Journal of Environmental Quality 30:140-146 (2001)
© 2001 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
TECHNICAL REPORT
PLANT AND ENVIRONMENT INTERACTIONS
Bioavailability of Biosolids Molybdenum to Corn
G.A. O'Connor,
T.C. Granato and
R.H. Dowdy
USDA-ARS, Univ. of Minnesota, St. Paul, MN 55108
Corresponding author (gao{at}gnv.ifas.ufl.edu)
Received for publication April 7, 2000.
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ABSTRACT
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This study was part of a larger effort to generate field data appropriate to the assessment of biosolids molybdenum (Mo) risk to ruminants. Corn (Zea mays L.) is an important component of cattle diet, and is a logical crop for biosolids amendment owing to its high N requirement. Paired soil and corn stover samples archived from two unique field experiments were analyzed to quantify the relationship (uptake coefficient, UC) between stover Mo and soil Mo load. Both studies used biosolids with total Mo concentrations typical of modern materials. Data from long-term (continuous corn) plots in Fulton County, IL confirm expected low Mo accumulation by corn stover, even at very high biosolids loads and soil Mo loads estimated to be near 18 kg Mo ha-1. Uptake slopes were actually negative, but USEPA protocol would assign UC values of 0.001. Data from plots in Minnesota also suggested essentially no correlations between stover Mo and soil Mo loads for continuous corn. However, greater Mo accumulation in corn grown following soybean [Glycine max (L.) Merr.] suggests the possibility of enhanced Mo bioavailability to corn in corn–soybean rotations. Nevertheless, molybdenosis risk to cattle consuming corn stover produced on biosolids-amended land is small as stover Mo concentrations were always low and stover Cu to Mo ratios exceeded 2:1, which avoids molybdenosis problems.
Abbreviations: APL, allowable pollutant concentration • HEI, Highly Exposed Individuals • ICP–AES, inductively coupled plasma–atomic emission spectroscopy • RPc, reference pollutant loading • UC, uptake coefficient
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INTRODUCTION
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IN 1993, the USEPA promulgated regulations (40 CFR Part 503) that, along with state regulations, govern biosolids recycling (USEPA, 1993). The federal rule is risk based, assessing exposure of Highly Exposed Individuals (HEI)—animals, humans, and the environment—to 10 metals in biosolids through 14 exposure pathways. A reference pollutant loading (RPc) is calculated for each metal in each pathway, which is designed to avoid detrimental effects on the HEI (USEPA, 1995). The smallest RPc for any of the investigated pathways becomes the limiting value for a particular metal, and is used to calculate an allowable pollutant concentration (APL) in biosolids for each metal. The accumulated APL values represent Table 3 of Part 503. A biosolids meeting Table 3 values for all metals (and appropriate pathogen and vector attraction reduction criteria) is termed Class A, or "Exceptional Quality" ("EQ"), and has no restrictions on use.
Pathway 6 (biosolids to soil to plant to animal) is the limiting pathway for Mo. The HEI is a ruminant that develops molybdenosis (an Mo-induced Cu deficiency) as a result of consuming excessive amounts of Mo in the diet. Ingested Mo is transformed in the rumen to tetrathiomolybdate, which can form an insoluble and unavailable compound with Cu (Suttle, 1991). Thus, a severe Mo-induced Cu deficiency can occur in sensitive livestock (cattle, Bos taurus) with borderline Cu deficiencies; a common condition in many cattle grazing environments worldwide (Gartell, 1981; McDowell, 1985).
The allowable (RPc) Mo load in Pathway 6 is calculated from an algorithm that depends heavily upon the UC. The coefficient represents the ratio of Mo in the feed (mg Mo kg forage-1) exposed to a given soil biosolids Mo loading (kg Mo ha-1). Uptake is assumed to be linearly related to soil Mo load, and no other factors (e.g., forage type or condition, soil pH, nature of the biosolids, other components of animal diet, etc.) are directly considered.
