CADMIUM DISTRIBUTION IN HILL PASTURES AS INFLUENCED BY 20 YEARS OF PHOSPHATE FERTILIZER APPLICATION AND SHEEP GRAZING

Cadmium (Cd) accumulating in the kidneys of older grazing animals thre atens the continued access of New Zealand offal products to traditiona l markets. Phosphatic fertilizers represent the largest input of Cd in to 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 fa rmlets which had received either high superphosphate (HF) inputs (765 kg P ha(-1)), low superphosphate (LF) inputs (113 kg P ha(-1)) or no p hosphate fertilizer (UF) inputs over the previous 20 years. The averag e 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 re ceived 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 decre ased with soil depth. The concentration of Cd in mixed herbage reflect ed 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 C d kg(-1) dry matter) farmlets. Soil Cd accumulation was highest on low slopes (0-12 degrees) and lowest on high slopes (> 26 degrees) in bot h LF and HF farmlets, whereas pasture Cd concentration in the HF farml et increased with slope from 0.178 +/- 0.035 mg Cd kg(-1) on low slope s 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 far mlet, demonstrating the link between phosphate fertilizer use and Cd a ccumulation. Zinc to Cd ratios were much lower in this layer compared with those in LF and UF sites, providing further evidence of the contr ibution of phosphate fertilizer to soil Cd. A model originally develop ed to explain the distribution of soil P in sheep-grazed fertilized hi ll 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 graz ing and camping behaviour. The model on an average overpredicted the m easured soil Cd by 17% in the high slope sites and by 10% in the low s lope sites.