The effect of irrigation on cadmium, uranium, and phosphorus contents in agricultural soils

Cadmium (Cd) is a toxic metal which has accumulated in New Zealand agricultural soils due to phosphate fertilizer application. Understanding the contribution of plant uptake or leaching of Cd to observed Cd losses from soil is important. The concentration and distribution of Cd in irrigated and unirrigated soils with the same phosphate fertilizer history were investigated. Twenty-two pairs of soil samples from four depths (0-0.1, 0.1-0.2, 0.2-0.3 and 0.3-0.4 m) were taken from irrigated and unirrigated areas in the same field on dairy farms in three regions of New Zealand. The mean concentration of Cd at depths of 0-0.1 m and 0.1-0.2 m, as well as the cumulative masses of Cd (0-0.2, 0-0.3 and 0-0.4 m) in unirrigated soils were significantly higher (P < 0.05) than in irrigated soils. The concentration of phosphorus (P) at all depths (except for 0.2-0.3 m), as well as the cumulative mass of P in all depths of unirrigated soils, was also significantly higher (P < 0.05) than irrigated soils. However, no significant difference was detected in the concentrations of uranium (U) between irrigated and unirrigated soils. Irrigation induced a ∼7% Cd loss from topsoil (0-0.1 m), with the average rate of Cd loss from the top 0.1 m (due to irrigation) being 2.3 g ha−1 yr−1. This study therefore confirms that irrigation can enhance Cd mobilization, however Cd is mainly adsorbed to the surface soil.