PLANT VARIATION IN CD, PB, ZN AND AS ACCUMULATION AND BIOAVAILABILITY AND METHODS TO LIMIT RISK
Title: Zinc Fertilization Plus Liming to Reduce Cadmium Uptake by Romaine Lettuce on Cd-Mineralized Lockwood Soil
Submitted to: Plant Nutrition Colloquium Proceedings
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 17, 2009
Publication Date: August 25, 2009
Citation: Chaney, R.L., Green, C.E., Ajwa, H., Smith, R.F. 2009. Zinc Fertilization Plus Liming to Reduce Cadmium Uptake by Romaine Lettuce on Cd-Mineralized Lockwood Soil. Proceedings of the International Plant Nutrition Colloquium 16, Paper 1252, 7 pp. On line.
Interpretive Summary: Romaine lettuce grown on certain California soils derived from marine shale parent rocks contains higher cadmium (Cd) levels than would be allowed in international commerce under proposed CODEX rules. No US limits for Cd in crops have been established by the US Food and Drug Administration. The soils contain 10-fold or greater Cd than normal US soils, but soil zinc (Zn) is not increased; this is very unusual because most soils contain 200-times more Zn than Cd. Previous studies have shown that liming the Cd-mineralized soil did not reduce cadmium accumulation by lettuce, in contrast with many studies of soils contaminated by Zn smelter or mine wastes, or biosolids amended soils. The present study was undertaken to better understand Cd uptake by Romaine lettuce grown on the Lockwood soil from Salinas Valley, CA. In Experiment 1, addition of limestone to make the soil calcareous and up to 100 mg Cd/kg soil caused a strong reduction of lettuce Cd from excessive to acceptable levels for international commerce. In Experiment 2, higher Zn levels were tested, but they did not cause further reduction in crop Cd. In Experiment 3, the soil was adjusted to a range of pH; both lower and higher pH than the original soil, pH 7 caused the lettuce to accumulate higher levels of Cd, confirming that if the soil is limed without Zn fertilizer, lettuce Cd may increase. The outcome of the studies identified soil management practices which can reliably produce Romaine lettuce with low enough Cd levels to meet constraints for international marketing, giving the growers methods to protect the quality of their produce.
Lockwood shaly loam (Pachic Argixerolls) and similar Cd mineralized soils derived from marine shale in California contain higher Cd levels and higher Cd:Zn ratios than uncontaminated US soils, and produce leafy vegetables with considerably higher Cd than is normal for US lettuce. Previous work by Burau et al. suggested that, in contrast with geogenic Zn+Cd enriched soils, liming the Cd-mineralized soils did not effectively reduce crop Cd concentration. Our previous studies found for high Cd:Zn soils, liming may not reduce crop Cd but liming plus Zn fertilizer can strongly reduce crop Cd. Greenhouse pot trials were undertaken with addition of 0-500 mg Zn kg-1 to Lockwood soil (5.3 mg Cd and 54 mg Zn kg-1). All pots were made calcareous with reagent CaCO3 (harvest pHW 7.8), or adjusted to a range of pH without Zn fertilizer. The zero Zn calcareous treatment produced lettuce with 13.2 mg Cd kg-1 DW and only 10.5 mg Zn kg 1 DW; adding 100 mg Zn kg-1 soil produced lettuce containing 2.53 mg Cd kg-1 and 35 mg Zn kg-1; adding 250 mg Zn kg-1, 6.3 mg Cd kg-1 lettuce; and 500 mg Zn kg-1, 3.9 mg Cd kg-1 lettuce. For the 0 Zn treatment, acidification or liming increased lettuce Cd. The key finding is that if Zn is added to raise Cd:Zn ratio, liming these soils can be highly effective in reducing lettuce Cd, much like found for other Cd+Zn enriched soils.