Soil phosphorus forms and their availability in six typical plantations at the southern foot of the Taihang Mountains, China

Assessing the differences in soil phosphorus (P) components between pure and mixed plantations is crucial for understanding the mechanisms of P retention and release in forest ecosystems. This study aimed to evaluate the forms and availability of soil P across different plantation types to inform sustainable forest management practices.We employed a modified Hedley phosphorus fractionation scheme to analyze soils from six plantation stands: pure forests of Robinia pseudoacacia (R), Platycladus orientalis (P), and Quercus variabilis (Q), and their mixed combinations (RQ, RP, PQ). The results demonstrated that: (1) Organic P was the dominant form across all forest types, with significantly higher levels in mixed forests than in pure forests. Both inorganic and organic P decreased with increasing soil depth, with the highest total P concentrations found in the 0–10 cm layer of the RQ forest. Notably, the maximum occluded P (OP) content (193.23 mg·kg⁻¹) was recorded in the 20–30 cm layer of the RQ forest. (2) Phosphorus fractions followed the order: occluded P (OP) > moderately active P (MAP) > readily reactive P (RRP) ≈ available P (AP). (3) Inorganic and organic P showed significant positive correlations with OM, TC, TN, TK, TP, and TOC (P < 0.01). AP, RRP, and MAP were also positively correlated with these soil properties, whereas OP was negatively correlated. Our findings suggest that mixed forests, particularly the R. pseudoacacia–Q. variabilis combination, were associated with enhanced soil P retention and availability. Therefore, forest management strategies in this region should consider mixed-species plantations to optimize P utilization efficiency. These findings provide a scientific basis for improving soil fertility and sustainable P management in afforestation projects.