Diverging origins and converging fates of copper and zinc after 11 years of continuous soil amendments with pig slurry and deep litter: An XAS study.

Copper (Cu) and zinc (Zn) occur in distinct chemical forms in organic waste, yet their long-term behavior and fate in agricultural soils remain poorly understood. We used synchrotron-based X-ray absorption spectroscopy (XAS) to investigate Cu and Zn speciation in contrasting organic wastes derived from pig farming (pig slurry and pig deep litter) and to track their decade-long fate in amended soils. Pig slurry contained Cu(I) and Zn sulfides, whereas pig deep litter contained Cu(II) bound to organic matter and amorphous Zn phosphate. Over the long term, these initially contrasting metal species converged toward similar species and displayed comparable accumulation patterns in soil. Cu and Zn likely released from sulfide dissolution were complexed by organic matter and phosphate ligands in soil, resembling the species introduced via pig deep litter that persisted after application. Under the field-realistic conditions studied, Cu and Zn associated with soil minerals such as phyllosilicates and Fe/Mn (hydr)oxides did not increase in response to organic waste inputs. In addition, the long-term stability of amorphous Zn phosphate is highlighted. These findings provide new insight to address Cu and Zn contamination in an integrated manner in agroecosystems.