Spatio-Temporal Variations in Soil Phosphorus Pools Indicate Their Bioavailability

Long-term soil phosphorus (P) supply is sustained by soil P pools with varying bioavailability. Theoretically, the more bioavailable a soil P pool is, the more easily it can be influenced by spatio-temporal variations in biological P utilization and abiotic factors (e.g., lithology), and therefore the greater spatio-temporal variation it exhibits. However, this hypothesis has not been explicitly tested. Here, we tested this hypothesis by examining the empirical relationships between spatio-temporal variations and the theoretical bioavailability of soil P pools, as defined by the Hedley fractionation procedure, both in a subtropical forest and at the global scale. As hypothesized, spatial and temporal variations in Hedley P pools generally increased with their theoretical bioavailability in the selected subtropical forest and globally. Specifically, spatio-temporal variations in inorganic P pools were ranked as occluded Pi < moderately labile Pi < labile Pi < readily available Pi, and organic P pools were ranked as occluded Po < moderately labile Po < labile Po. Spatial variation in Pi pools exceeded that in Po pools from the same extracts, whereas temporal variation in Pi pools was typically lower than that in Po pools. Additionally, primary mineral Pi exhibited significant spatio-temporal variation, ranking second only to readily available Pi, and was likely due to stronger influences from abiotic factors such as pedogenesis and lithology. These findings demonstrate a general consistency between spatio-temporal variations and the bioavailability of soil P pools, supporting the reliability of the Hedley fractionation procedure in distinguishing soil P pools with different bioavailability.