Biosolids increase phosphate adsorption of semi-arid Mediterranean soils

The impact of biosolid compost on the adsorption of orthophosphate (IP) to Mediterranean-type soils was studied. Eight soils were amended with a stable biosolid compost (ADSC) at 9:1 and 97:3 ratios (w/w). Four soils were amended with the dissolved organic matter (DOM) fraction of the ADSC at the amount added at the 9:1 mixture (810 mg C kg^-1). Soils and their 9:1 soil‒ADSC mixtures were incubated for seven years. The maximum ADSC IP-adsorption capacity (S_MAX, Langmuir model) at native pH (≈7.5) was 850 mg P kg^-1. Mixing the ADSC with the soils increased their S_MAX values by ca. 150 and 190 mg P kg^-1 in the 9:1 and 97:3 mixtures, which exceeded additivity by 50% and 575%. The addition of DOM similarly increased the S_MAX of three out of the four soils. Following the incubation, the soils' organic-C decreased by 34% and the ADSC-derived OC decreased by 60%. Still, the corresponding soil's and mixtures' average levels of labile IP either increased (by 60%) or remained steady (at ̴30% of total-P). Incubation increased the S_MAX of three soils and five soil‒ADSC mixtures and decreased their binding affinity (k), trends which were also reflected in the quantity/intensity parameters. This study showed that amending semi-arid Mediterranean soils with stable biosolids, and their long-term oxidative co-stabilization is conducive to increase their IP binding capacity and bioavailability. Finally, the often similar effects of the compost and its DOM on IP adsorption merits further research regarding the role of cation (Ca⁺²) bridging in IP‒DOM‒solid phase interactions.