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Abstract
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.
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