Fungal and Bacterial Dynamics in the Lettuce Rhizosphere Responding to Successive Additions of Cd and Zn. A. M. I. D. Amarakoon * and R. M. C. P. Rajapaksha Department of Soil Science, Faculty of Agriculture, University of Peradeniya, Peradeniya, Sri Lanka. Introduction Results and Discussion Cadmium and Zinc are introduced to soil ecosystems mainly through agricultural inputs (e.g. fertilizers, manures, pesticides etc.) and industrial emissions. Metals in soils are known to persist long-run and eventually introduce to biological systems and aquatic environments. Elevated concentrations of metals alter soil microbial community structures towards a more metal resistant populations eliminating sensitive species.1 Such changes in microbial communities affect soil quality and thereby plant growth. Response of mycorrhizal fungi to elevated metals varied depending upon their sensitivity to metals, bioavailability and threshold concentrations of metals in soil. 2 Thus, heavy metal uptake by food crops grown in contaminated soils may be governed by mycorrhizal infection to a greater extent. 3 Application of 500 mg of Zn and 25 mg of Cd resulted in about 90 and 10 mg of DTPA extractable Zn and Cd, in experimental soil respectively (Fig. 1). Plant uptake of Zn and Cd increased continuously with increasing metal dose and maximum uptake remained lower than 10% of the contamination (Fig. 2). Presence of Zn reduced Cd uptake by lettuce and vice versa. Objectives Figure 1: DTPA extractable Zn and Cd in soil Figure 2: Tissue Zn and Cd contents To assess the response of arbuscular mycorrhizal fungi (AMF) inhabiting lettuce rhizosphere to successive additions of Cadmium and Zinc To assess the Cd and Zn uptake by lettuce plants Methodology Lettuce (Lactuca sativa) plants were grown in pots for a period of 10 weeks on limed or non-limed tropical Entisol. Cadmium, zinc and a combination of both were introduced to one month old plants and then two successive additions were made as given below: All treatments replicated three times. The experimental design was complete randomized design. One week after imposing each metal dose, plants were uprooted along with their rhizosphere soil. Soils were analyzed for DTPA extractiable Zn and Cd Plants were analyzed for Root mycorrhizal colonization, Cd and Zn uptake Figure 5: Relationship between tissue and soil Cd contents At 4 weeks At 6 weeks At 8 weeks Treatment Dose 1 Dose 2 Dose 3 kg-1 soil Zn 2.5 mg 12.5 mg 25 mg Cd 50 mg 250 mg 500 mg Zn + Cd 2.5 mg Cd + 50 mg Zn 12.5 mg Cd + 250 mg Zn 25 mg Cd + 500 mg Zn Control - Figure 3: Dynamics in total and Cd resistant bacterial population Figure 4: Dynamics in mycorrhizal root colonization The dual metal treatment showed the highest bacterial population and AMF infection percentage, with a 19% increase, following application of the first metal dose (Figs. 3 and 4). The second metal dose reduced bacterial population but enhanced AMF infection in single metal treatments. The third metal dose affect bacteria and AMF adversely but enhanced growth of Cd-resistant bacteria population. About 90 and 10 mg of DTPA extractable Zn and Cd kg-1 soil, respectively were critical for AMF when they were added alone. The level of tolerance declined to 75 and 1 mg of DTPA extractable Zn and Cd in the presence both metals. The observed critical levels were lower than those reported previously suggesting that AMF of experimental soils are sensitive to metals. Plant uptake of Cd increased with increasing metal dose (Fig. 5) despite a significant reduction in the AMF infection from dose 2 to 3. This may indicate that plant protective mechanisms of AMF do not operate at those metal levels. Conclusions References DTPA extractable Zn and Cd concentrations of 90 and 10 mg kg -1 soil, respectively were critical for bacteria and AMF and lead to a development of metal resistant bacterial population. Zinc and Cd levels beyond these levels resulted in metal accumulation in lettuce at a toxic level that retard crop growth. Landi, L., Renella, G., Moreno, J. L., Falchini, L., Nanniperi, P., 2000. Influence of cadmium on the metabolic quotient, L-:D-glutamic acid respiration ratio and enzyme activity: microbial biomass ratio under laboratory conditions. Biology and Fertility of Soils 32:8-16 Val, D., C., Barea J., M., Aguilar, C., 1999. Diversity of Arbuscular Mycorrhizal Populations in Heavy Metal Contaminated Soils. Applied and Environmental Microbiology 65: 718-723. Chen, B., D., Liu, Y., Shen, H., Christie, X., L., Li., P., 2004. Uptake of Cadmium from an experimentally Contaminated Calcareous soil by Arbuscular Mycorrhizal Maize (Zea mays L.). Mycorrhiza 14:347–354. * Present affiliation Department of Soil Science, Faculty of Food and Agriculture, University of Manitoba, Winnipeg R3T 2N2 umamarai@cc.umanitoba.ca –