Particulate rather than mineral-associated organic matter pool determines soil phosphorus bioavailability in uncultivated black soil

Increasing depth drove a consistent decline in labile P (LP) and moderately labile P (MLP) and a rise in non labile P (NLP) within both particulate organic matter (POM) and mineral-associated organic matter (MAOM), indicating a shift of the soil P pools toward more stable, less available forms in uncultivated black soils. Clear cross-pool differences emerged: POM contained higher concentrations of all P fractions-especially LP and MLP-than MAOM, and LP contributed most to P availability within POM. The determinants of P availability differed by pool: LP and MLP in POM were primarily influenced by organic carbon, whereas LP and MLP in MAOM were jointly influenced by organic carbon and iron oxides.