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Published in J. Environ. Qual. 33:133-140 (2004).
© ASA, CSSA, SSSA
677 S. Segoe Rd., Madison, WI 53711 USA

TECHNICAL REPORTS

Heavy Metals in the Environment

Effect of Alkaline-Stabilized Biosolids on Alfalfa Molybdenum and Copper Content

Richard C. Stehouwer* and Kirsten E. Macneal

Department of Crop and Soil Sciences, The Pennsylvania State University, University Park, PA 16802-3504

* Corresponding author (rcs15{at}psu.edu).

Received for publication May 30, 2002.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Agricultural utilization of biosolids poses a potential risk to ruminant animals due to transfer of Mo from biosolids to forage to the animal in amounts large enough to suppress Cu uptake by the animal. Alkaline-stabilized biosolids (ASB) must be given particular consideration in assessment of Mo risk because the high pH of these biosolids could increase Mo and decrease Cu uptake by forage legumes. In this 3-yr field experiment, ASB and ground agricultural limestone (AL) were applied based on their alkalinity at rates equivalent to 0, 0.5, 1.0, and 2.0 times the lime requirement of the soil and alfalfa (Medicago sativa L.) was grown. Alfalfa yield was similar with AL and ASB except in the second year when ASB produced larger yields, apparently due to increased B availability with ASB. Application of ASB did not detectably increase extractable soil Mo (0- to 15-cm depth), but increased alfalfa Mo uptake in all cuttings with yield-weighted uptake coefficients (UCs) of 8.07 and 7.11 following the first and second ASB applications, respectively. Although ASB increased extractable soil Cu, and alfalfa Cu content was greater with ASB than with AL, yield-weighted alfalfa Cu to Mo ratio was decreased by ASB to levels near 3. These results suggest that ASB may have a greater effect on Mo uptake and Cu to Mo ratio of forage legumes than do other biosolids. Additional research is needed to determine implications of larger Mo cumulative loading with ASB for Mo risk, particularly in the soil pH range of 7 to 8.

Abbreviations: ASB, alkaline-stabilized biosolids • AL, ground agricultural limestone • UC, uptake coefficient


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
IN 1993 the USEPA promulgated regulations that govern the land application of biosolids in the United States (often referred to as Part 503; USEPA, 1994). The rule was developed from risk assessments that considered exposure of humans, animals, and environmental receptors to possible contaminants in biosolids via 14 possible pathways. Nine trace elements, including Mo, were regulated under the rule. In the case of Mo, the most limiting pathway was plant uptake of Mo and transfer to animals eating the plants. Increased dietary intake of Mo, by ruminant animals in particular, can inhibit Cu uptake and induce a Cu deficiency known as molybdenosis if the Cu to Mo ratio in the ruminant diet falls below 2:1 (McDowell, 1985; Ward, 1994). Part 503 originally contained a cumulative pollutant loading rate of 18 kg Mo ha–1 (lifetime loading rate from biosolids application) and a pollutant concentration limit of 18 mg Mo kg–1 (exceptional quality biosolids limit). Data used in the Mo risk assessment came from three experiments (Soon and Bates, 1985; Pierzynski and Jacobs, 1986a, 1986b). This risk assessment has been criticized, however, because the studies by Pierzynski and Jacobs used biosolids with excessively high Mo concentrations and thus the biosolids and soil Mo loadings were not representative of present-day biosolids and land application practices. The risk assessments have also been criticized because of the limited data set used, because only two of the studies were field studies (neither of which included forage legumes), and because plant uptake coefficients from the three studies were averaged using a geometric mean (McBride et al., 2000). Several companies involved in Mo production and use brought legal action against the USEPA, which led to the temporary suspension of all Mo limits except the ceiling concentration limit (O'Connor et al., 2001a).

