Biosolids Constituents Persistently Reduce the Phytoavailability of Biosolids Applied Cadmium.

Over the last 40 years of research, our understanding of the fate and potential effects of biosolids applied Cd has greatly improved. In early years, some highly Cd contaminated materials caused high Cd uptake by crops, and high Cd bioavailability of the crop Cd. But regulations and advice from the agricultural community has achieved much lower concentrations of Cd and ratio of Cd:Zn which prevent biosolids Cd risks. In early research, some interpreted their multi-year studies to suggest that crops responded to applied Cd only during the year after application of biosolids; this was later shown to result from changes in pH during the study. More and more studies showed that high quality biosolids caused only small changes of Cd in crops, but that those increases above background may persist over decades. Mahler and Ryan conducted pot tests of Cd phytoavailability in long term biosolids amended field soils, adding Cd salt at 3 rates which showed that Cd uptake from historically applied biosolids was strongly affected by soil pH and Cd level in the biosolids, and that crop increase was linear with added salt Cd. Others claimed alternative explanations of these findings. Ryan et al. then showed that biosolids applied Cd adsorption capacity added by biosolids persisted long after application and persistently controlled Cd phytoavailability, and that the inorganic constituents of biosolids were important in this persistent Cd adsorption and reduced phytoavailability. Subsequently, Kukier, Chaney, Ryan and cooperators conducted tests on long term biosolids field plot soils (>20 years post applications) by adding stable isotope labeled Cd salt at 5 levels, adjusting all soil mixtures to equal pH, and growing Romaine lettuce and later durum wheat. They found that with equal pH, lettuce and wheat shoot increase in Cd was unequivocally linear with salt Cd addition for all unamended and biosolids amended soils, and that the nature of the biosolids strongly affected salt Cd uptake slope. High Fe biosolids compost reduced the slope of salt Cd uptake up to 10-fold compared to the unamended soil illustrating the low phytoavailability of Cd added in high quality biosolids (at 13 mg/kg, still higher than modern biosolids. GM=2.0 mg Cd/kg). Further, higher rates of biosolids caused greater reduction in the salt Cd uptake response slope. Taken together, all this research strongly supports the original interpretations of Ryan et al. about the long term adsorption and phytoavailability of biosolids applied Cd. Ryan’s leadership in risk assessment research for beneficial use of biosolids and of the US-EPA technical advisory group formed to develop the scientific bases for the CWA-503 Rule, have been a remarkable benefit to the Nation and our Society.