DOES TERMINATION OF LONG-TERM ANNUAL BIOSOLIDS APPLICATIONS TO LAND CAUSE A “TIME BOMB” OF INCREASED METAL UPTAKE BY CORN?
The Metropolitan Water Reclamation District of Greater Chicago (District) has owned and operated a 6,300 ha land reclamation site in Fulton County, Illinois for the past 28 years. Long term monitoring data from a set of eight fields, averaging 17.1 ha in size and amended from 1974 through
1984 with a total of 543 Mg of biosolids ha−1, were analyzed to test the hypothesis that mineralization of organic C in the amended soil following cessation of biosolids applications would produce a “biosolids time bomb” effect, leading to dramatic increases in
the phytoavailability of metals associated with the biosolids.
During a 13-year period following the cessation of biosolids applications, mean soil organic C decreased from 51.0 to 38.0 g Kg−1 and it was estimated that as much as 43.6 percent of the total organic C added
to the amended fields was mineralized between 1974 and 1997. This large net mineralization of soil organic C led to significant increases in the ratios of soil metal concentrations to soil organic C concentration (relative metal concentration). The mean relative metal concentration increased
in the soil by 25.8, 21.6, 33.3 and 37.2 percent for Cd, Cu, Ni, and Zn, respectively, from the first three years following cessation of biosolids applications to the final three years of the study. This indicates that metals are being released from some binding sites of the organic C matrix
associated with the biosolids applied during the study period, a condition necessary for the biosolids time bomb to occur.
However, contrary to the hypothesis that the mineralization of soil organic C would lead to a release of metals into more phytoavailable forms, this study indicated
that mean metal concentrations in plant tissue of corn grown from 1995 through 1997 were not significantly (p>0.05) different (Cu, and Zn in corn grain) or were significantly (p<0.05) lower (Cd and Ni in corn grain, and Cd, Cu, Ni, and Zn in corn leaves) than they were from 1985 through
1987. This indicates that significant mineralization of soil organic C does not lead to increased phytoavailability of metals associated with biosolids. Modeling of organic C mineralization in biosolids amended fields indicated that net mineralization had either ceased by the 12th
year following the cessation of biosolids applications or is proceeding so slowly that soil organic C levels in these amended fields would not reach their original unamended levels for another 103 years. Therefore, it does not appear that a time bomb effect will ever occur in these fields.
On the contrary, the results of the large scale field studies indicate that, net mineralization of biosolids organic C in amended soils leads to decreased phytoavailability of metals to corn.
While there was no evidence of a time bomb effect following cessation of biosolids applications,
biosolids amendments however, produced a small increase (p<0.05) in the mean Cd and Zn concentrations in corn grain, as compared with grain harvested from corn grown on unamended fields (0.01 to 0.10 mg Kg−1 for Cd, 22.7 to 28.4 mg Kg−1 for Zn). Mean Ni
concentrations in corn grain from biosolids-amended and unamended fields were not significantly different (p<0.05). Mean Cu concentration in corn grain from biosolids-amended fields were significantly lower (p<0.05) than in grain from unamended fields (1.5 mg Kg−1 and
1.9 mg Kg−1, respectively).
Biosolids amendments significantly increased (p<0.05) Cd, Ni, and Zn concentrations in corn leaves as compared to unamended fields (0.3 to 5.6 mg Kg−1 for Cd, 0.2 to 0.5 mg Kg−1 for Ni, and 32.0 to 87.0 mg
Kg−1 for Zn). Biosolids amendments had no significant effect (p<0.05) on the Cu concentration of corn leaves.
Any increase in the metal uptake of corn tissue noted in this study due to biosolids application on land is far less than that predicted by the risk assessment
models used in the Part 503 Regulations. Thus, the Part 503 Regulations were found to be extremely conservative and very protective of human and animal exposure to metals from the ingestion of corn grain and leaves containing Cd, Cu, Ni, and Zn.
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