Metal bioavailability dynamics during a two-year trial using ryegrass (Lolium perenne L.) grown in soils treated with biosolids and metal salts

Abstract

A 24-month field lysimeter experiment using ryegrass (Lolium perenne L.) grown in three soil types was used to investigate metal bioavailability dynamics following amendment with biosolids and metal salts (Cd, Cu, Ni, Zn). Common surrogates of soil metal bioavailability (total soil metal, EDTA, Ca(NO₃)₂, total dissolved, diffusive gradient in thin film, and modelled free ion activity) were determined on soil samples taken every 6 months. Ryegrass was also harvested every 6 months and analysed for metal concentrations. Across soils and treatments dissolved organic carbon (DOC) and pH decreased, whereas dissolved Ca and Mg increased with time. The free ion activity concentrations of each metal also increased over 24 months, whereas Ca(NO₃)₂-extracted metals were unchanged. Zinc presented the most changes in bioavailability status, with total Zn concentration decreasing over time, and EDTA-extractable and soil solution Zn increasing significantly by 1.82 mg kg⁻¹ (1.1%) and 1.52 mg L⁻¹ (29%), respectively. Shoot concentration of Zn increased by 1.32 mg kg⁻¹ (2.7%), whereas shoot Ni concentration decreased by 0.65 mg kg⁻¹ (4%). The findings of this study clearly demonstrated that over 24 months, soil metal bioavailability and shoot metal concentrations register only minor changes and appear to be unaffected by soil DOC and pH fluctuations.