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Study tests the staying mettle of copper, zinc
By Charmain Tan Courcelle
MISSISSIPPI STATE -- A study of the practice of land application of poultry litter suggests copper and zinc may accumulate in amended soils using current nutrient management strategies.
Billy Kingery, soil scientist with the Mississippi Agricultural and Forestry Experiment Station, has completed a study to determine the effect of long-term application of poultry litter on the amount and distribution of these metals in soil.
Metals, such as copper and zinc, and the metalloid arsenic, are added to poultry feed in trace amounts as part of a diet designed to optimize bird growth and performance. Because these compounds are excreted in poultry waste, there have been concerns that long-term land application of poultry litter could lead to metal contamination of surface and groundwater supplies through runoff and leaching, Kingery said.
"Copper and zinc can accumulate in the food chain and are potentially toxic to organisms at high levels," Kingery said. "We began this work because the potential for copper and zinc to accumulate at high levels in soils amended with poultry litter over long periods warranted study."
To determine the effect of litter application on the accumulation and movement of metals in soil, Kingery and researcher Feng Xiang Han compared soil samples from a pasture where poultry litter was added for 25 years with that from an adjacent, nonamended forest soil. They worked with a poultry producer in Neshoba County who owns the land surveyed in this study.
The Mississippi State University team collected soil from 130 sites in the pasture and forest. Soil samples were analyzed for concentrations of copper and zinc, as well as nickel, chromium, lead and manganese -- metals that also cause environmental and health problems at high levels.
"We found that metals do accumulate in waste-amended soil over time," Kingery said. "But even with recent heavy applications of litter on this farm, the total concentration of these elements was still below limits set by the Environmental Protection Agency."
Kingery said these results were encouraging, but total metal concentrations only provide one piece of the puzzle.
"To fully understand metal behavior in soil, you need to know where they're found. Are they in the organic or inorganic components? Are they available to plants? Or, are they bound strongly to soil particles?" Kingery explained.
Kingery's team collected soil samples at different depths up to 180 centimeters to determine the location and levels of metals in the profiles of litter-amended soils. They also passed the soil samples through a series of chemical assays to evaluate metal mobility into the environment and availability to plants.
"Our results so far suggest that current recommended nutrient management practices allow safe management of metals," Kingery said. "The metals are available to plants, and over time, they are less able to move into the environment. No extra management practices seem to be required."
But different soil characteristics and climate can have an effect on metal accumulation. Consequently, Kingery has begun a collaboration with Agricultural Research Service scientist Karamat Sistani and the Natural Resources Conservation Service to examine metal accumulation following long-term litter application on farms located in other counties.
"Soil components can affect metal availability to plants and mobility in the environment. Some components, like chelators, make metals more available to an organism; others bind metals strongly and prevent metal absorption," Kingery said.
Kingery also is working with National Sedimentation Laboratory scientists to evaluate metal mobility in different agronomic systems that use poultry litter.
"We're looking at the actual runoff from plots planted with different row crops," he said. "Management can have a strong influence on what happens to metals. These studies will give us an idea of the economics and sustainability of litter application using different management practices."
Kingery's research is funded by the MAFES Special Research Initiative program. In addition, this project has received backing from the Mississippi Poultry Association and the Mississippi Farm Bureau.