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a Biological & Agricultural Engineering Dep., Univ. of Georgia, Athens, GA 30602
b Crop & Soil Science Dep., Univ. of Georgia, Athens, GA 30602
c USEPA Region 8, 999 18th Street, Suite 300, Denver, CO 80202-2405
* Corresponding author (jgaskin{at}engr.uga.edu)
Received for publication December 19, 2001.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: MTL, maximum tolerable level • NS, no biosolids applied • <6YR, biosolids applied for less than six years • >6YR, biosolids applied for more than six years
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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There are relatively few studies of biosolids land application effects on both soil metal concentrations and forage quality. Two studies of land application of biosolids in arid rangelands indicate that the addition of organic matter and trace metals can benefit rangeland production (Fresquez et al., 1991; Pierce et al., 1998). Forage production and quality on a semiarid rangeland also increased due to a one-time application of biosolids (22.5–90 Mg ha-1) compared with the unamended control (Fresquez et al., 1991). Diethylenetriaminepentaacetic acid (DTPA) Cu, Cd, Pb, and Zn concentrations in the soil were significantly different from the control at the highest biosolids loadings (45 and 90 Mg ha-1) in the fifth growing season, but metal concentrations in the forage [blue grama, Bouteloua gracilis (Kunth) Lag. ex Griffiths, nom. illeg.] were not significantly different from the control (Fresquez et al., 1991).
Forage quality and quantity increased in native grass species in Colorado rangelands receiving a one-time application of biosolids from 5 to 40 Mg ha-1 (Pierce et al., 1998). Grasses had low Cu to Mo ratios (<1.2:1) before biosolids application, which can induce Cu deficiencies in cattle and sheep (Miltmore and Mason, 1971). Biosolids application increased Cu to Mo ratios above the 2:1 ratio recommended by Miltmore and Mason (1971).
In a Florida study, increased metal concentrations were observed in both the soil and bahiagrass (Paspalum notatum Flugge) forage from plots treated with biosolids (179 and 358 kg N ha-1) compared with the commercially fertilized control, but no toxic levels of plant metals were observed over the grazing season (Tiffany et al., 2000b). A parallel study reported that biosolids applied at double the agronomic rate did not affect the mineral status of cattle (Angus x Hereford cross), except for Cu (Tiffany et al., 2000a). Lower Cu levels in cattle from the biosolids-treated fields reflected the low Cu levels in the forage and possibly the higher forage S content, which interferes with Cu utilization in ruminants (Tiffany et al., 2000a).
Copper and Zn uptake increased in bermudagrass hay from a one-time application of biosolids at rates of 8.4 to 25.2 Mg ha-1 compared with comparable rates of commercial fertilizer (Lane, 1988). These metals did not accumulate to the upper chronic dietary exposure level for cattle.
These studies suggest there is little potential for metal concentrations to accumulate in forages from one to three years of biosolids applications, even at relatively high rates (i.e., greater than agronomic rates). However, there is limited data on the effects of long-term land application of biosolids on pasture or hayfields, particularly from existing land application programs. Our study evaluated the effect of repeated biosolids application over periods of up to 12 yr on metal concentrations in soil and bermudagrass forage from fields that participated in a biosolids land application program by a City of Augusta–Richmond County (Georgia) wastewater treatment plant. Some of these hayfields had received biosolids that periodically contained elevated metal concentrations. Our objective was to determine if long-term land application of biosolids with periodically elevated metal concentrations increased soil and forage metal concentrations, and whether forage metal concentrations were at levels that could present risks to ruminant health.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Biosolids have been land-applied in Burke County since 1978. In July 1993, the USEPA Part 503 regulations were promulgated (USEPA, 1993). These regulations required monitoring of biosolids for metals content and set levels at which biosolids could not be land-applied (Table 1 in Part 503) and levels below which metals content would not restrict land application (Table 3 in Part 503). Due to the change in regulations in 1993, we decided to group fields into those that had received biosolids for more than six years and those that had received biosolids for less than six years. The criteria for the field selection included: number of years biosolids were applied, bermudagrass hayfields, and well-drained soils with a loamy sand surface. The Burke County Cooperative Extension Service agent contacted producers in the land application program that met the criteria and determined which producers were willing to participate in the study. Each producer was assigned a code by the county extension agent so that producers could remain anonymous. Ten bermudagrass hayfields were selected in each of the following categories: fields that received biosolids for six or more years (>6YR), fields that received biosolids for less than six years (<6YR), and fields that never received biosolids application (NS).
