Submitted to: Journal of Environmental Quality
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/15/2005
Publication Date: 11/7/2005
Citation: Sukkariyah, B.F., Evanylo, G., Zelazny, L., Chaney, R.L. 2005. Cadmium, cu, ni, and zn availability in a biosolids-amended piedmont soil years after application. Journal of Environmental Quality. 34(6):2255-2262. Interpretive Summary: Concern about metals in land-applied biosolids has raised because metals will persist in the amended soils, while the organic matter of the biosolids is biodegraded over time. It has been assumed that if the organic matter is lost from the amended soil, that metal phytoavailability must increase compared to soon after biosolids application. Many publications on this topic are from short-term greenhouse pot tests, and short-term field experiments. It has been widely agreed that long term tests on well managed field plots are needed to show that regulations are appropriate. The present study was undertaken to evaluate metal uptake by plants growing on field plots established in 1978 using biosolids with relatively high levels of Zn and Cu. These plots where confined by wooden separators in the field to prevent mixing with soil from adjacent plots. A previous study showed that nearly all the metals applied in the biosolids had remained in the plow layer depth of soil. The soils had become strongly acidic and were limed two years before the present tests. Lettuce, radish and barley were grown to assess the persistence of metal phytoavailability long after the biosolids had been applied. Although lettuce and radish are recognized as quite sensitive to excessive soil metals, no evidence of phytotoxicity was observed, either visible symptoms of injury, or yield reduction with increasing biosolids rate of application. Foliar metals did not exceed threshold concentrations for phytotoxicity. In addition, until the soil became more strongly acidic in the last year of the test, plant accumulation was a curvilinear (plateauing) function of soil metals for most elements and crops, while at greater acidity, plant uptake was a linear function of added metals. Soil extractions by commonly used reagents gave concentrations correlated with uptake by the plants. The uptake slopes (mg element in plant shoots per kg element applied per ha) were found to compare well with the uptake slope for leafy and root vegetables used by the US-EPA in the Clean Water Act Section 504 rule. No evidence was found of an increase in the phytoavailability of biosolids-applied trace elements long after biosolids applications.
Technical Abstract: Biosolids-derived organic matter is hypothesized to contribute considerably to the metal adsorption capacity of biosolids-amended soil. Concerns over the possible increase in phytoavailability of biosolids-applied trace metals to plants have been raised based on the assumption that decomposition of applied organic matter would increase phytoavailability. The objectives of this study were to assess the effect of time on chemical extractability and concentration of Cd, Cu, Ni, and Zn in plants on plots established by a single application of biosolids with high trace metals content in 1984. Biosolids were applied to 1.5 x 2.3 m confined plots of a Davidson clay loam (clayey, kaolinitic, thermic, Rhodic Kandiudults) at 0, 42, 84, 126, 168, and 210 Mg ha-1. The highest biosolids application supplied 4.5, 760, 43, and 620 kg ha-1 of Cd, Cu, Ni, and Zn, respectively. Radish (Raphanus sativus L.), Romaine lettuce (Lactuca sativa var. longifolia) and barley (Hordeum vulgare) were planted at the site for 3 consecutive years 17-19 years after biosolids application. Extractable Cd, Cu, Ni, and Zn (as measured by DTPA, CaCl2, and Mehlich1) were determined on 15-cm depth samples from each plot. Simple linear regression between plant metal concentration and biosolids-added trace metals were computed to determine uptake coefficients (UC) of crops for each metal as outlined by the USEPA Technical Support Documents for land application of sewage sludge. Soil organic matter content increased from 18 to 65 g kg-1 with the application of 210 Mg ha-1, subsequently decreased to 42 g kg-1 by 1992, and remained unchanged thereafter. DTPA-extractable Cu and Zn decreased by 58 and 42% respectively 17 years after application despite the reduction in organic matter content. Biosolids treatments had no significant effect on the yield of the crops. Plant uptake of trace metals varied among crops. Plant tissue metal concentrations increased with biosolids rate but were within the normal range of these crops. Trace metal concentrations in plants generally correlated well with the concentrations extracted from soil with DTPA, CaCl2 and Mehlich-1. Mehlich-1 gave the best correlation coefficients for Cu and Zn and, therefore, was the most reliable in predicting their concentrations in the crops grown. Metal concentrations in plant tissue exhibited a plateau response in most cases. Several linear increases were observed in some cases in 2003 when the soil pH decreased below 5.5. The plant uptake coefficient values generated for the different crops were in agreement with the values set by the Part 503 Rule.