Phosphorus Footprint in the Whole Biowaste–Biochar–Soil–Plant System: Reservation, Replenishment, and Reception

Two phosphorus (P)-rich biowastes, sewage sludge (SS) and bone dreg (BD), were selected to clarify P footprints among biowaste, biochar, soil, and plants by introducing a novel "3R" concept model. Results showed that pyrolysis resulted in P transformation from an unstable-organic amorphous phase to a stable-inorganic crystalline phase with a P retention rate of 70–90% in biochar (P reservation). In soil, SSBC released more P in acid red soil and alkaline yellow soil than BDBC, while the opposite result appeared in neutral paddy soil. The P released from SSBC formed AlPO4 by combining with Al in soil, whereas P from BDBC transformed into Ca5(PO4)3F(or Cl) in conjunction with Ca in the soil (P replenishment). Various plants exhibited an uptake of approximately 2–6 times more P from biochar-amended soil than from the original soil (P reception). This study can guide the application of biochar in various soil–plant systems for effective nutrient reclamation.