Cadmium accumulation in agricultural soils

Abstract

Cadmium (Cd) has accumulated in New Zealand (NZ) soils as a result of phosphate fertiliser application. Cadmium is a biotoxic heavy metal and can be adsorbed by soil and enter the human food chain. Three objectives were investigated in this thesis: 1. Determine if the distribution of Cd varies between soils with contrasting mineralogy and drainage characteristics, but the same phosphate fertiliser history, 2. Evaluate the utility of Cd stable isotope ratios (δ¹¹⁴/¹¹⁰Cd) to trace the sources of Cd in NZ soils through time and distinguish the contribution of different sources of Cd in NZ soils, and, 3. Determine whether there is a difference in the concentration of Cd in irrigated and unirrigated soils within the same paddock. The concentration of Cd was measured in three soils, with contrasting mineralogy and drainage characteristics within the same paddock, and thus same fertiliser history. The mean concentration of Cd in topsoil (0-7.5 cm) samples was 0.77 mg kg⁻¹ (range 0.56-0.99) in the Horotiu soil (Orthic Allophanic Soil in NZ soil classification, Typic Hapludand in US soil taxonomy), 0.83 mg kg⁻¹ (range 0.60-1.11) in the Bruntwood soil (Impeded Allophanic Soil in NZ soil classification, Aquic Hapludand in US soil taxonomy) and 0.78mg kg⁻¹ (range 0.46-0.96) in the Te Kowhai soil (Orthic Gley Soil in NZ classification, Typic Humaquept in US soil taxonomy). There were no significant differences in the concentration, and/or the total mass of Cd between the three soils. Cadmium was mainly adsorbed to the near surface soil regardless of soil mineralogy and drainage characteristics. Thus, it was concluded that it is appropriate to apply the same Cd management approach (The Tiered Fertiliser Management System) to the investigated soil types. Isotope ratios of Cd (δ¹¹⁴/¹¹⁰Cd) were used to trace the sources of Cd in a longterm irrigation and fertiliser trial at Winchmore, Canterbury, New Zealand. The isotopic composition of pre-2000 fertilisers (δ¹¹⁴/¹¹⁰Cd = 0.10 ± 0.05 to 0.25 ±0.04) was comparable to the Nauru source rocks used in fertiliser manufacture (δ¹¹⁴/¹¹⁰Cd = 0.22 ± 0.04), but distinct from the control subsoil (δ¹¹⁴/¹¹⁰Cd = -0.33± 0.04) and post-2000 fertilisers (δ¹¹⁴/¹¹⁰Cd range of -0.17 ± 0.03 to 0.01 ± 0.05). The isotopic compositions of fertilised soil samples ranged from δ¹¹⁴/¹¹⁰Cd = 0.08± 0.03 to δ¹¹⁴/¹¹⁰Cd = 0.27 ± 0.04, which were comparable to pre-2000 fertilisers. Thus, it becomes possible to distinguish the sources of Cd within the soil using isotopes. The fractional distribution of Cd sources confirmed the main contribution of Nauru-derived phosphate fertilisers (pre-2000 fertilisers) in increasing the amount of Cd in soils at the Winchmore research farm. The concentration and distribution of Cd in adjacent irrigated and unirrigated soils with the same phosphate fertiliser history were investigated. Twenty-two pairs of soil samples from 4 depths (0-10, 10-20, 20-30 and 30-40 cm) were taken from irrigated and unirrigated areas in the same paddocks on different dairy farms from three regions of New Zealand (Bay of Plenty, Manawatu-Wanganui, and Canterbury). The mean concentration of Cd (depth of 0-10 cm and 10-20 cm) and the cumulative mass of Cd (depths of 0-20, 0-30, and 0-40 cm) were higher (P < 0.05) in unirrigated soil than in irrigated soil. Total Cd was about 10% less abundant in the 0-40 cm depth range in irrigated soil (mean of 0.63 kg ha⁻¹) than unirrigated soil (mean of 0.73 kg ha⁻¹), with the average difference of 7.2 g ha⁻¹yr⁻¹ for the 0-40 cm depth. The significant difference (P < 0.05) in the cumulative mass of Cd between irrigated and unirrigated soils demonstrated that irrigation may have enhanced the mobility of Cd. However, overall the results demonstrate that Cd was generally immobile and mainly absorbed to the near surface of the soils studied.