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Composite soil and forage samples were collected from each field during late June and July of 1999. None of the fields had fresh biosolids on the surface at the time of sampling. Each composite consisted of 12 subsamples randomly located throughout the field. At each subsample site, a 10-cm-deep soil core was taken with an Oakfield punch (Oakfield Apparatus Co., Oakfield, WI) and placed in a labeled plastic bag and a 5-cm2 sample of forage was clipped by hand and placed in a labeled paper bag. Forage samples were collected just before the first harvest. Most of the forage samples were collected after the bermudagrass had headed out (22 of 30 samples). The remaining forage samples were in the vegetative stage. All samples in the vegetative stage were in the NS group except one. The soil subsamples were air-dried, thoroughly mixed, and sieved (2 mm), and an aliquot was taken for analysis. The unwashed forage samples were dried at 60°C, mixed, and ground in a Wiley mill (1-mm screen), and an aliquot was taken for analysis.
Analytical
Soil samples were analyzed for recoverable metals (nitric acid digestion), soil test metals (Mehlich I), water soluble metals, total C, N, and S, pH, cation exchange capacity (CEC), NO3–N, NH4–N, and electrical conductivity. Recoverable metals were digested with boiling concentrated nitric acid (USEPA Method 3050; USEPA 1994). Water-soluble metals were determined by shaking a 1:5 soil and water solution for 8 h, filtering the supernatant, and analyzing the extract (Amacher, 1996). Metals were analyzed on an inductively coupled plasma mass spectrophotometer (ICP–MS). Standard reference materials (SRM) with certified metal levels and spikes were run with the digests. One SRM and one spike was run for every 20 samples analyzed. Every fifth sample was replicated through the digest, and calibration was checked every 10th sample. Total C, S, and N were analyzed on a LECO (St. Joseph, MI) analyzer (Nelson and Sommers, 1996). Electrical conductivity and pH were determined on a 2.5:1 soil and water paste (Rhoades, 1996 and Thomas, 1996, respectively). Soil test P, Ca, K, Mg, Mn, and Zn were extracted with Mehlich I solution (AOAC Method 968.08; Cunniff, 1996) and analyzed on an emission ICP by USEPA Method 200.7 (USEPA, 1994).
Forage samples were analyzed for metals, total C, N, and S, crude protein, neutral detergent fiber, total digestible nutrients, and percent moisture. Metals were analyzed with the same methods used for soils. Nitrates in the forage samples were determined by double distillation with MgO-Devarda's alloy (Bremner, 1965). Percent crude protein was analyzed by combustion (Cunniff, 1996) on a Elementar Rapid N analyzer (Elementar Americas, Mt. Laurel, NJ). Neutral detergent fiber was analyzed by the Van Soest method with an Ankom fiber analyzer (Ankom Technology, Fairport, NY) (Van Soest et al., 1991). Total digestible nutrients were calculated from crude protein and crude fiber analysis based on the forage and animal type by the University of Georgia Feed and Environmental Water Laboratory. Percent moisture was determined by weighing the forage samples, oven-drying at 60°C overnight (approximately 16 h), and reweighing the samples. Percent moisture is reported as water weight over fresh weight.
Biosolids Characterization and Field Histories
Monitoring data on biosolids chemistry, application rates, and number of fields in the land application program reported to the Georgia Department of Natural Resources, Environmental Protection Division were obtained and summarized for a general overview of the land application program and the amounts of biosolids that were applied to the fields in the study.
The City of Augusta–Richmond County wastewater treatment plant began land application of biosolids to agricultural fields surrounding Augusta in 1978. The biosolids are produced through anaerobic digestion and are applied in a liquid state, typically at about 30 g kg-1 total solids. From 1996 through 1999, the City–County surface-applied liquid biosolids to about 100 sites per year. The fields in the land application program range from 4 to 120 ha. Biosolids are applied on a plant-available N basis. Mean total Kjeldahl nitrogen from 1985 to 1999 was 19 kg Mg-1. This allowed the application of up to 9 Mg ha-1 yr-1 of biosolids for a bermudagrass field. Typical field application rates averaged 0.47 Mg ha-1 per application with a maximum of 4 Mg ha-1 based on information in regulatory reports to the Georgia Environmental Protection Division. Some fields received two applications per year.
