UPTAKE OF HEAVY METALS BY FORAGE CROPS FROM SLUDGE AMENDED SOILS

Mark R. Noll*, Nicole C. Perry, Department of the Earth Sciences, State University of New York College at Brockport, Brockport, NY 14420, and Marie J. Delobbe, Genie de l'Environment, Institut Universitaire de Technologie Louis Pasteur, Strasbourg, France.

mnoll@weather.brockport.edu

Abstract

Approximately 5.3 million metric tons of municipal sewage sludge is generated in the United States annually with 36% being disposed of by land application. Much of the land-applied sludge is spread on agricultural lands used for the production of forage crops. The potential risks associated with this practice include the increased levels of toxic heavy metals in the forage crop. This results in potential adverse affects to plant growth and ruminant animal health. This study investigated the uptake of heavy metals by alfalfa in a greenhouse setting. Alfalfa was grown in soil with sludge amendments ranging from 0.5 wt% to 10 wt%, with or without the addition of lime. Metals studied included Zn, Cd, Cu, Mo, and Pb. Results show little or no correlation of plant uptake to soil pH or total mass of sludge added. A strong correlation exists between the total mass of sludge added and the plant concentration factor (CF), where CF equals the concentration of a heavy metal in plant tissue divided by the concentration in the soil. Concentration factors for Zn and Cu show a linear decrease in CF with increasing mass of sludge added. CF values range from 1.97 to 1.27 and 1.98 to 0.65 for Zn and Cu, respectively. Concentration factors for Cd and Mo exhibit a logarithmic decrease with increasing sludge addition. Values for CF range form 215 to 48 and 95 to 16 for Cd and Mo, respectively. Results from field studies will also be discussed.

Introduction

Over one third of the 5.3 million tons of municipal sewage sludge generated each year in the U.S. are disposed of by application to agricultural lands. This practice has both beneficial and adverse impacts to the soil-plant system where applied. However, the long term impacts to soil, groundwater, and surface water systems, and the biota they support has not been thoroughly studied.

USEPA regulations (Clean Water Act part 503) permit heavy metal loading on soil to exceed one hundred times or more the local background level (McBride, 1995). These levels are justified by the sludge protection hypothesis which suggests that plant uptake reaches a maximum as sludge application increases (Ryan and Chaney, 1993). This phenomenon is characterized by the sequestration of metals by the increased levels of organic matter added with the sludge. Based on this hypothesis, the worst case scenario would involve a linear increase in heavy metal availability with increasing sludge addition. Contrary to this, the relationship between the plants ability to take-up heavy metals, and the availability or speciation of these metals may significantly alter the relationship between soil loading and plant uptake (Pierzynski and Jacobs, 1984, Ross, 1994a, McBride, 1995, Sloan et al., 1997).

This study investigated the uptake of heavy metals (Cu, Mo, Pb, Zn, Cd) by alfalfa in a greenhouse setting, and by soybeans on agricultural fields that have received periodic application of sewage sludge over at least ten years. In the later case, these fields received sludge amendments following standard USEPA protocol.

Methods

Greenhouse studies were completed using a commercial topsoil mixture amended with an alkaline stabilized sewage sludge (N-viro) (Richards, et al., 1997, McBride, 1998). Alfalfa was grown in four-inch diameter pots in duplicate for five different series of amendments. The five series consist of A) soil only, B) soil plus lime, C) soil, sludge, and lime, D) soil, sludge, and lime with added N,P, and K fertilizer, and E) soil plus sludge. Six different sludge amendment rates were used corresponding to 1) 0.5 wt%, 2) 1 wt%, 3) 2 wt%, 4) 3 wt%, 5) 5 wt%, and 6) 10 wt%. For those samples receiving both sludge and lime, the amount of lime added was determined to be equivalent to make up for the lower amounts of sludge added. These resulted in soil pH values being consistent regardless of the amount of sludge added (Fig.1).

