Saturday, 15 July 2006

Crop Rotation and Crop Residue Biocycling Affect on the Availability of Heavy Metals to the Subsequent Crop.

Lisa M. Eastley1, Cynthia Grant2, Don Flaten1, and Mario Tenuta1. (1) Dept of Soil Science, Univ of Manitoba, Winnipeg, MB R3T 2N2, Canada, (2) Agriculture & AgriFood Canada, Brandon Research Centre, Grand Valley Road, Brandon, MB R7A5Y3, Canada

Many processes are involved in the movement of heavy metals in the soil solution; however, adsorption and desorption are the most important chemical processes that control heavy metal availability. These processes are regulated by soil factors such as cation exchange capacity, pH and redox potential. Soil properties that regulate the availability of heavy metals can be altered by agricultural practices. Changes to various soil properties will alter the chemical and biological balance of the soil solution resulting in the increase or decrease of heavy metal availability. Diverse crop rotations are very beneficial in reducing diseases, weeds and insect pests, thus encouraging higher crop yields. They may also be used to improve soil physical and chemical factors in the achievement of optimal yields. However, the changes in the soil environment due to crop rotations may have a significant effect on the availability of heavy metals for the subsequent crop in the rotation. For example, difference in crop growth can impact the pH of the soil solution, Santonoceto et al. (2002) stated canola can acidify the soil through the deep rooted roots removal of cations for biomass and seed, thus demonstrating the effects crops have upon the soils condition and mineral availability for the subsequent crop in a rotation. Crop rotations can also change the biological component of the soil since the population of mycorrhizae and microorganisms depends on the species of the crop growing. Thus, the incorporation of a non-host crop species in the rotation would decrease the population of mycorrhizal fungi. This indirectly increases the uptake of some heavy metals by plants, since evidence suggests mycorrhizae may bind some heavy metals, thereby decreasing the concentrations available to the plant. However, this characteristic varies between fungal isolates. The incorporation of a non-host crop also would decrease the mycorrhizal fungi population for the subsequent crop. Crop residue biocycling could also contribute to increased levels of heavy metals available to the subsequent crop. The impact of crop residue biocycling may be related to the type of residue being incorporated into the soil. The incorporation of the residue of crops identified as high heavy metals accumulators, could enhance heavy metal phytoavailability for the next crop. However, the magnitude of heavy metals released from the residue would depend on the decomposition of the straw and the ability of the soil to retain the heavy metal released from the straw. Andersson and Siman (1991) observed Cd concentration was higher in crops when the residue had been returned to the soil as compared to when it was not. However, the significance of the role of crop residue biocycling in the accumulation of Cd is not well known. Determining the significance of crop rotation and crop residue biocycling in the availability of heavy metals could be very beneficial in lowering or enhancing the concentration of any heavy metal in the subsequent crops. As a result, proper crop management could be a method to alter the concentration of heavy metals. This option could adjust the availability of heavy metals within the soil solution with minimal cost.

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