Impact of sewage irrigation on speciation of nickel in soils and its accumulation in crops of industrial towns of Punjab.

Analysis of soil samples collected from sewage and tube well irrigated soils of Ludhiana, Amritsar Jalandhar and Mandi Gobindgarh, revealed that Diethylene triamine pentaacetic acid extractable nickel ( DTPA-Ni) was found to be higher in sewage fed soils. Sewage irrigation increased soil DTPA-Ni content by 3.04 times over the tube well irrigated soils. The content of DTPA-Ni showed decreasing trend with depth. Hydrogen concentration (pH) was negatively and significantly correlated with DTPA-Ni nickel whereas, organic carbon and total Ni show positive and significant correlation. Sequential fractionation was carried out to partition Ni in to fractions namely exchangeable and water soluble, organic bound, carbonate bound, Mn oxides bound, amorphous Fe oxides, crystalline Fe oxides bound and residual. Plant availability of these fractions is believed to decrease in the above order. Sequential fractionation indicated that every extracted fraction exhibited increase in Ni content with sewage irrigation with most prominent increases occurring in the organic and oxide fractions. The lowest amount of Ni in exchangeable and water soluble and the highest in residual pools testify that plants grown on these soils may not suffer from Ni toxicity. Though all the crops irrigated with sewage water had appreciably higher concentration of Ni as compared to the crops raised with tube-well water yet raya (Brassica juncea) and toria (Brassica campestris) accumulated higher content of heavy metals as compared to other crops, with higher content in roots than shoots. Transport index suggested that major part of taken up Ni is translocated to top parts of plant. Based on values of transport indices, different crops maybe arranged as toria > raya = maize > bajra > lady finger. As the plants take up nickel readily and there is danger of its excessive accumulation in plant organs and devaluation of the plant products. This is topical issue particularly in crops used for direct consumption.