O'Connor et al. (2001a) presented a modified risk assessment to establish Mo standards for land application of biosolids. The modified risk assessment used data from numerous studies (many of which were field studies), from several crops including forage legumes, and from studies that used modern biosolids with relatively low Mo content. One criticism of the 503 risk assessment that was not specifically addressed in the modified risk algorithm presented by O'Connor et al. (2001a) was the effect of different types of biosolids on the plant availability of Mo in soil, and resulting effect on plant uptake coefficients. Alkaline-stabilized biosolids may well have greater Mo availability than other biosolids, and such biosolids were underrepresented in the data used by O'Connor et al. (2001a) for their modified risk assessment. Increased Mo uptake in response to increases in soil pH has been observed in many studies (Gupta, 1997; Kabata-Pendias and Pendias, 2001). Availability of Mo also appears to be greater in soils with high phosphate and organic matter content (Jones et al., 1990). Each of these are conditions that could result from application of alkaline-stabilized biosolids. Richards et al. (1997) and McBride (1998) reported that a large amount of total Mo in alkaline-stabilized biosolids was water soluble or easily extractable. Because solubility and phytoavailability of Cu decreases with increasing pH, plant uptake of Cu could be suppressed by alkaline biosolids or alkaline soils even though most biosolids contain substantial amounts of Cu. Each of these characteristics of alkaline-stabilized biosolids could combine to more strongly influence Mo uptake and plant Cu to Mo ratios than would occur with other types of biosolids. Finally, forage legumes appear to be particularly responsive to changes in soil Mo availability, and readily accumulate Mo (McBride et al., 2000; O'Connor et al., 2001a). Thus, combinations of forage legume production with application of alkaline-stabilized biosolids could represent the greatest likelihood for increased Mo uptake and decreased forage Cu to Mo ratio. Remarkably, almost no field experiments have been conducted to investigate such a scenario. McBride et al. (2000) reported a UC of 4.3 for red clover (Trifolium pratense L.) grown with alkaline-stabilized biosolids, but this was a greenhouse pot study.

The experiment reported in this paper was conducted to address the need for data concerning alkaline-stabilized biosolids (ASB) and forage legume production in any revised risk assessment for Mo in biosolids. A 3-yr field study was conducted to examine Mo, Cu, and S uptake by alfalfa following applications of alkaline-stabilized biosolids used as a liming material.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
This 3-yr experiment was conducted on a Berks channery loam (loamy-skeletal, mixed, active, mesic Typic Dystochrept) located in Lehigh County, Pennsylvania on a farm production field that had been planted to soybean [Glycine max (L.) Merr.] in the previous growing season. The soil had a pH of 6.2; a lime requirement of 4.5 Mg CaCO3 ha–1 to increase pH to 7 (according to the SMP buffer test; Eckert and Sims, 1995); Mehlich 3–extractable (Wolf and Beegle, 1995) P, Ca, and Mg of 56.6, 1253, and 182 mg kg–1 respectively; and a cation exchange capacity of 10.5 cmolc kg–1 (summation of Mehlich 3–extractable cations and 1 M KCl extractable Al).

The ASB used in this experiment was produced by the N-Viro (Toledo, OH) process, which involves treating dewatered sewage sludge with an alkaline admixture to generate high pH and temperature followed by accelerated drying. Chemical characteristics of the ASB are given in Table 1. The alkaline admixture consisted of flue gas desulfurization by-products supplemented with additional quicklime (CaO) or hydrated lime [Ca(OH)2] to achieve necessary pH and temperature limits. Flue gas desulfurization by-products generally consist of coal combustion ash, gypsum, and alkalinity in the form of Ca(OH)2 and CaCO3 (Stehouwer et al., 1995). The differing amounts of Ca and S in the ASB used in 1999 and 2000 reflect differing amounts and compositions of the flue gas desulfurization by-products added in the ASB production process.


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  		Table 1. Chemical characteristics of alkaline-stabilized biosolids used in 1999 and 2000.

 
Seven treatments were applied in the spring of 1999 (Table 2). The biosolids was applied on the basis of its alkalinity to supply 0.5, 1, and 2 times the liming requirement of the soil. Three additional treatments supplied equivalent rates of alkalinity from ground agricultural limestone (AL). These rates of alkaline addition were selected for experimental purposes to provide a range of soil pH and Mo addition. Inorganic fertilizer N (NH4NO3), P (CaHPO4), and K (KCl) were added to the limestone treatments to provide nutrients equivalent to N, P, and K supplied by the sewage sludge. We applied N equivalent to all NH4–N and 20% of organic N in the ASB. The 20% first-year mineralization factor for ASB was recommended by Baker et al. (1985) and is still used by the Pennsylvania Department of Environmental Protection as a guideline for biosolids application. A control treatment with no alkaline amendment or inorganic fertilizer was also included. All treatments were surface-applied to 6.1- x 7.6-m plots arranged in four randomized complete blocks. Blocks were separated by a 4.5-m alleyway to prevent cross contamination of plots during tillage, planting, and harvesting operations. Surface-applied treatments were incorporated by chisel-plowing to an approximate depth of 15 cm, followed by disking to an approximate depth of 10 cm. Alfalfa was planted on 15 Apr. 1999. All treatments, including inorganic fertilizers, were surface-applied a second time on 5 July 2000 without incorporation. In the spring of 2001, K fertilizer (as KCl) was surface-broadcast uniformly over all plots at a rate of 230 kg K ha–1.