The RPc value is sensitive to the UC value chosen, and the choice was a major area of disagreement in the derivation of RPc for Mo. Unfortunately, high quality field data were limited for Pathway 6 assessment. One set of data (e.g., Soon and Bates, 1985) used biosolids with typical total Mo concentrations of <100 mg Mo kg-1, and yielded low calculated UC values. Another data set (Pierzynski and Jacobs, 1986) used a specialty sludge containing 1500 mg Mo kg-1, and yielded UC values about 10-fold greater. Lacking additional suitable data (especially field studies), the USEPA calculated a geometric mean of data from both types of studies to arrive at UC = 0.423, RPc = 18 kg Mo ha-1, and APL = 18 mg Mo kg-1. They also derived a ceiling concentration for Mo that disqualifies biosolids containing >75 mg Mo kg-1 from land application (USEPA, 1993). Modern biosolids, nationally, typically contain 20 to 30 mg Mo kg-1 (R.B. Brobst, personal communication, 1999). Following a legal challenge, the USEPA delayed setting an APL value for Mo, pending additional field data for representative biosolids. The ceiling concentration remains in the rule.
This study was part of a larger effort (O'Connor and McDowell, 1999) that addressed the need for such additional field data. The focus here is on Mo uptake by corn grown in soils amended with typical biosolids over long times in the field. Corn (grain or silage) is a dominant component of cattle diets for both grazing and confined animals (Mowery and Spain, 1999). Corn does not usually accumulate excessive amounts of Mo (Gupta, 1997a, b), but plant tissue Mo concentrations in several forage materials are highly variable, depending on soil pH, soil wetness, climate, and regionality (Kubota et al., 1963; Miltimore and Mason, 1971; Kabata-Pendias and Pendias, 1992; Johansen et al., 1997). Further, available field data suggest corn (leaf or stover) UC values for biosolids-amended soils vary widely from 0.001 to 0.44 (Soon and Bates, 1985), and from 0.68 to 3.2 (Pierzynski and Jacobs, 1986). Corn grain UC values are uniformly low (0.001 to 0.08) (Soon and Bates, 1985; Pierzynski and Jacobs, 1986; Gupta, 1997b).
Our objective was to provide additional field data for corn stover uptake of biosolids Mo to improve UC value estimation. Data were collected from archived stover and soil samples collected in two long-term field studies where corn was grown on near neutral pH soils amended with biosolids containing Mo at concentrations typical of modern products.
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MATERIALS AND METHODS
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Archived soil and corn stover samples were obtained from two long-term field studies, referred to hereafter as the Chicago and Minnesota data sets. The Chicago data represent samples from unique plots maintained for >25 yr by the Metropolitan Water Reclamation District of Greater Chicago in Fulton County, IL. Plots began receiving anaerobically digested biosolids (unknown Mo concentration) in 1973, and have received annual applications at maximum, half-maximum, and quarter-maximum rates ever since. Analysis of the more recent biosolids samples suggest a total Mo concentration of 
12 mg Mo kg-1. Details of the experiment are available elsewhere (Pietz et al., 1982, 1983). For convenience, we selected paired soil and corn stover samples representing Years 3, 5, 9, 13, 17, 21, and 25 from the control, half-maximum, and maximum treatments for Mo analysis. Cumulative biosolids loads (maximum treatment) for these years are given in Table 1. Plots were established on calcareous strip-mine spoil (pH 7.8) in a randomized block design (Pietz et al., 1982).
Soil samples (0–15 cm) were collected each spring (six cores per plot) with a stainless steel probe, and then composited to form the sample for analysis. Soils were air-dried, sieved (<2 mm), and digested according to USEPA Method 3050A (USEPA, 1986). Digests were analyzed for Mo by inductively coupled plasma–atomic emission spectroscopy (ICP–AES), with careful attention to matrix matching (yttrium used as an internal standard) and interelemental interferences in standards and samples. Standards, spikes, and standard biosolids materials (IRM 007) were included for quality assurance. Molybdenum determinations were within 10% of expected values.
Corn (`Pioneer 3517') was grown on lister ridges 73 cm apart at a population of 50000 plants ha-1. Stover (vegetation remaining following harvest of corn cob and grain) samples were hand-harvested from the center two rows of each plot, dried for 48 h at 65°C, and ground in a Wiley mill equipped with a 20-mesh stainless steel screen. Ground tissue (2 g) was digested in concentrated HNO3, then evaporated to dryness, and brought to final volume (50 mL) in 1% HCl. The diluted digest was analyzed for Mo by a colorimetric dithiol method (Clark and Axley, 1955), and for Cu by ICP–AES. Analysis included appropriate standards, spikes, and standard reference plant material (SRM 1515) for quality assurance, and all analyses were within 10% of expected values.
