پیامد آلودگی هم‌زمان کادمیم و سرب بر شاخص‌های بیوشیمیایی و میکروبیولوژیکی خاک

آلودگی خاک به کادمیم و سرب از جمله تنش های مهم و متداول رو به گسترش در محیط است که رشد و فعالیت جمعیت میکروبی خاک را تحت تاثیر قرار می دهند. این دو فلز سمی ممکن است به صورت منفرد و یا هم زمان در خاک وجود داشته باشند. گرچه اثر منفرد آن ها بر ویژگی های بیوشیمیایی و میکروبیولوژیکی خاک شناخته شده است، اما اثر مشترک آن ها بر کارکرد میکروبی هنوز مشخص نیست. از این رو، هدف پژوهش حاضر بررسی اثر برهم کنش کادمیم و سرب بر برخی ویژگی های بیوشیمیایی و میکروبیولوژیکی در یک خاک آهکی طی 120 روز انکوباسیون بود. آزمایش به‌صورت فاکتوریل (شامل دو سطح کادمیم و دو سطح سرب) در قالب طرح کاملاً تصادفی و در شرایط آزمایشگاهی اجرا شد. نتایج نشان داد که حضور هم زمان کادمیم و سرب موجب افزایش غلظت قابل جذب این فلزات و ضریب ویژه تنفسی در مقایسه با خاک های تیمار شده با حضور منفرد این فلزات گردید. همچنین حضور هم زمان دو فلز موجب کاهش بیشتر معدنی شدن کربن و نیتروژن، آمونیفیکاسیون آرژنین، سرعت نیترات سازی، کربن و نیتروژن زیست توده میکروبی، تنفس پایه، تنفس ناشی از سوبسترا و فعالیت آنزیمی (اوره آز، آریل سولفاتاز، فسفومنواستراز قلیایی، دهیدروژناز، کاتالاز و هیدرولیز فلوروسین دی استات) در مقایسه با تیمارهای با حضور منفرد کادمیم و سرب شد. بنابراین، حضور هم زمان دو فلز، اثرات منفی و بازدارندگی یکدیگر را بر رشد و فعالیت جمعیت میکروبی و کیفیت زیستی خاک تشدید می کنند. این یافته ها نشان می دهند که هنگام ارزیابی خطر اکولوژیکی کادمیم و سرب در محیط های آلوده باید اثر متقابل آن ها بر جامعه خاکزیان نیز مورد توجه قرار گیرد.

Consequences of Co-contamination of Cadmium and Lead on Soil Biochemical and Microbiological Properties

Introduction: Heavy metals contamination of soils is an important environmental concern which has specially longterm hazardous effects on soil biogeochemical and microbiological properties (including microbial and enzyme activity, microbial community structure, and the contents of organic compounds). Among heavy metals, cadmium (Cd) and lead (Pb) are the two highly toxic, nonbiodegradable and often coexisted anthropogenic pollutants in contaminated sites. Numerous earlier studies have demonstrated a detrimental influence of Cd and Pb, both individually and jointly, on microbial and biochemical properties through reduction of microbial activity, microbial biomass and enzyme activity in polluted soils. Metal cocontamination has a greater negative effect on soil microbial community and enzyme activity compared to individual ones. Although the individual effects of Cd and Pb on soil biological functions are generally wellknown, their combined effects on microbial growth, population and functions are largely uncertain. The main aim of this study was to investigate the interactive effects of Cd and Pb pollutants on biochemical and microbiological properties in a contaminated soil. It was hypothesized that combined Cd and Pb would increase mobility and availability of Cd and Pb, which subsequently results in further reductions in soil biochemical and microbiological properties. Materials and Methods: The study was conducted under controlled laboratory conditions. A factorial experiment with two levels of cadmium (0 and 10 mg kg1) and two levels of lead (0 and 150 mg kg1) was conducted using a completely randomized design with three replications. The soil was artificially spiked with cadmium chloride and lead chloride to attain the above mentioned concentrations. To reactivate the microbial population and for the aging effect, soil moisture was set at 70% of field capacity, and containers were preincubated at room temperature for 20 days. Soil samples were incubated under standard conditions (70% of field capacity and 25±1 oC) for 120 days. At the end of the soil incubation the concentration of DTPATEA (diethylene triamine penta acetic acidtriethanol amine)extractable Cd and Pb, biochemical and microbiological properties including nitrification rate (NR), cumulative N mineralization (CNM), cumulative C mineralization (CCM), microbial biomass C (MBC), microbial biomass N (MBN), arginine ammonification (AA), basal respiration (BR), substrate (glucose)induced respiration (SIR), metabolic quotient (qCO2) and the activities of soil urease (URE), alkaline phosphatase (ALP), arylsulphatase (ARY), dehydrogenase (DEH), catalase (CAT) and fluorescein diacetate hydrolysis (FDA) were determined. In this experiment, the Bliss independence model was used to determine the type and nature of the interaction between Cd and Pb pollution (i.e., synergistic and antagonistic). Results and Discussion: Results showed that the DTPAextractable metal (Cd and Pb) concentrations were considerably higher under the combined metals compared with the singlemetals. In cocontaminated soils, a metal may contribute to release of other metals to soil solution and consequently would enhance the availability of the released metals. Compared to individual metal, the qCO2 was greater in Cd+Pb contaminated soil. Microbial and biochemical properties (MBC, MBN, AA, NR, CNM, CCM, BR, SIR) and enzyme activity (URE, ARY, ALP, DEH, CAT and FDA) significantly decreased in the presence of Cd or Pb pollutant than the control. Generally, the negative effects of Cd and Pb coexistence on biochemical and microbiological properties were higher than Cd or Pb alone because of synergistic interaction in the metal combinations. The results of Bliss independence model indicated the synergistic effect of Cd and Pb on microbial and biochemical functionalities in metalcocontaminated soils. In soil ecosystem, heavy metals exhibit toxicological effects on soil microbes which may lead to the decrease of their function and activities. Conclusion: Heavy metals can effectively change the soil biochemical and microbiological properties. This study provided strong evidence revealing that combined Cd and Pb can increase the mobility and availability of heavy metals, and intensify their toxicity effects on microbial community and enzyme activity in cocontaminated soils. The coexistence of Cd and Pb reduced soil biochemical and microbiological properties more than their individual presence. Soil microorganisms are an important indicator of soil fertility and health and thus would improve the accuracy of the ecological risk assessment of toxic metals at multimetal contaminated sites. However, further information on responses of microbial indicators to the joint effect of heavy metals under longterm and realistic field conditions is required.