus availability in the plantation stands of Baishanzu National Park.

Key words: stand types, available phosphorus, inorganic phosphorus fractions, aluminum phosphate, Baishanzu National Park, plantation, influencing factors