POTENTIAL PHYTOAVAILABILITY OF HEAVY METALS AS A FUNCTION OF SOIL PROPERTIES IN BIOSOLIDS-AMENDED SOILS

Application of biosolids (formerly known as sewage sludge) to agricultural land can improve soil fertility by increasing, among other properties, nutrient and organic matter contents. However, repeated land applications of biosolids may also increase the heavy metal concentrations in soils, which may pose a significant human health risk. Recently, over-application of biosolids (in the order of decades) has been identified as a major problem in the agricultural soils of areas surrounding New Delhi, the capital city of India, where the real estate is being rapidly developed for residential purposes. The present study is aimed at evaluating the effects of long-term application of biosolids on the geochemical fate, and hence potential phytoavailability of heavy metals in such agricultural soils. Phytoavailability (i.e., availability of metals for plant uptake) is an integral function of soil physico-chemical properties, such as pH, salinity, clay and organic matter content, etc. Soil properties determine the availability of metals to plants by controlling their geochemical speciation; certain metals are strongly retained in soils, and hence, are unlikely to be available for plant uptake, whereas certain others are either not retained at all or are loosely retained, and hence, are potentially phytoavailable. For the reported study, soil samples were collected from ten spatially variable locations within the aforementioned agricultural area in New Delhi; uncontaminated control soils were collected from one location in the vicinity (a residential garden soil) for comparison purposes. The soil samples were analyzed for physico-chemical properties such as texture, pH, salinity, cation exchange capacity, organic matter content, and exchangeable Ca, Mg, P and oxalate extractable Fe and Al contents. The samples were also analyzed for total concentrations of Cr, Cu, Cd, Ni, Mn, Zn, Pb, Fe, and Co. However, since total concentrations of metals in soils are not a good indicator of phytoavailability, a widely used chemical extraction scheme, namely the Mehlich III method was employed to identify plant-available forms of heavy metals in soils. Phytoavailable metal concentrations were correlated with the soil properties to quantify their relative effects on metal uptake potential by plants.