Organic complexation of copper in throughfall and open field bulk deposition: Influence of the tree canopy of Mediterranean forest ecosystems

The complexing capacity for Cu ions (L_T) and the apparent stability constant values (logK_app) were measured in throughfall (TF) and open field bulk deposition (BD), at two Mediterranean forest ecosystems of western Greece. L_T was measured for the first time in the TF of natural forests. Concentrations of L_T were three-to-four-fold higher in TF (mean ± st.dev: 2014 ± 769 nM for Varetada and 1565 ± 595 nM for Karpenissi) compared to those in BD (531 ± 517 nM and 468 ± 321 nM, respectively). In all TF and BD samples, L_T concentrations were significantly higher than the corresponding total Cu concentrations, indicating that Cu was fully complexed. The L_T/TOC ratios in TF were found comparable between the two study sites (235 ± 149 nM mg^-1 C for Varetada and 256 ± 233 nM mg^-1 C for Karpenissi) and with those of BD (226 ± 257 and 163 ± 163 nM mg^-1 C, respectively). The determined mean logK_app values were almost identical in TF (6.8 ± 0.7 at Varetada; 6.8 ± 0.6 at Karpenissi) and BD (6.5 ± 0.6 at Varetada; 6.8 ± 0.3 at Karpenissi), pointing to the fact that regardless of the enrichment of TF in ligands (L_T(TF) > L_T(BD)) the type of binding sites remain the same. The logK_app values which were obtained herewith are similar to those obtained for humic-like substances (HULIS), indicating them as the most widespread type of ligands in BD and TF.