Phytoavailability of nickel (Ni) in contaminated soils in response to soil-applied amendments

Abstract

Nickel (Ni) contamination of soils is a widespread problem as it enters into soils and environment through different anthropogenic activities. Nickel enters into human food chain through plant uptake and its entry into food chain can be restricted by immobilizing Ni in soil. A series of experiments including one incubation, three pot studies in soil and one pot experiment in sand culture was conducted in the wire house, Institute of Soil and Environmental Sciences, University of Agriculture, Faisalabad. Soil used for all the experiments was collected from an agricultural field irrigated with raw sewage at village 217/RB, Kajlianwala, Faisalabad. The first study was conducted to evaluate the effect of incubation and type of organic amendments on AB-DTPA extractability of Ni and other metals in contaminated soil. Activated carbon (AC) increased OM in soil despite that it had lower OM contents compared with that of pressmud (PrM) and others. The maximum decrease in soil pH was caused by PrM 4 (pressmud at the rate of 4 %) while maximum increase in it was observed with PM 2 (poultry manure at the rate of 2 %) after one-month incubation. The AC 4 (activated carbon at the rate of 4 %) decreased Ni in soil during incubation to maximum extent followed by PM 4 (poultry manure at the rate of 4 %). The Ni concentration in soil increased over time of incubation and changes in metal mobility seems mostly due to transformation of soil-applied OM with time. All the amendments except AC increased AB-DTPA extractable Ni in the post- experiment soil. The maximum increase in Ni compared with that of control was observed with PrM while AC decreased it. Only, PM increased Ni concentration in maize shoots while all other amendments decreased it. The AC and FM (farm manure) decreased Ni uptake by maize shoots while PM and PrM increased it over control. Over all, AC remains good immobilizer of Ni in soil that decreased its phytoavailability, its concentration and uptake by maize shoots compared with that of control plants. The second study was conducted in pots to investigate the effect of organic amendments on phytoavailability of Ni and other metals and their uptake by maize fodder. Organic amendments significantly increased OM contents in the post-experiment soil and maximum OM was observed with PrM. Farm manure (FM), PM and PrM increased Ni availability in the post-experiment soil while AC decreased it. Differences 12for Ni uptake due to amendments were non-significant, however, amendments increased Ni concentration and its uptake with an increase in Ni availability in soil except PrM that decreased it. The third study was conducted to investigate the residual effect of organic amendments on growth, phytoavailability of Ni in soil and its uptake by berseem. The organic matter in post-berseem soil decreased compared with that of post-maize soil and decrease was maximum with AC and minimum with PM. Residual organic matter significantly increased phytoavailable Ni at the end of berseem crop, however, phytoavailable Ni was negatively correlated with soil pH. All the amendments increased Ni concentration in berseem shoots while AC decreased it when compared with that of control. Phytoavailabilty of Ni decreased in post-berseem soil compared with that of post-maize soil and decrease was maximum with control and minimum with PM. The fourth study was conducted in sand culture to investigate the effect of Ni application on Ni concentration, its uptake and its effect on ionic composition of maize plants. In this study, application of Ni increased shoot and root growth significantly compared with that of the control. Nickel concentration in maize shoots and roots increased with increasing Ni application rate. However, roots accumulated more Ni compared with that in shoots. Increasing rates of Ni application increased distribution of Ni towards roots compared with that of shoots. Generally, Ni application (> 1 mg kg -1 ) decreased Mn concentration in maize shoots and roots but it increased in roots at lower Ni rates (≤ 1 mg kg -1 ). The effect of Ni on Cu concentration in shoots was not consistent as it decreased with the lowest (1 mg kg -1 ) and highest Ni (> 4 mg kg -1 ) rates but increased with other rates. The fifth study was conducted to investigate the effect of organic acids on phytoavailabilty of Ni and other metals in soil during decomposition of OM. Application of organic acids affected plant growth, particularly EDTA and oxalic acid decreased shoot fresh weights. In the present study, organic acids differed for AB-DTPA extractable Ni in post-experiment soil. The EDTA increased Ni in soil solution to a greater extent than other organic acids. The Ni concentration in maize shoots increased with the application of organic acids with the exception of acetic acid that decreased it. 13Over all, it is concluded that AC increased OM in soil despite that it had lower OM contents compared with those of other amendments, but decreased AB-DTPA extractable Ni in soil during incubation to maximum followed by PM 4 . Consequently, AC decreased AB-DTPA extractable Ni while other amendments increased it in the post- experiment soil. The PM increased Ni concentration in maize shoots while all other amendments decreased it. The AC and FM decreased Ni uptake by maize shoots while PM and PrM increased it over that with the control. The AC remains good immobilizer of Ni as it decreased Ni phytoavailability and uptake by maize shoots compared with that of control plants. The residual OM contents in post-berseem soil decreased compared with that of post-maize soil, decrease was maximum with AC and minimum with PM. Residual effect of amendments increased Ni concentration in berseem shoots while that of AC decreased Ni concentration in berseem shoots compared with that of the control plants. Phytoavailabilty of Ni decreased in post-berseem soil compared with that of post- maize soil and decrease was maximum with the control and minimum with PM treatment. Roots accumulated more Ni compared with that in shoots. Increasing rates of Ni application increased distribution of Ni towards roots compared with that of shoots. Generally, Ni application (> 1 mg kg -1 ) decreased Mn concentration in maize shoots and roots but it increased in roots at lower Ni rates (≤ 1 mg kg -1 ). Organic acids differed for AB-DTPA extractable Ni in post-experiment soil. The EDTA increased Ni in soil solution to a greater extent than that with other organic acids. The Ni concentration in maize shoots increased with the application of organic acids with the exception of acetic acid that decreased it. It is worth to note that AB-DTPA extractable Pb remained higher with EDTA application than other treatment, but reverse pattern of Zn and Cu concentration was recorded. This indirectly signifies the Pb leaching probability with EDTA application if EDTA used for Pb decontamination of soils through phytoextraction.