Mobility and Uptake of Zn, Cd, Ni and Pb in Sludge-Amended Soils Planted to Dryland Maize and Irrigated Maize-Oat Rotation.

Mobility and Uptake of Zn, Cd, Ni and Pb in Sludge-amended Soils Planted to Dryland Maize and Irrigated Maize-Oat rotation

ZM Ogbazghi¹, EH Tesfamariam¹, JG Annandale¹, PC De Jager¹, and I Mbakwe¹

¹University of Pretoria, Department of Plant Production and Soil Science, P.Box: X20, Hatfield 0028, Pretoria, South Africa, E-mail: u11350921@tuks.co.za

 

ABSTRACT

Sludge application to agricultural lands is often limited, mainly because of concerns about metal accumulation in soils and uptake by crops. The objective of the study was to test the hypotheses that in the short to medium term (5-10 years) the application of good quality sludge according to crop nitrogen requirements: i) will not lead to significant accumulation of water soluble metal fractions in soil, ii) mobility and uptake of metals is higher under irrigated than dryland systems, and iii) metal concentrations in plant tissue could reach phytotoxic levels before the soil reaches environmental threshold levels. Field plots were arranged in a randomized complete block design comprising four replications of three treatments (0, 8, and 16 Mg ha^-1 yr^-1 anaerobically digested municipal sludge) planted to dryland maize and irrigated maize-oat rotation. Soil and plant samples were collected following 7 years of treatment application for selected metal analyses. A large fraction of the Zn, Ni, and Pb in the soil profile was EDTA extractable (46 to 79%). Saturated paste extractable fractions of Cd and Pb were <1 mg kg^-1. Plant uptake of Cd, Pb and Ni under irrigation was double that for dryland systems. Concentrations of the metals considered in plant tissue of both cropping systems remained well below phytotoxic levels, except for Zn under dryland maize that received 16 Mg sludge ha^-1 yr^-1. Metal concentrations in the soil remained far below total maximum threshold levels. Therefore, hypotheses 1 and 3 were accepted for the metals considered and hypothesis 2 was rejected for Zn.

Keywords: Sludge; heavy metals; phytotoxic; soil threshold level, maize