Sustainable phosphorus management in soil using bone apatite

Soil fertility and phosphorus management by bone apatite amendment are receiving increasing attention, yet further research is needed to integrate the physicochemical and mineralogical transformation of bone apatite and their impact on the supply and storage of phosphorus in soil. This study has examined bone transformation in the field over a span of 10-years using a set of synchrotron-based microscopic and spectroscopic techniques. Transmission X-ray microscopy (TXM) observations reveal the in-situ deterioration of bone osteocyte-canaliculi system and sub-micron microbial tunneling within a year. Extensive organic decomposition, secondary mineral formation and re-mineralization of apatite are evident from the 3rd year. The relative ratio of (v₁ + v₃) PO₄^3- to v₃ CO₃^2- and to amide I increase, and the v_3c PO₄^3- peak exhibits a blue-shift in less than 3 years. The carbonate substitution of bone hydroxyapatite (HAp) to AB-type CHAp, and phosphate crystallographic rearrangement become apparent after 10 years' aging. The overall CO₃^2- peak absorbance increases over time, contributing to a higher acid susceptibility in the aged bone. The X-ray Photoelectron Spectroscopy (XPS) binding energies for Ca (2p), P (2p) and O (1s) exhibit a red-shift after 1 year because of organo-mineral interplay and a blue-shift starting from the 3rd year as a result of the de-coupling of mineral and organic components. Nutrient supply to soil occurs within months via organo-mineral decoupling and demineralization. More phosphorus has been released from the bones and enriched in the associated and adjacent soils over time. Lab incubation studies reveal prominent secondary mineral formation via re-precipitation at a pH similar to that in soil, which are highly amorphous and carbonate substituted and prone to further dissolution in an acidic environment. Our high-resolution observations reveal a stage-dependent microbial decomposition, phosphorus dissolution and immobilization via secondary mineral formation over time. The active cycling of phosphorus within the bone and its interplay with adjacent soil account for a sustainable supply and storage of phosphorus nutrients.