Copper bioavailability plays an important role for soil quality since it is both known as essential trace element and as potential contaminant being toxic for microorganisms and plants at higher concentrations. In soil, Cu is predominantly adsorbed to organic matter (OM) by formation of very stable ring chelates.
Material and Methods
We examined the Cu sorption behavior of organic-rich upper soil from a mofette (natural CO2 degassing site) by conducting laboratory studies with Cu spikes up to 12 mmol kg‑1 on material from a transect with decreasing soil p(CO2): mofette center (Mof, 56% CO2), transition (Tra, 7%), and non-CO2-influenced reference (Ref, 1.6%).
Surprisingly, the sorption isotherms revealed that Cu adsorption increased in the order Mof<Tra<Ref (linear sorption coefficients: 960, 1240, 1400 L kg‑1, respectively) despite decreasing organic carbon content (191, 148, 112 g kg‑1, respectively). We attributed this to differences in OM quality of the natural samples since humification is slowed down under increased soil p(CO2) and less decomposed material accumulates. This affects OM stability: formation of dissolved OM increased in the order Ref<Tra<Mof in our sorption study, indicating that the higher Cu mobility in Mof compared to Tra and Ref was caused by formation of soluble Cu-OM complexes. X-ray absorption spectroscopy (XAS) proved that Cu was predominantly adsorbed to OM. Additionally, XAS analysis as well as sequential extraction gave first hints, that even the sorption mechanism may differ between Mof and Ref.
Our study indicates that high OM content in soil does not necessarily imply strong Cu adsorption. Organic matter formed under conditions that inhibit humification processes (e.g. high p(CO2)) seems to be a less favorable binding partner for Cu and favors the formation of dissolved Cu-OM complexes due to a lower stability of the solid OM. To assess the Cu immobilization potential of a natural soil, OM quality has to be taken into account.