Phosphorus Speciation Suggests Contrasting P Supply Mechanisms Across Aggregates in Subtropical Forest Soils With Different Lithology

ABSTRACT Background Phosphorus (P) is a limiting element in highly weathered subtropical/tropical soils. Depending on lithology, tropical soils differ strongly in soil chemistry and aggregate structure. Aims This study investigates how differences in lithology shape the P speciation and distribution and hence P availability across soil aggregates in subtropical forests. Methods The total P (TP), exchangeable P (Ex‐P), and P fractions (e.g., iron [Fe]–P, aluminum [Al]–P, calcium‐bound Au‐P, detritus De‐P, labile [LP], moderately labile [MLP], and stable organic P [SP]) were quantified. Microbial biomass P (MBP) and phosphatase activities (acid phosphatase and phytase) were measured in bulk soils and soil aggregates from two adjacent forests, one on acidic red soil and the other on limestone soil. Results (1) Red soil was lower in TP (353.32 mg kg −1 ) and Ex‐P (2.22 mg kg −1 ) than limestone soil (706.09 and 8.8 mg kg −1 , respectively). In red soil, Fe–P dominated inorganic P (69.7%–85.0%), whereas limestone soil allocated more inorganic P to Au‐P (10.4%–15.9%) and De‐P (23.1%–28.4%). (2) Red soil was dominated by SP which was slightly enriched in macroaggregates, while limestone soil held a higher proportion of LP and all organic P fractions were more equally distributed across aggregate sizes. (3) Red soil MBP concentrated in macroaggregates (1.5%–2.2% of TP) and correlated positively with labile P forms (e.g., Al–P and LP). Limestone soil MBP was small (<1% of TP) and concentrated in microaggregates, correlating negatively with P availability in the macroaggregates. (4) Phytase activities correlated positively with organic P forms in both macro and microaggregates of the red soil but not in the limestone soil. Conclusion The findings imply contrasting roles of microorganisms for P supply: Microorganisms mediate a tighter P cycle in the P‐poor red soil, whereas in the P‐rich limestone soil, macroaggregates appear to limit microbial P uptake by calcium adsorption and occlusion, hence contributing to P sequestration. These findings underscore the critical role of lithology in shaping P storage and availability in subtropical forests, offering insights for targeted soil management in conservation and restoration efforts.