Figure 1. Soil pH values measures in 1:1 soil:water.

Alfalfa was planted at the beginning of January 1999. Cuttings of above ground biomass were collected in March 1999 and September 1999. Biomass samples were dried at 65^oC to constant weight. The dried tissue was ashed at 450^oC, and the residue was dissolved in 6M HNO₃. Samples were analyzed by ICP-MS.

Samples of the topsoil material were analyzed at Midwest Laboratories for a complete set of basic soil properties, and DPTA extractable heavy metals. The concentration of heavy metals in the sludge material was provided with the N-viro product (Richards, et al. 1997)

Soybean samples were collected from adjacent fields with or without sludge amendments. The beans were collected and processed in the same fashion as the alfalfa. Soil samples were also collected from these fields in the same location as the plant samples. Samples were collected from 0-8 inches and 8-16 inches below ground surface. Soil samples were extracted with a 50:50 mixture of 6M HCl and 6M HNO₃.

Results and Discussion

Results of the greenhouse studies showed little relationship between heavy metal uptake and either soil pH or the sludge amendment. Figure 2 shows the relationship between Zn uptake and soil pH, and is representative of all of the heavy metals studied. Furthermore, no significant difference in metal uptake is seen between samples from series C and D (soil-sludge-lime) and series E (soil-sludge) receiving the same sludge amendment. It is also significant to note that there is no significant difference in metal uptake between soil receiving sludge amendments and the control soil. It is suggested that the elevated soil pH values, and narrow range of approximately 1 pH unit have either minimized (Cu, Pb, Cd, Zn) or maximized (Mo) uptake.

According to the sludge protection hypothesis, heavy metal uptake should not be related to the amount of the sludge amendment. Figure 3 shows the uptake of Cd as a function of sludge amendment, and are representative of all metals studied. Although a slight increase in uptake is suggested for low amounts of added sludge, the overall results show no apparent correlation between these two factors. These results appear to support the sludge protection hypothesis for short time durations.

An alternative means of evaluating the data is to evaluate the effective concentration of heavy metals by the plant. A concentration factor (CF), the concentration in the plant tissue divided by the concentration in the soil, was calculated for each heavy metal studied. Results found metals falling into three groupings. Lead was not concentrated by the alfalfa. Values for CF fell from 0.043 to 0.005 following a log function as the sludge amendment increased from 0.5 wt% to 10 wt%. Copper and zinc were found slightly concentrated in the alfalfa except for Cu at the highest sludge amendment. Values for CF fell in a linear fashion from 1.97 to 1.27 and 1.98 to 0.65 for Zn and Cu, respectively, as sludge amendment increased. Cadmium and molybdenum were found to be highly concentrated by the alfalfa. Values for CF fell from 215 to 25 and 95 to 8 for Cd and Mo, respectively, following a logarithmic function (Fig. 4).

Results for Pb, Zn, and Cu are expected. At the elevated soil pH under which the experiments were conducted, these metals are expected to be sequestered by the soil for a variety of reasons (Ross, 1994b). Cadmium would also be expected to be sorbed by various soil solid phases. However, competition with higher concentrations of Ca⁺², Zn⁺², and other divalent species may be limiting Cd sorption. Contrarily, Mo uptake by the alfalfa should be optimized as pH increases. This is evidenced by the high CF values, but not supported by the relationship between plant uptake and soil pH.

Plant uptake of the heavy metals studied does not appear to be effected by soil pH in the range 7.2 to 8.4. Further, the total amount of sludge added does not influence plant uptake. The exception to this is at very low sludge amendments where a small increase in uptake is observed. These results appear to support the sludge protection hypothesis. An alternative hypothesis relates the rate of breakdown of sludge during the growing season. At low sludge additions, the organic matter in the sludge that sequesters the heavy metals may breakdown and release metals into the soil solution. At higher sludge additions, the amount of organic matter present exceeds that which may be broken down during one growing season.

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