Field histories were determined through producer interviews conducted by the county extension agent. The producers were asked about the following: the use of biosolids including years applied and number of applications per year; recent use of commercial fertilizer and lime; the use of other organic sources of fertilizer such as manures, other soil amendments, and other industrial by-products; whether fields were in cotton (Gossypium hirsutum L.) during 1940s and 1950s when lead arsenate was used as a pesticide; year when the fields were converted to pasture or hayfields; and previous crop history and land use.
Data Analysis and Statistics
Soil metal concentrations were compared with typical values for mineral soils in Georgia (Holmgren et al., 1993) and an adjusted USEPA cumulative pollutant loading rate. The USEPA loadings were converted to soil concentrations assuming a 10-cm depth and a bulk density of 1.5 g cm-3. This bulk density is typical for the series found at the study sites (Perkins, 1987).
We assumed a normal distribution because there were too few samples to test for the underlying distribution. The variances for most of the recoverable metals in the three groups were similar, and statistical analysis was performed on untransformed data. Variances for the water-soluble metals appeared to increase with increasing means, so these data were lognormally transformed (Ott, 1984). The MANOVA procedure in Number Cruncher Statistical System (Hintz, 2000) was used to determine if there were significant differences in metal concentrations in the three groups. The MANOVA procedure also performed an ANOVA for each metal. Duncan's Multiple Comparison Test was used as a conservative test of groups that were significantly different from each other for each metal (Hintz, 2000). The covariants used for the soil analysis were Al, Mn, Fe, pH, electrical conductivity, C, S, and N. The covariants used for the forage analysis were Al, Mn, Fe, C, S, N, NO3, crude protein, neutral detergent fiber, total digestible nutrients, and percent moisture. The means used to test the null hypothesis were corrected for covariance in this test and will differ slightly from the uncorrected means. All tests were conducted at the 0.05 probability level.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Soil chemical characteristics were similar in the three groups (Table 1). Soil pH and total soil carbon concentrations were not significantly different. Cation exchange capacity (CEC) was significantly higher in the >6YR group.
Biosolids Characterization
The metal characteristics of the biosolids changed over the course of the land application program (Table 2). Before July 1993, when the Part 503 regulations were promulgated, municipal sewage treatment plants were not required to analyze biosolids for metals. Although the data are sparse, metal concentrations in the biosolids were higher from 1987–1993 compared with 1994–1997. Maximum results for Cd, Ni, and Se exceeded the 99th percentile for the 1988 National Sewage Sludge Survey (USEPA, 1990) and the Part 503 Table 1 ceiling concentrations. Land application of biosolids with these maximum concentrations would not be permitted under current regulations. All fields in the >6YR group would have received biosolids with these high metals content. Two fields (same producer) in the <6YR group received biosolids with the high metal concentrations. These fields had biosolids applied every other year beginning in 1990 and only had four applications.
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Soil Metals Concentration
Recoverable metal concentrations were similar to values reported by Holmgren et al. (1993) for mineral soils in Georgia and were low compared with the USEPA Part 503 cumulative pollutant loading (Table 3). The >6YR group was significantly higher in both Cu and Mo than both the <6YR and NS groups. There were no differences indicated between the groups for Cd, Pb, Hg, Ni, or Zn. The <6YR and the NS groups were significantly different for As with the mean As concentration lowest for the <6YR group. This group received biosolids from the period when the mean concentrations of As were low (Table 2), but also had the highest potential use of lead arsenate. Due to the low soil As concentrations, differences reported may also be due to natural soil variability.
Most water-soluble metals increased in soils having more than six years of biosolids applied (Table 4). Copper, Mo, and Ni concentrations in the >6YR group were significantly higher than the <6YR and NS groups. Cadmium concentrations in the >6YR group were significantly higher than the NS group, but were not different from the <6YR group. Zinc concentrations in the >6YR and <6YR groups were significantly higher than the NS group. There were no observed differences between groups for water-soluble As, Pb, or Se.
