Effect of Heavy Metals Contamination of Soil By Industrial / Domestic Effluents On Size And Activity of Soil Microbial Biomass

Experiments were conducted in order to evaluate the effect of effluent irrigation on heavy metals accumulation in soil and soil microbial properties. The study comprised of three parts, In the first experiment twenty five soil samples each at depths of 0-15 cm and 15-30 cm were collected from Islamabad, Adiala, Taxilla and Wah factory areas and analyzed for physiochemical properties, AB-DTPA extractable and total heavy metals and microbial parameters like soil microbial biomass C, microbial biomass N, soil respiration and activities of enzymes such as dehydrogenase and alkaline phosphatase. Effluent samples from these areas were also collected and analyzed for heavy metals contents and other water quality parameters.The results of effluent analysis revealed that EC, total dissolved solids (TDS) and heavy metals such as Cr, Ni, Cd, Cu, and Fe were above permissible limits while Zn and Pb were within the permissible range.In soil samples, AB-DTPA extractable Fe, Cu and total Cu contents were markedly higher than the critical limits at all the sites. AB-DTPA extractable and total Pb contents were within permissible limits. AB-DTPA extractable Zn was high at 24 sites whereas total Zn was higher than the critical limits at two sites only.Soil pH, organic C, CEC, clay and CaCO3 contents showed negative but weak correlations with the AB-DTPA extractable metals in soil at both soil depths, while the soil EC had positive. There were negative correlations of total and AB-DTPA extractable heavy metals in soils with soil microbial biomass C, microbial biomass N and the activities of dehydrogenase and alkaline phosphatase, but a positive relationship with soil respiration. Total toxic heavy metals had shown strong correlations with microbial activity parameters. In the second experiment, a laboratory incubation experiment was carried out in order to evaluate decomposition pattern of different organic amendments like pea straw and maize straw in soils with variable metal contents, designated as low-, medium- and high- metal soils. Each soil (600 g oven dry equivalent) was incubated at 25 oC and 50 % water holding capacity for 56 days with the following treatments: 1) Control, 2) pea straw 1 %, 3) maize straw 1 %, with four replications. Soil samples were collected at 0, 7, 14, 28, 42 and 56 days after incubation for the measurement of microbial biomass C, microbial biomass N, dehydrogenase, alkaline phosphatase and N mineralization. Soil respiration was measured 1, 2, 3, 5, 7, 10 and 14 days after incubation and thereafter weekly. Results showed a significant decrease in microbial biomass C, biomass N and N mineralization with increasing metal load in soil. Soil respiration rate was in the order: high metal soil > medium metal soil > low metals soil. Significant increases in microbial biomass C, biomass N, NO3-N, NH4-N and enzymes activities (Dehydrogenase and alkaline phosphatase) were noted with the addition of organic amendment (pea and maize straw) as compared to control in all three soils but the magnitude of increase was maximum in low metal treatment. In the third part of study, a greenhouse experiment was carried out to elucidate the effects of organic amendments on soil microbial parameters, heavy metals bioavailability and the growth of maize plants in two soils with variable metal contents. The treatments applied were 1) control, 2) pea straw 1 %, and 3) maize straw 1 % with 4 replications. Soil samples were collected at the time of pot filling and at sowing, 14, 28 & 56 days after sowing (DAS) of maize and analyzed for soil microbial biomass C, microbial biomass N, dehydrogenase, alkaline phosphatase, NO3-N and NH4-N. Plant samples were harvested at 56 DAS and data regarding plant height, fresh & dry weight were recorded. Soil and plant samples at harvest were also analyzed for N, P, K, Ca, Mg, Na, and heavy metals contents. Microbial parameters were markedly lower in high metal than in low metal soils. Organic amendments significantly increased all microbial parameters, soil NO3-N and NH4-N in both the soils. Plant growth in low metal soil was significantly higher than the high metal soil. A significant increase in microbial parameters was noticed with pea straw addition as compared to maize straw and control treatments. Addition of pea straw and maize straw significantly reduced AB -DTPA extractable (Cu, Zn, Fe, Mn, Pb, Cd, Cr & Ni) metals in the soils and their uptake by maize plants. Significant reduction in plant metal uptake was noticed with organic amendments, specifically by maize straw treatment. Overall, in study 2 and study 3, the interaction effects of organic amendments with soils depicted a significantly higher microbial activity parameters in soils amended with pea straw than the maize straw. On the basis of this study the main conclusions drawn are, 1) most of the soils in effluent irrigated field of Rawalpindi/ Islamabad areas possess heavy metals above critical limits 2) microbial biomass and activity parameters in these soils showed a decline with increasing concentrations of total heavy metal in soil and 3) addition of organic amendments improved microbial parameters in metal polluted soils but the magnitude of improvements decreased with increasing heavy metal load of the soil.Thus, the application of organic amendments such as pea and maize straw in effluent irrigated areas is recommended for reducing metal toxicity to microbes, and plants. A close monitoring of the soil and crops in agricultural field irrigated with untreated wastewater is required on regular basis in order to prevent the degradation of soil and entry of heavy metals into food chain.