| چکیده انگلیسی مقاله |
Introduction: Salinity and pollution are two environmental stresses that individually influence the population, growth and activity of earthworms as soil bioengineers. It is well-known that the population and activity of these organisms are mostly reduced or even their activity and growth can be stopped in polluted and saline soils. The individual effects of these abiotic stresses on earthworms, however, depend on the level of salinity, pollution and organic matter. Nonetheless, the joint or combined effect of these stresses on earthworms, especially in arid and semi-arid areas, is poorly known. Because of the importance of earthworms in soil ecosystem, the study of salinity and pollution interactions on earthworm population and activity to reduce their detrimental effects using organic materials is essential. The aim of this study was to examine how salinity and lead (Pb) stresses simultaneously affect the earthworms in soil ecosystem. Materials and Methods: In this research, the interaction effect of salinity stress using sodium chloride (NaCl) and Pb stress using lead nitrate (PbNO3) on the population, weight and activity of the earthworm Eisenia fetida was studied under greenhouse conditions. This factorial experiment was carried out using 3 factors, including Pb pollution (control and 30 mg kg-1 Pb), salinity (control, 4 and 8 dS m-1) and cow manure (control and 4% by weight) arranged in a completely randomized design with four replicates. The experiment lasted 13 weeks and earthworm’s population and activity including the number of adult worms, total earthworms, wet and dry weights, and wet and dry weights of casts produced by earthworm were measured at the end of the experiment. Concentration of DTPA (di-ethylene-triamine-pentaacetic acid) extractable Pb was also determined to assess how salinity influences the accessibility of this metal in the soil. The Fisher’s least significant difference test was used to determine the significance of any difference between the means values at 5% level with the STATISTICA 8 software. The Bliss Independence Model was used to determine the type of interaction between salinity and Pb pollution for each manure treatment. Results and Discussion: The current results showed that increasing salinity level enhanced the accessibility of Pb and subsequently its toxicity for earthworms. In contrast, addition of cow manure reduced the accessibility of Pb by 22-50% at all salinity levels. Earthworm population, wet and dry body weights, and wet and dry weights of casts produced by worms were all significantly (p< 0.05) influenced by the interaction of salinity, Pb pollution and manure application. Earthworm characteristics in Pb-polluted soils had the lowest values in the presence of 8 dS m-1 salinity. Compared with the control, earthworm population tended to decline by 59%, wet weight by 28%, dry weight by 18%, cast wet weight by 18% and cast dry weight by 10% in Pb-polluted soils spiked with the highest salinity level. However, a reverse trend was observed with addition of cow manure, and the effect of salinity was greater in polluted than unpolluted treatments. In polluted soils amended with cow manure, 8 dS m-1 salinity reduced earthworm population (57%), wet weight (18%), and cast wet (38%) and dry (41%) weights compared with non-saline soils. The nature of interaction between salinity and Pb pollution on earthworm characteristics was often additive or synergistic in the absence of cow manure, but antagonism in the presence of cow manure. Manure addition to this calcareous soil reduced to a large extent the harmful effect of Pb pollution and soil salinization on survival of adult earthworms. Conclusions: The earthworm species E. fetida was very sensitive to the interaction between salinity and Pb stresses. The harmful effect of Pb pollution on earthworm depended largely on the salinity level and manure addition. Salinity could synergistically increase the mobility, bio-availability and toxicity of soil Pb. Polluted and saline environments have a great potential to damage this earthworm species, however, manure addition could reduce the positive interaction of salinity and pollution on E. fetida, and improve its population and activity. It is concluded that the type of interaction between salinity and Pb pollution on E. fetida was additive or synergism in manure-unamended soils, but was changed to antagonism in manure-amended soils. Materials and Methods: In this research study, the interaction effect of salinity stress using sodium chloride (NaCl) and Pb stress using lead nitrate (PbNO3) on the population, weight and activity of the earthworm Eisenia fetida was studied under greenhouse conditions. This factorial experiment was carried out using 3 factors, including Pb pollution (control and 30 mg kg-1 Pb), salinity (control, 4 and 8 dS m-1) and cow manure (control and 2.5% by weight) arranged in a completely randomized design with four replicates. The experiment lasted 13 weeks and earthworm’s population and activity including the number of adult worms, total earthworms, wet and dry weights, and wet and dry weights of casts produced by earthworm were measured at the end of the experiment. Concentration of DTPA (di-ethylene-triamine-pentaacetic acid) extractable Pb was also determined to assess how salinity influences the accessibility of this metal in the soil. The Fisher’s least significant difference test was used to determine the significance of any difference between the means values at 5% level with the STATISTICA 8 software. The Bliss Independence Model was used to determine the type of interaction between salinity and Pb pollution for each manure treatment. Results and Discussion: The current results showed that increasing salinity level enhanced the accessibility of Pb and subsequently its toxicity for earthworms. In contrast, addition of cow manure reduced the accessibility of Pb by 22-50% at all salinity levels. Earthworm population, wet and dry body weights, and wet and dry weights of casts produced by worms were all significantly (p< 0.05) influenced by the interaction of salinity, Pb pollution and manure application. Earthworm characteristics in Pb-polluted soils had the lowest values in the presence of 8 dS m-1 salinity. When compared with the control, earthworm population tended to decline by 59%, wet weight by 28%, dry weight by 18%, cast wet weight by 18% and cast dry weight by 10% in Pb-polluted soils spiked with the highest salinity level. However, a reverse trend was observed with addition of cow manure, and the effect of salinity was greater in polluted than unpolluted treatments. In polluted soils amended with cow manure, 8 dS m-1 salinity reduced earthworm population (57%), wet weight (18%), and cast wet (38%) and dry (41%) weights compared with non-saline soils. The nature of interaction between salinity and Pb pollution on earthworm characteristics was often additive or synergistic in the absence of cow manure, but antagonism in the presence of cow manure. Manure addition to this calcareous soil reduced to a large extent the harmful effect of Pb pollution and soil salinization on survival of adult earthworms. Conclusions: The earthworm species E. fetida was very sensitive to the interaction between salinity and Pb stresses. The harmful effect of Pb pollution on earthworm depended largely on the salinity level and manure addition. Salinity could synergistically increase the mobility, bio-availability and toxicity of soil Pb. Polluted and saline environments have a great potential for damage to this earthworm species, however, manure addition could help to reduce the positive interaction of salinity and pollution on E. fetida, and to improve its population and activity. It is concluded that the nature of the effective interaction between salinity and Pb pollution on E. fetida was additive or synergism in manure-unamended soils, but was changed to antagonism in manure-amended soils. |