Evaluating Sodium Pyrophosphate Extraction to Probe Soil Organo‐Mineral Interactions Across Contrasting Pedogenic Stages

Sodium pyrophosphate (NaPPi) is widely used in wet‐chemical extraction methods for assessing organo‐mineral interactions (OMIs) in soils. However, its effectiveness remains debated due to challenges in achieving specific solubilization and issues such as peptization and solubilization of organic carbon (OC) not directly associated with minerals. This study evaluated the effectiveness and specificity of Na‐PPi extractions at different pH levels (7 and 10) and concentrations (0.1 and 0.2 M) across eight soil types, minerals, and synthetic organo‐mineral complexes. Peptization was significantly reduced at pH 7 in relation to pH 10, especially with 0.2 M Na‐PPi, resulting in more reliable determination of OC involved in OMIs. The amount of OC extracted with 0.2 M Na‐PPi at pH 7 showed a positive correlation with mineral‐associated organic carbon in the studied soils ( R adj 2 = 0.52, p ‐value = 0.03), highlighting the significance of the OMIs targeted by NaPPi for OC accrual in soils. In general, Na‐PPi extractions targeted adsorption complexes with Al and Fe oxides, particularly affecting amorphous Al(OH) 3 and ferrihydrite. It also assessed Ca 2+ and Mg 2+ more tightly bound than the exchangeable fraction, suggesting disruption of cation‐bridging associations and organometallic complexes. The extractions also solubilized coprecipitation complexes with Al and Fe. However, some limitations still occurred with NaPPi at pH 7 as it mobilized small amounts of poorly crystalline Al and Fe phases (not bound to OC) and did not fully extract the mineral‐bound OC pool. Finally, by using soils from different parent materials and with varying degrees of weathering, we inferred the contribution of distinct OMIs across these soils. Base‐rich and less developed soils showed higher contributions of Ca 2+ and Mg 2+ . In moderately weathered soils, amorphous Al and poorly crystalline Fe were more significant, while acidic and highly weathered soils were more influenced by Al 3+ and crystalline Fe and Al oxides. Together, these findings demonstrate that the Na‐PPi extractions provide mechanistically meaningful information on organo‐mineral interactions across contrasting soils, provided their operational constraints and selectivity are explicitly recognized.