Cadmium in plants on polluted soils: Effects of soil factors, hyperaccumulation, and amendments
Cadmium (Cd) is a heavy metal that is of great concern in the environment, because of its toxicity to animals and humans. This article reviews recent papers showing how soil factors (such as pH, phosphate, zinc, and organic matter), Cd hyperaccumulation, and soil amendments affect Cd availability. The studies confirm that the pH of the soil is usually the most important factor that controls uptake, with low pH favoring Cd accumulation, and that phosphate and zinc decrease Cd uptake. The work reveals that the availability of Cd is increased by the application of chloride and reduced by application of silicon. The most striking result of this review is the elevated levels of Cd in plants that are being reported in recent studies. Data for concentrations of Cd in soils and plants under variously polluted conditions are presented in a table and show that all plants have Cd concentrations ≥0.1 mg/kg, the normal concentration in plants. Concentrations ranged from two low concentrations of 0.1 mg/kg Cd (in grain of corn, Zea mays, on an abandoned sludge disposal site that had not received sludge for 10 years, and in roots of hybrid poplar, Populus deltoides x P. nigra, at a 25-year old active sludge farm) to 380 mg/kg Cd in leaves of penny-cress (Thlaspi caerulescens). Plants that hyperaccumulate Cd (i.e., have 100 mg/kg Cd in the tissue or more) belong to the genus Thalspi, the only known Cd hyperaccumulator. Of particular concern for humans are the high concentrations of Cd in rice grain and tobacco leaves. Even if Cd availability is decreased by adding amendments, it is still in the soil and a potential hazard. The best solution for maintaining non-contaminated soils and plants is to remove the sources of Cd in the environment. Given that that is essentially impossible at this time, further research needs to determine how soil and plant factors affect Cd availability on polluted soils.