Studies on the origin, distribution and mobility of cadmium in pastoral soils : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science at Massey University
Cadmium (Cd) is a toxic heavy metal with no known biological function. Exposure of the human population to Cd is predominantly through cigarette smoke and Cd-containing foodstuffs. Many phosphatic fertilizers contain Cd and their application to land used for food production results in increased concentrations of Cd in the soil. The fate of this Cd in soils is poorly understood. In this study, factors influencing the concentration and residence time of Cd in pastoral soils were investigated. Total Cd concentrations in topsoil samples (0 - 7.5 cm) from a regularly fertilized farm were found to range between 0.07 and 0.91 mg Cd kg-1 (arithmetic mean of 0.36 mg Cd kg-1). The top soil Cd concentration was unrelated to soil type or parent material but correlated well with total phosphorus and total carbon. In a study of the distribution of total Cd throughout the profiles of 17 soil types on the farm, Cd concentrations generally decreased with depth and soil parent material contributed little to topsoil Cd loads. A comparison of pedologically matched fertilized and unfertilized soils on the same farm confirmed that phosphatic fertilizer was the dominant source of Cd. Fertilized sites showed a 3- to 20-fold increase in soil Cd loading. Increased Cd concentrations were detectable to 15 cm. These increased Cd loads were easily accounted for by an estimated phosphatic fertilizer input of 6 g Cd ha-1 yr-1 over 7 decades. Soil Cd concentrations were also influenced by stock camping behaviour, although the magnitude of this effect was much less than that due to fertilizer. To investigate conditions under which Cd may move deeper in the soil profile, laboratory-based leaching studies on repacked soil columns using various electrolyte leaching solutions were conducted. Columns leached with 2.5 mM CaCl2, CaSO4 and Ca(NO3)2 showed some limited movement of Cd, but columns leached with 2.5 to 7.5 mM KCl showed no significant redistribution of the added Cd. Under the soil conditions studied (pHH2O 5.45), cation exchange appeared the predominant driving force behind Cd mobility, not anion complexation. On the same soil, Cd mobility increased linearly as leachate CaCl2 concentration increased. By 50 mM CaCl2 most Cd was leached from the column. In view of the dominance of Cl in coastal soil solutions, this mobility under the influence of CaCl2 may be important. In a further experiment, four soils differing in physical and chemical properties were leached with 2.5 mM CaCl2. The wide range of Cd mobility seen, could be explained by differences in soil pH (pHH2O 4.95 - 6.02). The movement observed in these columns was adequately modelled using a simple convection-dispersion equation and adsorption isotherms. The movement of Cd observed and modelled in these laboratory studies suggests that leaching is likely to be a much more important mechanism of Cd loss from the soil profile than removal of animal products. This work was supported by findings from a field-based mass balance study of Cd inputs and accumulation in soil developed under pasture on a Wharekohe podzol. Despite clear accumulation of fertilizer-derived Cd in the surface of these pastoral soils, up to 44% of the applied Cd remained unaccounted for. The most likely reason for this was leaching. Accurate modelling of the residence time and plant availability of Cd in soils will rely on a quantitative understanding of the factors influencing Cd mobility in soils. This is an area requiring further research.