Biological Assessment of Natural Attenuation of Metals in Soil
Natural attenuation, also known as aging, of metals in soil refers to the processes by which the mobility, bioavailability, and toxicity of soluble metals in soils decline with time. These processes are important in terms of both risk assessment and crop production because of their effects on metal toxicity and micronutrient availability, respectively. However, most of the research related to natural attenuation of metals in soil has been conducted using a chemocentric approach. Adsorption and desorption methods were used over 30 years ago by Tiller et al. (1972) to investigate longterm binding of Co in soil. Single or sequential extractions have also been widely used to assess aging of a variety of metals and metalloids in soil (see Chapter 1). More recently, isotopic dilution techniques have been employed to assess changes in metal lability over time (see Chapter 2). These chemical techniques provide information regarding changes in soil-solution partitioning, extractability, and lability of metals in soil, but do not directly measure changes in biological availability or toxicity of metals (Stevens and McLaughlin 2001). Therefore, biological and chemical assessment of aging must be integrated to adequately understand both the mechanisms and the effects of natural attenuation of metals in soil.