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  		Table 2. Description of treatments and Mo and Cu loadings from alkaline-stabilized biosolids (ASB) applications.

 
Soil samples were collected every spring and fall during the experiment. Five soil cores were collected from the 0- to 15-cm depth of each plot using a 1.5-cm-i.d. soil probe, and composited. Samples were air-dried and ground to pass a 2-mm sieve before analysis. Samples were analyzed for pH (1:2 in water), Mehlich 3–extractable P, K, Ca, and Mg, and total extractable Cu, Mo, Ni, and Zn by USEPA 3051 digestion (USEPA, 1986). Inductively coupled plasma emission spectrophotometry (ICP) was used to analyze all elements.

Alfalfa yield was measured by hand-cutting at a height of 5 cm from 1-m2 quadrats randomly located within the experimental plots. Alfalfa was harvested twice in 1999 and three times in both 2000 and 2001. Following each hand harvest the entire field, including all experimental plots, were cut and baled by the farmer. All harvested tissue from the 1-m2 quadrats was dried at 60°C, weighed, and ground to pass a 2-mm sieve. Ground tissue samples were subjected to microwave digestion in closed vessels and digests were analyzed for Ca, Mg, S, Al, Fe, Zn, B, and Cu by ICP (Miller, 1998). Analysis of alfalfa tissue for Mo was done using a modification of the dry-ashing procedure described by Dahlquist and Knoll (1978). To improve detection limits, a 1-g tissue subsample was ashed and the ash brought up to a 10-mL volume in nitric acid followed by ICP analysis.

Analysis of soil and plant digests for Mo was conducted at a wavelength of 202.030 nm with interelement correction factors applied for Fe and Al and an instrument detection limit of 0.008 mg Mo L–1. An interferent solution containing 500 mg L–1 Al, Ca, and Mg and 200 mg L–1 Fe was analyzed before each ICP run and yielded an average Mo value of 0.008 mg L–1. We also ran an interferent solution containing 1000 mg S L–1, which yielded an average Mo value of 0.0075 mg L–1. Analysis of National Institute of Standards and Technology (NIST, Gaithersburg, MD) citrus leaf (1572) and orchard leaf (1571) gave average concentrations of 0.12 and 0.23 mg Mo kg–1, respectively, compared with the certified values of 0.17 and 0.30 mg Mo kg–1, respectively, and acceptable ranges of 0.08 to 0.26 and 0.20 to 0.40 mg Mo kg–1, respectively. Analysis of certified sewage sludge ERA WWW 26-2 gave an average concentration of 88.14 mg Mo kg–1 compared with the certified value of 93.1 mg Mo kg–1, and an acceptable range of 73.6 to 112.0 mg Mo kg–1. Recoveries for spikes of 2, 5, and 10 mg Mo kg–1 alfalfa tissue were 86 and 101% for alfalfa tissue with low (0.77 mg kg–1) and high (9.01 mg kg–1) Mo content, respectively. Recovery of a 2 mg Mo kg–1 soil spike added to soil with a background Mo content of 1.15 mg kg–1 was 85%. Recovery of a 2-µg Mo spike added to the soil digest blank was 88%.