The Minnesota data represent soil and corn stover samples from the I-9 site at the Rosemount Experiment Station, which has been the subject of biosolids research for 20 yr (Sloan et al., 1998). Plots received waste-activated, dewatered biosolids (unknown Mo concentration) for three consecutive years, beginning in 1977 at low, medium, and high rates. Cumulative biosolids loadings for the high treatments are given in Table 1 (Dowdy et al., 1983). The low and medium rates were 0.25 and 0.5 of the high rate, respectively. The plots were maintained for an additional 16 yr with regular fertilization, but no further biosolids additions (Sloan et al., 1998). We selected paired soil and corn stover samples representing 1977, 1979, and 1995 (Years 1, 3, and 19) to investigate possible changes in biosolids Mo bioavailability over time, especially long after biosolids applications had ceased. Soil pH at the Rosemount site was 6.2 initially, and remained at this level, or slightly higher [6.5 (±0.4 range)], throughout the 20-yr study (Sloan et al., 1997). Variations in soil pH were not a function of biosolids treatment.
Three 5-cm soil cores were collected from an area near the center of each 0.22-ha plot following corn harvest. Samples of the 0- to 15-cm depth were air-dried, crushed to pass a 2-mm sieve, and homogenized to form a composite sample for each plot. Digestion and Mo analysis of the digests was the same as described for the Chicago soil samples.
Corn (`Pioneer 3780') was grown each of the three years selected for study at an average plant population of 75000 ha-1. Corn was, in fact, grown in 12 of the 20 cropping seasons, and was continuously grown in the 1977 to 1984 seasons. Corn grown in 1995 followed soybean grown in 1994. Stover samples were hand-harvested from 10 m of row adjacent to soil sampling areas, dried at 70°C, and ground in a stainless steel Wiley mill to pass a 1-mm sieve. Plant tissue (1 to 2 g) was ashed in a muffle furnace for 3 to 4 h at 650°C. The ash was dissolved in 50% HCl, evaporated to dryness, redissolved in HCl, filtered, and brought to volume (50 mL) with distilled water. Molybdenum analyses was the same as described for the Chicago plant samples. Regression analyses were computed using SAS (SAS Institute, 1996). The linear regression slopes are defined by the USEPA (USEPA, 1995) as the UC values, irrespective of statistical significance. It is within this context, and the objective of this study, that the data are discussed.
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RESULTS AND DISCUSSION
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Chicago Data
Molybdenum concentrations in corn stover collected from the long-term corn fertility plots at Fulton County, IL are plotted against soil Mo loads in Fig. 1 and 2
. Soil Mo loads were estimated by two methods. Method 1 assumes an average biosolids total Mo concentration of 12 mg Mo kg-1 (representative of modern biosolids applied to the site). Soil Mo load then represents the product of biosolids Mo concentration and biosolids loads (Table 1). Method 1 ignores changes in biosolids Mo concentrations that may have occurred over 25 yr and any possible Mo leaching losses from the site. Estimated soil Mo loads reach a maximum of about 20 kg Mo ha-1 (Fig. 1), which slightly exceeds the original USEPA-calculated RPc of 18 kg Mo ha-1 (USEPA, 1995). Method 2 calculates soil Mo load from measured total soil Mo concentrations in the 0- to 15-cm depth samples. Load equals the product of soil concentration and the sum of mass of biosolids applied to each hectare and mass of soil mixed with biosolids (2240 Mg). This procedure accurately accounted for other trace element (Zn, Cd, Ni, and Cu) loadings to the plots (Granato et al., 1999). Method 2 resulted in slightly lower, but similar, estimates of soil Mo loads (Fig. 2) than Method 1. Both methods of calculation ignore possible contribution to plant uptake of soil Mo from depths >15 cm.
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Fig. 1. Linear model of stover Mo concentrations vs. soil Mo load for Chicago samples. Soil Mo loadings estimated from current biosolids Mo concentration and biosolids loads applied (Method 1)
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Fig. 2. Linear model of stover Mo concentrations vs. soil Mo loadings for Chicago samples. Soil Mo load estimated from soil Mo concentration of the 0- to 15-cm depth (Method 2)
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Regardless of how soil Mo loads are calculated, UC values are small. Despite the statistical significance of the linear models, the UC values are probably best considered to be zero. The USEPA risk assessment policy guidelines assign a default value of 0.001 to negative or zero estimates of UC values (USEPA, 1995). Thus, despite the uniquely high soil Mo loads attained over many years of repeated biosolids application, the stover Mo concentrations are essentially unchanged from that in the control plots. Stover Mo is low regardless of treatment, even at high soil pH (7.8). Stover Cu concentrations (Table 2) were almost always <10 mg Cu kg-1, but individual-year stover Cu to Mo ratios were routinely >10:1, well above the 2:1 value regarded as critical for cattle feed to avoid molybdenosis (Gartell, 1981).
