Effect of pH on the release of soil colloidal phosphorus

Purpose Colloid-facilitated phosphorus (P) has been proved as a significant contributor to eutrophication. Release of colloidal phosphorus (Pcoll) depends on the released soil colloids and their P adsorption abilities, both of which are greatly affected by pH. The aim of the study was to assess the effect of pH in a wide range on Pcoll loss of the top silt loamy soil rich in organic matter and P for the successful description and prediction of P transport. Materials and methods In batch experiment, soil samples were shaken with deionized water in a wide range of pH from 1.4 to 9.9 for 24 h. Then water-dispersible colloids (WDCs) were extracted by pre-centrifugation, microfiltration, and ultracentrifugation processes orderly and determined gravimetrically. The Pcoll values at each pH were calculated as the difference between the concentration of total P in non-ultracentrifuged and ultracentrifuged samples. The same method was applied for the released colloidal mineral elements (Al, Fe, Ca, Si, Mg, Mn). Colloid morphology and P distribution on colloid surface were directly illustrated by transmission electron microscope (TEM) and scanning electron microscope-energy-dispersive X-ray spectroscopy (SEM-EDS) mapping scanning analyses. Results and discussion Over pH 4.6-6.0, the released Pcoll was below 5.5 mg kg⁻¹ soil, while up to 66.8 mg kg⁻¹ soil and 28.5 mg kg⁻¹ soil at pH 1.4 and 9.9, respectively, indicating that both high and low pH values enhanced the mobilization of Pcoll. At the low pH, the potential loss of Pcoll mainly resulted from the dissolution of inorganic encasing cement such as Al oxides and clay mineral. However, besides electrostatic repulsion, dissolution of organic coating at high pH enhanced the Pcoll release as revealed by the SEM-EDS results. The P was heterogeneously distributed on the WDCs and might be associated with soil organic matter, Al oxides, Fe oxides (oxyhydroxide), and clay mineral at the low pH, while mainly with Fe oxides and less clay mineral at the high pH. Conclusions Both high and low pH enhanced colloid and Pcoll releases. This study first visually revealed that the dissolution of organic cement at high pH enhanced the release of WDCs and their facilitated Pcoll release. The exceptional loss risk of Pcoll may be caused by soil acidification due to the dissolution of inorganic cements, especially in Al-rich soils, and by the enhanced high soil pH due to the dissolution of organic coating in the organic matter-rich soil