Data were analyzed using analysis of variance to determine treatment effects and the following planned orthogonal contrasts: (i) control plots contrasted with ASB- and AL-treated plots (control vs. treated); (ii) ASB treatments contrasted with AL treatments (ASB vs. AL); (iii) linear effect of ASB and AL treatments (ASB and AL linear); and (iv) linear interaction of ASB and AL treatments (ASB x AL linear interaction). Molybdenum uptake coefficients were determined for each cutting by linear regression of alfalfa tissue Mo against cumulative Mo loading from ASB. Yield-weighted average Mo uptake coefficients were determined for two alfalfa cuttings that followed the first ASB application (second cutting of 1999 and first cutting of 2000), and for the five alfalfa cuttings that followed the second ASB application. Yield-weighting was done by multiplying the fraction of total yield by the corresponding tissue Mo concentration for each treatment and then summing all cuttings within the weighting period. Yield-weighted average Cu to Mo ratios were similarly calculated.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Soil Chemistry
Application of ASB increased soil P, Ca, Mg, S, and Cu (Table 3). These effects were also noted in the first year of the study following one application of treatments, and persisted in the third year of the study even though treatment application was not repeated in 2001 (data not shown). The large increases in soil Ca and S with ASB reflect the use of flue gas desulfurization by-products as one component of the alkaline admixture used to prepare the biosolids. Increases in soil P were larger with AL than with ASB. Since we matched fertilizer P application in the AL treatments with total P application in the ASB treatments, this difference suggests lower Mehlich-3 extractability of the biosolids P. Increases in soil Cu reflect the loading of Cu from ASB application (Table 2)


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  		Table 3. Effect of alkaline-stabilized biosolids (ASB) and agricultural limestone (AL) on soil chemical parameters measured in fall 2000, following the second application of treatments.

 
The effects of ASB and AL on soil pH were relatively minor following the first treatment application (spring 1999–spring 2000) (Table 4). Only in the spring 2000 sampling did the treatments increase pH, with ASB causing a larger increase than AL. Because the 1 x LR rates of ASB and AL did not increase soil pH to 7 we decided to repeat treatment applications during the summer of 2000. Following the second treatment application, both ASB and AL increased soil pH at all sampling dates, and in two of the three sampling dates the linear soil pH increase from ASB was greater than that from AL. Higher pH with ASB could result from more rapid and complete reaction of ASB than AL or could indicate analytical error in determination of ASB calcium carbonate equivalency.


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  		Table 4. Effect of alkaline-stabilized biosolids (ASB) and agricultural limestone (AL) on soil pH and extractable Mo.

 
Although the changes in soil pH were small (less than one pH unit), they are important in this context given the well-established effect of increasing soil pH on increased soil Mo solubility and plant availability in the circumneutral pH range (Goldberg et al., 1998; Gupta, 1997; Kabata-Pendias and Pendias, 2001; O'Connor et al., 2001a). In our experiment the second application of ASB clearly added alkalinity in excess of soil liming requirement and increased soil pH to levels above 7. However, application of excess alkalinity can occur with commercial agronomic application of ASB, particularly if the biosolids are applied to supply crop N needs rather than for soil liming. In a 3-yr assessment of 18 farms in Pennsylvania receiving commercial agronomic biosolids applications (Shober et al., 2003), mean soil pH (0- to 10-cm depth) of the three farms where ASB was applied was 6.5, 7.2, and 7.7.

Although up to 0.51 kg Mo ha–1 was added with the two ASB treatments (Table 2), we could not detect any increases in soil Mo in the 0- to 15-cm depth (Table 4). Our inability to consistently measure changes in soil Mo reflects the small amount of added Mo in relation to overall experimental error. The maximum possible estimated soil Mo increase from the largest amount of ASB addition (assuming all Mo remained in the upper 15 cm and a soil bulk density of 1.5 g cm–3) would be 0.26 mg Mo kg–1 soil. By comparison, standard deviations for soil Mo in the control plots over the six soil sampling events ranged from 0.22 to 0.34 mg Mo kg–1 due to field variability as well as sampling and analytical error sources. The only evidence of an ASB effect on soil Mo occurred in the fall 2000 sampling that was conducted approximately 4 mo after the second ASB application. There was a significant linear interaction between AL and ASB indicating that soil Mo was decreased more by ASB than by AL. This result provides some indication that the soil pH increases with ASB may have resulted in increased Mo leaching. It must be emphasized, however, that this effect was observed in only one of six sampling events. Although increased Mo leaching with increased pH is consistent with known effects of soil pH on Mo solubility, Mo leaching could not be verified in our study because soils were not sampled below 15 cm and no leachate samples were collected. McBride (1998) reported that 11 and 13% of total Mo in two ASB materials (similar to the ASB used in our study) were soluble in 2:1 (w/w) water extracts. Such high water solubility of Mo indicates not only the potential for rapid Mo leaching, but also for initially high plant availability of Mo.