Minnesota Data
Molybdenum concentrations in corn stover from the Rosemount, MN site are plotted against soil Mo loads in Fig. 3, 4, and 5
. Soil Mo loads were calculated as two times the measured soil Mo concentrations in the 0- to 15-cm samples. The multiplier of two is used by the USEPA (USEPA, 1995) when the soil bulk density is reasonably assumed to be 1.3 Mg m-3 
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Fig. 5. Linear model of corn stover Mo concentrations vs. soil Mo loadings for combined 1977, 1979, and 1995 Minnesota data
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The soil Mo loads attained in the Minnesota study were much smaller than those attained in the Chicago study, and Mo uptake by stover was small. In Year 1, the maximum load was <1 kg Mo ha-1, and the UC estimate was negative (Fig. 3). By Year 3, the maximum load was still <1.4 kg Mo ha-1 (data not shown), and the linear model 
was not significant 
. In both years, corn stover Mo concentrations never exceeded 1 mg Mo kg-1, and high stover Mo concentrations were not associated with high soil Mo loads. The UC values are probably best estimated to be zero for both years.
Data for 1995, 16 yr after the last biosolids application, are given in Fig. 4. Soil Mo loads are slightly lower than in 1979 (maximum 1 kg Mo ha-1), probably reflecting some Mo leaching from the surface horizon. The linear model is not significant, but tissue Mo concentrations are much greater than in Years 1 and 3. Even the control samples reflect greater stover Mo concentrations 
, about threefold greater than in Years 1 and 3 when biosolids were applied. Corn grown in 1995 followed soybean grown in 1994. Soybeans (legumes in general) tend to accumulate Mo (e.g., Gupta, 1997a,b), and concentrations of 20 to 50 mg Mo kg-1 are common. Molybdenum availability may be enhanced when corn follows soybean and soybean-accumulated Mo is released via biomass degradation. Dowdy et al. (1999) reported increased Cd and Zn concentrations in corn stover following soybean. Similar unexplained increases in wheat grain Cd were noted by Oliver et al. (1993) when wheat followed lupins (a legume) as the preceding crop. Such an effect of soybean residuals on corn Mo uptake could account for the otherwise unexplainable increase in control treatment stover Mo concentrations. Soil pH changes were minimal during the study, and there were no unique weather phenomena in 1995, which could affect Mo uptake (e.g., Kubota et al., 1963).
Minnesota data for Years 1, 3, and 19 are combined in Fig. 5. The linear model is not significant and the UC value estimated for the combined Minnesota data would be zero. We conclude that despite the greater stover Mo concentrations measured in 1995, there is little risk of corn stover accumulating excessive Mo many years after biosolids applications cease. Mean stover Cu concentrations in biosolids-amended plots each year were 
5 mg Cu kg-1 (Table 3), and Cu to Mo ratios were >2:1 in all years, so the risk of molybdenosis to animals consuming the forage is small.
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SUMMARY AND CONCLUSIONS
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Corn (grain and silage [stover + grain]) is an important component of cattle diets, and is a logical crop for biosolids amendment because of its high N demand. Thus, evaluation of molybdenosis risk associated with biosolids-amended corn is important. We used archived samples from two unique field experiments to quantify corn UC values for biosolids Mo. Both studies used biosolids with Mo concentrations typical of modern materials.
Data confirm previously reported low Mo accumulation by corn stover, even at very high biosolids loads and soil Mo loads near 18 kg Mo ha-1. Chicago data suggest corn UC values of essentially zero in plots continuously in corn since 1973. Minnesota data yielded similar estimates of UC for continuous corn, but one year's data (1995) suggested greater Mo accumulation in corn following soybean. The possibility that soybean (perhaps, any Mo-accumulating crop) can enhance Mo bioavailability to the following crop deserves further study. Nevertheless, the risk of molybdenosis to cattle consuming corn grown in biosolids-amended soil is small as the tissue Mo concentrations were always low, and Cu to Mo ratios were above critical levels known to protect against molybdenosis.
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NOTES
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Florida Agricultural Experiment Station Journal Series no. R-07475.
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ABSTRACT
INTRODUCTION
