Applying biosolids to acid soils in NSW: Are guideline soil metal limits from other countries appropriate?

Abstract

In New South Wales (NSW) the use of biosolids (sewage sludge) as a soil amendment has become a major issue for regulatory authorities because of the potential impact of biosolids metal contaminants on agricultural produce. Although guidelines regulating the use of biosolids in other countries are based on extensive data sets, these were collected under different soil and climatic conditions than those found locally. This experiment was carried out to determine whether guideline soil metal limits from other countries are appropriate for regulating biosolids application under acid soil conditions that occur in NSW and whether a more intensive series of field experiments needed to be carried out. The uptake of biosolids Cd and Zn by silverbeet (swiss chard, Beta vulgaris) was assessed on soils differing in pH_C (4.2–5.8) (pH measured in 0.01 M CaCl₂) and soil metal loading (0.53–2.82 mg Cd/kg and 54–159 mg Zn/kg) in the glasshouse and field. Metal uptake by our field-grown silverbeet was >10-fold higher for Cd and >20-fold higher for Zn than was predicted from the slope of the metal uptake response curve for leafy vegetables used in US EPA biosolids guidelines. For some treatments, leaf tissue Cd levels exceeded the maximum permissible concentration for Cd in foodstuffs, and Zn levels were above phytotoxicity thresholds (with some yield reduction) when silverbeet was grown on soils with Cd and Zn concentrations well below soil metal limit concentrations in the United States biosolids guidelines and equal to levels set in the United Kingdom. In addition, biosolids metal uptake under glasshouse conditions was more than twice that in field-grown plants. These results clearly demonstrate the importance of developing NSW-specific biosolids guidelines. Biosolids regulations in NSW should be based on uptake data for a wide range of important food chain and pasture crop species collected in the field and grown under local conditions.