Alfalfa Yield
Yields of alfalfa were greater in the second growing season than in the first or third growing seasons (Table 5). Low yields in the first year were due to well-below-normal precipitation during the first half of the growing season. From 25 May to 12 Aug. 1999 only 40 mm of rainfall was recorded. Such extremely dry conditions within 30 d of alfalfa seeding resulted in very slow establishment and very low yields for the first cutting. Furthermore, most of the biomass produced in the first cutting of 1999 was weed species, not alfalfa. For this reason plant tissue analysis of the first cutting of 1999 was not included in the overall analysis of Mo and Cu uptake. Rains in August and September (390 mm) and chemical weed control resulted in good alfalfa growth before the second cutting of 1999, and nearly pure stands of alfalfa that persisted for the remainder of the experiment.


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  		Table 5. Effect of alkaline-stabilized biosolids (ASB) and agricultural limestone (AL) on alfalfa yield.

 
Alfalfa yield was greater with ASB than with AL in 2000 and 2001. The growth response to ASB appeared to be due to increased B availability. In the first cutting of 2000, visual B deficiency symptoms were apparent in alfalfa growing on all plots that did not receive ASB. Tissue B in alfalfa grown without ASB ranged from 9 to 20 mg B kg–1, just below to just above B sufficiency levels (data not included). Tissue B in alfalfa grown with ASB ranged from 27 to 40 mg B kg–1, and tissue B increased with increasing ASB application. Sulfur availability may also have affected yields as alfalfa S content was greater with ASB than with AL in most cuttings (Table 6). Differences in N availability between ASB and AL treatments does not appear to have been a strong yield factor. Alfalfa N content ranged from 35 to 50 g N kg–1, and in only two cuttings were there differences due to ASB or AL treatments; in one case N was greater with ASB, in the other N was greater with AL (data not included).


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  		Table 6. Effect of alkaline-stabilized biosolids (ASB) and agricultural limestone (AL) on S, Cu, Mo, and Cu to Mo ratio in alfalfa.

 
Alfalfa Molybdenum and Copper
Statistical analysis of alfalfa tissue Mo indicated significant interactions of treatments and cutting dates, therefore each cutting was analyzed separately. Alfalfa tissue Mo concentrations were larger with ASB than with AL application in all cuttings, and linear increases in tissue Mo were greater with ASB than AL in six of the seven cuttings (Table 6). Thus, despite the fact that ASB did not increase total extractable soil Mo, ASB application clearly increased alfalfa uptake of Mo. Increased Mo uptake is associated with liming and increased soil pH (Gupta, 1997; Kabata-Pendias and Pendias, 2001). In our experiment soil pH increases caused by ASB were larger than those caused by AL. This was most notable in sampling dates after the second treatment application (fall 2000–fall 2001) when soil pH reached as high as 7.5 with ASB treatments compared with a high of 6.9 with AL treatments (Table 4). Goldberg et al. (1998) found strong pH dependence of Mo adsorption in the pH range 3 to 10.5 on a variety of adsorbents. They reported maximum sorption at low pH and that Mo sorption decreased to near zero at pH 8. Thus, even though the soil pH differences between AL and ASB treatments were generally less than one pH unit, they occurred in a pH range that could strongly influence Mo solubility. The relationship between soil pH and alfalfa tissue Mo is illustrated in Fig. 1 . For control plots and AL-treated plots alfalfa Mo was not affected by changes in soil pH over the range 6.3 to 6.9, whereas for ASB-treated plots there was a significant increase in alfalfa Mo as soil pH increased over the range 6.5 to 7.5. However, the low r2 value of 0.32 for this regression, and the fact that up to pH 6.9 Mo uptake was significantly increased with ASB but not with AL, indicates that pH was not the only factor that affected Mo uptake.



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  		Fig. 1. Relationship between soil pH and alfalfa tissue Mo concentration for plots with various alkaline treatments.

