Organic amendments for reducing the plant uptake of cadmium from New Zealand soils

Abstract

Cadmium (Cd) is a naturally occurring element in soils. It is a non-essential trace element that can have toxic effects on fauna at relatively low concentrations. Several studies have reported above background levels of the metal in New Zealand (NZ) agricultural soils . This is thought to be due to the repeated applications of phosphate fertiliser where Cd is a commonly found impurity. Cd is taken up by plants relatively easily and can accumulate there to potentially dangerous concentrations without any effects on the plant itself. This has resulted in the NZ government imposing a 1.8 mg/kg limit on Cd in soil after which P fertiliser applications have to cease. The addition of organic amendments into the soil has been shown to reduce the plant uptake of Cd. This study has been carried out to gain better understanding of which specific organic amendment has the greatest potential for reducing plant uptake of Cd. The amendments used in the experiments were two types of peat, bio-solids from Kaikoura and Christchurch, coffee grounds, and municipal compost from Parkhouse Garden Supplies, Living Earth and Oderings. The study involved analyses of pH, conductivity, total organic carbon, water-soluble carbon (hot and cold water), elemental composition, cation exchange capacity (CEC) and the sorption potential for Cd of each amendment. Results indicated that the two bio-solid samples had Cd concentrations that were too high for potential application to the soil as this would risk accumulation at rates of up to 10 times higher than the other amendments. The findings from the Cd sorption experiment indicate that Parkhouse (PH) compost had the greatest capacity to adsorb Cd, the Kd value was 11317. This result was significantly larger than all other amendments, with Living Earth (LE) showing the second highest value of 578. This suggests the potential ability of Living Earth compost to be used as an amendment for soils contaminated with Cd. The significant sorption of Cd by PH compost could be attributed to the sulphur concentration of PH, as this was twice as high as any amendment (excluding bio solids). Parkhouse White (PHW) had the lowest S concentration but a similar CEC to PH, however the corresponding Kd was significantly lower (27). This suggests the potential addition of gypsum (CaSO4) to municipal composts can cause the associated S groups to form complexes with Cd, immobilising it and reducing plant uptake. Although, the sulphides or thiol groups are more likely to form complexes with Cd, rather than the sulphate groups. Overall, this study has indicated that the organic amendment with the greatest capacity to adsorb Cd is PH compost, with LE showing a reasonable capacity also. The next step would involve incorporating these composts into contaminated soils and to analyse the effect this has the plant uptake of Cd.