RISK ASSESSMENT AND REMEDIATION OF SOIL AND AMENDMENT TRACE ELEMENTS
Title: ZN DEFICIENCY PROMOTES CD ACCUMULATION BY LETTUCE FROM BIOSOLIDS AMENDED SOILS WITH HIGH CD:ZN RATIO
Submitted to: Journal of Residuals Science & Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 29, 2006
Publication Date: April 15, 2006
Citation: Chaney, R.L., Filcheva, E., Green, C.E., Brown, S.L. 2006. Zn deficiency promotes cd accumulation by lettuce from biosolids amended soils with high cd:zn ratio. Journal of Residuals Science & Technology. 3(2):68-75.
Interpretive Summary: An unusual case of contaminated sewage sludge application was observed at St. Marys, PA, in which a Cd pigment user caused high Cd contamination of the sewage sludge, but Zn levels remained at normal levels. Before regulations were developed, applications of farmers’ fields caused very high Cd contamination of soils, and at the same time this Cd had a high ratio to Zn. Normal Cd+Zn contamination from mine wastes and modern biosolids contains about 100-200 times more Zn than Cd. In this case, applied Zn inhibits uptake of Cd by plants, transport of Cd to edible crop parts, and reduces the absorption of Cd by animals which eat the crops. Leafy vegetables grown on the farmers’ fields had such high levels of Cd than remediation was required. But when we tested application of limestone to increase Cd binding by the soil matrix, lettuce Cd did not decline in strong contrast with many other experiments which have showed that limestone markedly reduced Cd uptake by plants. Greenhouse tests were conducted to identify why limestone did not reduce Cd uptake. Addition of Zn and Fe and Mn oxides to a biosolids compost were tested as remediation agents. The experiments revealed that liming the soil induced Zn deficiency even though the soil had adequate Zn to satisfy plant requirements according to normal agronomic measures. But when Zn was added with the limestone, foliar Cd in Romaine lettuce dropped remarkably. Adding biosolids composts rich in Fe and Mn, or Zn, Fe and Mn reduced lettuce shoot Cd, but not as effectively as did Zn sulfate. These studies identified a new class of Cd contamination of soils (high Cd:Zn ratio) which cannot be remediated by addition of limestone alone. Addition of Zn fertilizers, or byproducts rich in Zn along with limestone can effectively remediate the Cd risk of these soils.
Before regulations for biosolids utilization were developed, some highly contaminated sewage sludges were land applied on farmers’ fields. In one case, with a soil which had been amended with a high Cd:Zn ratio sludge, making the soil calcareous did not reduce Cd uptake by lettuce as raising pH had achieved in many earlier tests. We conducted an experiment to examine metal uptake by lettuce with different treatments (2% limestone, Zn, peat, or 10% addition of a biosolids compost enriched in Fe, Mn and/or Zn) would reduce the potential for Cd uptake by Romaine lettuce from soils which had been amended with high Cd biosolids in the 1960s-1970s.
An unexpected result was observed: liming two of the soils induced Zn deficiency in lettuce and raised lettuce foliar Cd concentrations. The high soil Cd appears to have interfered with Zn uptake, and foliar Zn was below the deficiency diagnostic level. Added Zn or peat+Zn prevented the Zn deficiency and greatly reduced lettuce Cd levels, while compost and Zn-amended compost were not quite as effective as the ZnSO4. No apparent effects of the added Fe and Mn oxides on lettuce Cd were observed, perhaps the oxides also adsorbed added Zn, reducing its ability to inhibit Cd uptake by lettuce. We conclude that application of excess limestone plus added Zn will reduce soil Cd risk to feed- and food-chain for soils with high Cd:Zn contamination such as biosolids amended soils at St. Marys, PA.