 
Alfalfa Mo uptake was also related to cumulative Mo loading from ASB (Fig. 2) and accounted for a greater amount of variability in alfalfa tissue Mo than did soil pH (r2 = 0.58). Multiple regression analysis using both Mo loading and soil pH showed that both terms were significant, but the r2 increased only to 0.59. Thus, Mo loading with ASB, even at the low rates in this experiment, appeared to contribute to increased alfalfa Mo uptake. Greater Mo solubility and availability with alkaline-stabilized biosolids relative to other types of biosolids have been reported (Richards et al., 1997; McBride, 1998; McBride et al., 2000); however, these studies also did not clearly isolate effects of Mo loading from the corresponding effects of soil pH.



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  		Fig. 2. Effect of cumulative loading of Mo from alkaline-stabilized biosolids (ASB) application on alfalfa tissue Mo content.

 
Because of the large variability among cuttings in alfalfa Mo uptake, and because season-long dietary Mo exposure provides a better indication of molybdenosis risk (O'Connor and McDowell, 1999), yield-weighted Mo alfalfa content was calculated and used to determine Mo uptake coefficients. Yield-weighted uptake coefficients calculated separately for cuttings following each ASB treatment application gave uptake coefficients of 8.07 and 7.11 (Table 7, Fig. 3) . The similarity of the UC values for alfalfa grown after one and two ASB applications is noteworthy because soil pH increased by approximately 0.5 pH units following the second ASB application. These UCs are higher than the UC reported by McBride et al. (2000) for red clover grown with an ASB-type material, and higher than the UC of 4.0 for fresh legumes used by O'Connor et al. (2001a) in their modified Mo risk assessment. One reason for the much higher UC values in our experiment could be the low Mo loadings used. The yield-weighted Mo uptake data plotted in Fig. 2 appear to be curvilinear since a quadratic regression increases the r2 value from 0.97 to 0.99. This suggests the possibility that UC may decrease as Mo cumulative loading increases. Such a "plateau" effect could result from Mo leaching and from diminishing effects of ASB on soil pH as the carbonate equilibrium of 8.2 is approached.


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  		Table 7. Parameter values for effects of cumulative Mo addition on yield-weighted Mo uptake (linear regression, y = ax + b) and on yield-weighted Cu to Mo ratio of alfalfa tissue [three-parameter exponential decay function, y = a exp(–bx) + y0]. Yield-weighting was done for the two cuttings that followed one alkaline-stabilized biosolids (ASB) application, and the five cuttings that followed the second ASB application. Regressions and yield-weighted data are plotted in Fig. 3.

 


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  		Fig. 3. Effect of cumulative loading of Mo from alkaline-stabilized biosolids (ASB) application on yield-weighted alfalfa tissue Mo content and Cu to Mo ratio. Solid lines are linear regressions of Mo content on Mo addition, and dotted curves are three-parameter exponential decay regressions of Cu to Mo ratio on Mo addition. See Table 7 for parameter values for each regression.

 
A shortcoming of the original Part 503 Mo risk algorithm and the modified Mo risk algorithm (O'Connor et al., 2001a) is that they only consider Mo uptake. Yet it is well known that the risk being protected against, molybdenosis in cattle, is influenced not only by Mo intake, but dietary intake of Cu and S, availability of Mo, Cu, and S, and other factors (Ward, 1994). Ward (1994) suggests that forage Mo concentrations predicted by UCs alone are unreliable predictors of the Mo effect on cattle. Use of forage Cu to Mo ratios rather than only Mo would provide an improved, though still inadequate, estimate of Mo risk to cattle (O'Connor et al., 2001b). No risk algorithm has been presented that does this, although a dietary intake Cu to Mo ratio of 2:1 has been regarded as a lower-limit threshold for cattle (Ward, 1994).

