Cadmium distribution in hill pastures as influenced by 20 years of phosphate fertilizer application and sheep grazing

Abstract

Cadmium (Cd) accumulating in the kidneys of older grazing animals threatens the continued access of New Zealand offal products to traditional markets. Phosphatic fertilizers represent the largest input of Cd into pastoral farming systems. Little research has been conducted on the cycling of Cd in grazed pastures. To estimate the distribution of Cd in grazed pastures, soil samples were collected from self-contained farmlets which had received either high superphosphate (HF) inputs (765 kg P ha-1), low superphosphate (LF) inputs (113 kg P ha-1) or no phosphate fertilizer (UF) inputs over the previous 20 years. The average total Cd concentration in the surface soil (0-30 mm; bulk density 0.64 g cm-3) of the farmlets which had received HF inputs was 0.40 mg kg-1 soil (range 0.18-0.60 mg kg-1), whereas farmlets which had received LF or no fertilizer inputs had an average Cd concentration of 0.10 mg kg-1 (range 0.02-0.19 mg kg-1). Cadmium concentration decreased with soil depth. The concentration of Cd in mixed herbage reflected the difference in soil Cd between the HF (mean ± standard error = 0.321 ± 0.033 mg Cd kg(-1) dry matter) and LF (0.063 ± 0.004 mg Cd kg(-1) dry matter) farmlets. Soil Cd accumulation was highest on low slopes (0-12°) and lowest on high slopes (> 26°) in both LF and HF farmlets, whereas pasture Cd concentration in the HF farmlet increased with slope from 0.178 ± 0.035 mg Cd kg-1 on low slopes to 0.487 ± 0.053 mg Cd ha-1 on high dopes. A strong relationship was obtained between soil Cd and P in the 0-30 mm layer of the HF farmlet, demonstrating the link between phosphate fertilizer use and Cd accumulation. Zinc to Cd ratios were much lower in this layer compared with those in LF and UF sites, providing further evidence of the contribution of phosphate fertilizer to soil Cd. A model originally developed to explain the distribution of soil P in sheep-grazed fertilized hill pastures gave good predictions of present day soil Cd distribution in the HF farmlet. The model predicts Cd accumulation in soil by using the amounts of Cd applied in fertilizer per unit surface area of land and functions which calculate the redistribution of Cd by animal grazing and camping behaviour. The model on an average overpredicted the measured soil Cd by 17% in the high slope sites and by 10% in the low slope sites.