Linda F. Benedict, Selim, M.
H. Magdi Selim
Understanding trace element interactions in the soilwater environment is essential in assessing their bioavailability and potential toxicity. Trace elements include several heavy metals such as zinc, copper, arsenic and cadmium. Several heavy metals, such as zinc and copper, are essential micronutrients required in the growth of both plants and animals. Micronutrients are often applied in the form of fertilizers or as a supplement in animal feed. Heavy metals also are used extensively as fungicides and as bactericides in numerous pharmaceuticals.
The bioavailability of trace elements in the soil-water environment depends on an array of soil properties, including soil pH, organic matter content, amount and type of dominant clay, and carbonates. In addition, several studies suggest varied interactions of heavy metals with phosphates in soils. Because phosphorus in the form of phosphate is a commonly used fertilizer in Louisiana, LSU AgCenter researchers studied the effects of phosphate on the bioavailability and mobility of heavy metals in soils – zinc, arsenic, cadmium and vanadium.
These findings are based on laboratory experiments measuring the retention of a heavy metal in the presence of phosphate. One type of experiment deals with the movement of a heavy metal in soil in the presence or absence of phosphate. Another investigated the retention or sorption of a heavy metal in the presence of various phosphorus concentrations over time.
Arsenic is a toxic element and is almost always present in soil, water, plants and aquatic systems. High concentrations of arsenic can be introduced in soils and wetlands through industrial waste disposal and mining activities as well as insecticide applications. Elevated levels of arsenic in many soils can be caused by applying arsenic compounds such as pesticides, herbicides, wood preservatives and livestock feed additives, which pose threats to surrounding surface water and groundwater quality.
Movement studies in all soils indicate that arsenic was more mobile when phosphate was present. This is likely due to the fact that the phosphate anion has similar chemical properties to arsenate. Therefore, competition for adsorption sites resulted in enhanced arsenic mobility and bioavailability in the soil environment.
Zinc is a micronutrient that produces yield responses for corn and other crops, particularly in alkaline and calcareous soils. AgCenter research indicates increased zinc adsorption by the soil in the presence of phosphate. Zinc release decreased significantly as the concentration of phosphate in the soil solution increased. This indicates that the presence of phosphate increases available sorption sites for zinc.
Movement studies indicate that the presence of phosphorus increased the amount of zinc adsorbed by the soil. This finding is opposite that for arsenate. Therefore, the presence of phosphate has the beneficial effect of reducing the toxicity and potential mobility in the soil and the groundwater.
Vanadium acts as a growth-promoting factor and helps in fixing and accumulating nitrogen in a plant, although a high concentration of vanadium may reduce productivity. Environmentally, the occurrence of vanadium in petroleum and coal is highly significant because they constitute major sources of vanadium emissions to the atmosphere. A large fraction of vanadium-rich atmospheric particles may enter the soil as particulate fallout or dissolved in rain. Vanadate and vanadyl ions are versatile at forming complexes that inhibit or stimulate activity of many enzymes.
AgCenter research indicates that the presence of phosphorus decreased the amount of vanadium sorbed by Sharkey soil. Most striking is that vanadium was immobile in Sharkey soil in the absence of phosphate. In contrast, the movement studies indicate extensive mobility of vanadium when phosphate and vanadium were applied to Sharkey soil. As a result, using phosphate in zones containing vanadium should be avoided to minimize vanadium movement in the soil-water environment.
Cadmium is perhaps the most important potential contaminant of food supplies on amended soils. Being relatively soluble in soils and chemically similar to zinc, it is readily taken up by crop plants and is quite toxic to humans. Cadmiumcontaminated rice has been documented in Japan. Natural processes such as forest fires, volcanic emissions and weathering of soil, sand, gravel and bedrock are sources of cadmium. Human activity also contributes to cadmium in the environment. Cadmium is present in coal and mineral fertilizers and can enter soil, water and air from mining, refining and other industries and from burning coal and other fossil fuels.
Research shows that the influence of phosphate on increased cadmium adsorption was manifested for all soils. Results from movement experiments indicate that lower concentrations and limited mobility of cadmium occurred in the presence of phosphorus. This observation was consistent for all soils.
Findings and recommendations
The presence of phosphate is beneficial in increased sorption and decreased potential movement of zinc and cadmium in soils.
For arsenic and vanadium, the presence of phosphate reduced their sorption capacity and enhanced their mobility in soils. It, therefore, should be avoided.
Future research will focus on the effect of phosphate on the state of multiple trace elements and pesticides in soils.
H. Magdi Selim is the A. George & Mildred G. Caldwell Professor of Soil Sciences in the School of Plant, Environmental & Soil Sciences.
(This article was published in the spring 2013 issue of Louisiana Agriculture magazine.)