Abstract: Phosphorus (P) is an indispensable nutrient in terrestrial ecosystems, and its environmental transfractionations and biogeochemical cycling plays a crucial role in sustaining ecosystem structure and functional stability. The distribution of P fractions in soils and their bioavailability constitute the core of soil P-cycling research, as plants require substantial amounts of P and the availability of different P pools directly constrains plant growth, reproduction, and overall ecosystem productivity. Thus, enhancing the bioavailability of different P fractions is essential for promoting nutrient cycling and overall ecosystem productivity. However, the extraction and identification of soil P species are still largely limited by analytical methodologies, and considerable uncertainty remains concerning the precise characterization of diverse P fractions and their transfractionation pathways. This review synthesizes current knowledge on soil P cycling, focusing on the chemical and structural characteristics of major inorganic and organic P fractions and their identification via sequential chemical extraction; the ecological significance of these P fractions and the environmental and biological factors regulating their bioavailability are highlighted. On this basis, the methodological constraints associated with current approaches to P speciation and cycling, and outlined the key scientific questions and future research directions are identified: including the need for detailed characterization of organic P molecular structures, full-process elucidation of P transfractionations at mineral–organic matter–microbial interfaces, and integrated assessments of how global change and emerging environmental stressors alter soil P cycling.