Effects of soil texture and structure and cultivation on Ni content in soil, effluent and plant

Sewage and wastewater from industry are used for irrigating farmland in many countries and contamination of these areas with heavy metals has become one of the major causes of concern for human. Contamination of soil by human activities is recognized as a widespread problem. The mobility of heavy metals is usually low due to their strong affinity for the solid phase, and thus they are very persistent contaminants in soils but physical, chemical and biological parameters can affect heavy metal mobility in soil. The aim of this study was evaluate the interactive effects of soil texture and structure and plant cultivation on Ni content of soils, effluent drainage and plant being irrigated by Ni-contaminated water.
Columns with 22.5 cm diameter and 30 cm height of disturbed (unstructured) and undisturbed (structured) soils of sandy loam (SL) and clay loam (CL) were used in this study. Half of columns (undisturbed and disturbed) were cultivated by radish (Raphanus sativus L.) and all soil columns were irrigated by Ni-contaminated water. The experiments were carried out using randomized complete design and factorial arrangement in three replications. The drained water was sampled after each irrigation cycle. Radish were harvested and divided into shoots and roots and the soil was sampled from 3 layers of 1-10, 10-20 and 20-30 cm. Then the concentration of nickel (Ni) in drained water, soil layers and plant (shoots and roots) was determined by atomic adsorption.
Result showed that the concentration of Ni in the undisturbed soil columns was greater about 34% in the SL than in the CL soil texture. This indicates a predominant role of pore continuity than retention at adsorptive site in structured clay loam soils. In the disturbed soil columns, average Ni concentration was greater about 1.5 times more than undisturbed soil columns, likely due to enhanced Ni/soil particle interactions due to presence of preferential paths for water flow. Accelerated water velocities through preferential flow pathways were likely responsible for the smaller adsorption of Ni in undisturbed soil columns. The effluent concentration under radish cultivation showed higher Ni concentration contents when compared to soil without plant. In the soils under plant cultivation, structural cracks and root channels were the cause of higher leached concentrations when compared to the soil without plant cultivation. Our results also showed translocation factor of Ni was more than one and hence, radish could be considered as an accumulator of Ni.
The high concentration of Ni in soil and effluent water drainage indicates the potential for pollution transfer from these media even in presence of radish as an accumulator; and also show that agricultural management can play an important role in soil and groundwater-heavy metals contamination.