Fig. 4. (A) Nickel K-edge X-ray absorption near edge structure spectroscopy (XANES) spectra of a Ni-enriched annual ring of black willow collected from Steed Pond and selected Ni bearing standards [Ni(NO₃)·6H₂O, NiSO₄, NiO, NiCl, Ni₃(PO₄)₂, Ni–citrate, and 2NiCO₃·3Ni(OH)₂·4H₂O)]. (B) Close up of the 1s 3d transition pre-edge for the same compounds. The dashed line represents the centroid of the pre-edge peak in black willow.