Tissue Cu content was greater with ASB than with AL treatments in six of the seven cuttings (Table 6). Significant linear interactions between ASB and AL indicated that AL addition tended to decrease alfalfa Cu while ASB addition tended to maintain or increase alfalfa Cu. Increased soil pH normally decreases Cu availability (Pierzynski et al., 2000, Ch. 7); however, this pH effect was overcome by the addition of Cu with ASB, and the consequent increases in soil Cu (Tables 2 and 3). Since both alkaline sources increased alfalfa Mo and either decreased, had no effect on, or caused relatively small increases in Cu, the net result was that both ASB and AL treatments decreased alfalfa Cu to Mo ratios (changes were not significant only in the third cutting of 2000) (Table 6). In each cutting, however, decreases in Cu to Mo ratios were larger with ASB than with AL. Yield-weighted Cu to Mo ratios for alfalfa grown following each ASB treatment application are plotted against cumulative Mo application in Fig. 3. The changes in Cu to Mo ratio are clearly curvilinear and are fit well by a three-parameter exponential decay model that predicts that the Cu to Mo ratio will asymptotically approach a lower limit value as a function of Mo cumulative loading (Table 7). Although this model cannot be used to predict Cu to Mo ratios beyond the low range of Mo loading in our experiment, it illustrates that changes in Cu to Mo ratio are diminishing as Mo loading increases.

In addition to Mo, increased dietary intake of S has also been reported to reduce Cu availability to cattle (Ward, 1994). We observed that alfalfa tissue S content was greater with ASB than with AL application in six of seven cuttings (Table 6). We did not analyze S in the ASB; however, it probably contained a large amount of SO4–S from gypsum contained in the alkaline admixture. The large increase in soil S (Table 3) provides further evidence of a large S content in the ASB and accounts for the increased S uptake by alfalfa. O'Connor et al. (2001a) reported a threshold of 4 g S kg–1 in forages, stating that concentrations above this could potentially be problematic. McBride et al. (2000) cited a somewhat lower threshold of 3 to 4 g S kg–1. Yield-weighted average S concentrations in our experiment ranged from 3.45 to 3.75 for ASB treatments and from 2.78 to 3.05 for AL treatments. Although applications of ASB increased alfalfa S to levels within or near a critical range, lack of a clear linear response to ASB loading rate and no evidence of higher S following the second ASB application suggest that repeated use of ASB as a liming material would not result in further tissue S increases. Because most of the S in the ASB material is present as highly water soluble gypsum, soil concentrations would be expected to decrease due to leaching. Stehouwer et al. (1999) found that soil S concentrations following application of a high-gypsum-content coal ash material decreased to near background levels within 18 mo. Alfalfa tissue S was increased to 4.6 g kg–1 during the growing season following ash application, but decreased to 2.8 g kg–1 the next growing season. In this experiment, however, we did not observe a similar decrease in alfalfa S one year after ASB application.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Alkaline-stabilized biosolids was effective as a soil liming material when applied on the basis of its alkalinity and soil liming requirement. The ASB also increased soil Ca, Mg, P, and S. Alfalfa yield with ASB was larger than with AL in two of three growing seasons, apparently due to increased B availability from ASB. Total extractable soil Mo was not increased by ASB application reflecting the relatively low maximum cumulative Mo loading (0.51 kg ha–1). Alfalfa Mo uptake was consistently increased by ASB, apparently in response to increased soil pH as well as increased Mo loading. Regression of yield-weighted alfalfa Mo against cumulative Mo loading gave a Mo UC of 7.11. This is a higher UC than has previously been reported for forage legumes, and higher than UCs used in previous or proposed Mo risk algorithms. Application of ASB also decreased alfalfa Cu to Mo ratio and increased alfalfa S, each to levels that approached suggested thresholds for molybdenosis risk. However, curvilinearity in the Mo and Cu to Mo ratio responses and lack of further S increase following the second ASB application suggested that subsequent ASB applications may result in smaller changes in these parameters. Additional study is needed to unambiguously separate the effects of increased soil pH and Mo cumulative loading on forage uptake of Mo when ASB is used. Further investigation is also needed to determine the effects of higher cumulative Mo loadings with ASB, particularly in the soil pH range of 7 to 8, as well as any decay in ASB effects on Mo uptake and Cu to Mo ratio with time after ASB application ceases. Because ASB may have a greater effect than other types of biosolids on forage parameters involved in molybdenosis risk to ruminant animals, data from such studies need to be included in a revised Mo risk assessment. Ranchers and dairies using forage produced with ASB should pay special attention to soil pH, overall feed Cu to Mo ratio, and Cu (mineral) dietary supplementation to avoid molybdenosis.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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The SCI Journals Agronomy Journal Crop Science
Journal of Natural Resources
and Life Sciences Education		 Vadose Zone Journal
Soil Science Society of America Journal Journal of Plant Registrations The Plant Genome