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The increase in forage Mo concentrations while Cu concentrations remain constant presents a potential nutritional concern. Low Cu to Mo ratios induce Cu deficiencies in ruminants (Miltmore and Mason, 1971). Copper deficiency is associated with scouring (Miltmore and Mason, 1971), anemia, depressed growth, and nervous disorders (Tiffany et al., 2000a). A range of Cu to Mo ratios that would be protective of ruminant health has been proposed: 2:1 (Miltmore and Mason, 1971), 4:1 (Alloway, 1973), and 5:1 (Suttle, 1991). The Cu to Mo ratios in forage decreased with increasing time of sludge application with an average ratio in the >6YR group of 5:1 compared with 12:1 in the <6YR group and 57:1 in the NS group. Six of the fields in the >6YR group had ratios below the critical level proposed by Suttle (1991), compared with none in the <6YR group and one in the NS group. Of the fields with forage below the 5:1 Cu to Mo ratio, only two had Mo concentrations near the MTL of 6 mg kg-1.
Evaluating Cu–Mo interactions is further complicated by the presence of S. Sulfur concentrations are known to influence the ability of cattle to use Cu and Mo (Suttle, 1991). High S concentrations can exacerbate Mo-induced Cu deficiencies (National Research Council, 1984). The MTL for S is 4 g kg-1 (National Research Council, 1984). The average S concentration for the three groups was just below this critical value (>6YR = 3.9 g kg-1, <6YR = 3.4 g kg-1, NS = 3.6 g kg-1), and concentrations in several fields were above the critical value. These S concentrations are higher than would be expected for bermudagrass forage in the southeastern United States (Kamprath and Jones, 1986). Although high S concentrations can be associated with biosolids application (O'Connor and McDowell, 1999), the high S concentrations in the forage in this study may not be related to biosolids application history as there was no significant difference between the groups.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Recoverable metal concentrations in soils with various biosolids application histories were low and generally similar to values reported for mineral soils in Georgia. Copper and Mo concentrations in the soil were significantly higher in the fields that had received biosolids for more than six years. The higher Cu concentrations could be nutritionally beneficial to ruminants because Cu is typically very low in southeastern United States soils, and cattle are routinely fed Cu supplements to prevent Cu deficiencies. Although Lane (1988) reported increased Cu concentrations in bermudagrass (‘Alicia’) with the use of biosolids as fertilizer, the higher Cu concentrations in the soil in our study did not translate to higher Cu in the bermudagrass forage, which is similar to results reported by Tiffany et al. (2000a) for bahiagrass.
Water-soluble metals clearly increased with long-term application of biosolids. The increased availability of the metals did not translate into significantly higher metal concentrations in the bermudagrass forage except for Mo, Ni, and Zn. Although the concentrations of these metals were significantly higher in the >6YR group compared with the NS group, the average forage concentrations were below the MTLs set by the National Research Council (1984). The Zn concentrations were within the mineral requirement range for beef cattle (National Research Council, 1984).
Cadmium concentrations in the forage or the soil were not significantly different between the three groups; however, three fields had forage Cd concentrations near or above the MTL for beef cattle. The Cd MTL is developed to be protective of human health and is below the level considered toxic for ruminants. These three fields received biosolids during 1990 when Cd concentrations in the biosolids were elevated. Although all these fields had higher Cd concentrations in the soil than the other fields sampled, there was not a consistent pattern of low pH, C, water-soluble Cd, P, or Zn that would explain the forage Cd concentrations.
The average Cu to Mo ratio in the forage decreased with increasing years of biosolids application due to increasing Mo concentrations. The increased Mo concentrations, coupled with high S concentrations, could create conditions conducive for ruminants developing copper deficiency; however, the effect can be ameliorated by feeding a Cu supplement (O'Connor and McDowell, 1999).
The study was conducted on farm fields participating in a biosolids land application program; consequently, results reflect the potential for metals contamination in a realistic though less controlled setting. Although biosolids containing higher metals concentrations than would currently be allowed were applied to the fields in the >6YR group, our study indicated that toxic levels of metals have not accumulated in the soils. The long-term application of biosolids has increased some metal concentrations in the soil, which was reflected in the forage samples. Overall, forage quality from fields with long-term application of biosolids was similar to that having only commercial fertilizer and should not pose a risk to animal health. However, the decreased Cu to Mo ratio and higher S values in the forage indicate that producers should be aware of potential effects on ruminant health. Land application programs should encourage producers to feed their animals Cu supplements. Further research is needed to assess the effect of the increase in water-soluble metals in the soil, particularly with Cd and Mo, and the field conditions under which these metals can accumulate in forage.
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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