On a site contaminated decades ago with Cu from a wood treatment facility, we can observe that the decomposition of soil organic matter has been slowed. This represents an exceptional data set, and it allows us to address many challenges faced by regulators and risk assessors who are trying to derive appropriate soil quality criteria. These data are representative of a field study with a very well-equilibrated contamination and allow the derivation of chronic toxicity threshold values for the inhibition of microbial respiration. Soil respiration is the main determinant of the carbon balance, and it is assessed using the accumulation of soil organic matter (SOM) in this case. Using data derived from a 70-year-old field study also has the advantage of not being subject to risk assessment uncertainty factors arising from the potential aging effects of spiked soil or to the uncertainty caused by laboratory-to-field differences, both of which are very difficult to address. The resulting toxicity thresholds for an inhibition of SOM degradation are 154, 193, and 285 mg Cu/kg dry soil for inhibition levels of 10, 20, and 50%, respectively. Setting those thresholds correctly is critical for a proper risk assessment relative to sustainable development and agriculture.