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Published online 1 May 2008
Published in J Environ Qual 37:798-807 (2008)
DOI: 10.2134/jeq2007.0214
© 2008 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
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TECHNICAL REPORTS

Heavy Metals in the Environment

Trace Element Changes in Soil after Long-Term Cattle Manure Applications

Mônica B. Benkea, Srimathie P. Indraratneb,c, Xiying Haoa,*, Chi Changa and Tee Boon Gohb

a Agriculture and Agri-Food Canada, Lethbridge Research Centre, P.O. Box 3000, Lethbridge, AB, Canada T1J 4B1
b Dep. of Soil Science, Univ. of Manitoba, Winnipeg, MB, R3T 2N2, Canada
c Permanent address: Dep. of Soil Science, Faculty of Agriculture, Univ. of Peradeniya, Sri Lanka

* Corresponding author (haoxy{at}agr.gc.ca).

Received for publication April 30, 2007.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 
Manure application supplies plant nutrients, but also leads to trace element accumulation in soil. This study investigated total and EDTA-extractable B, Cd, Co, Cu and Zn in soil after 25 annual manure applications. The residual effect of 14 annual manure applications followed by 11 yr with no applications was also investigated. Manure was applied at 0, 30, 60 and 90 Mg ha–1 yr–1 (wet weight) under rainfed (treatments Mr0, Mr30, Mr60, and Mr90) and at 0, 60, 120 and 180 Mg ha–1 yr–1 under irrigated conditions (Mi0, Mi60, Mi120, and Mi180). The manure applications had no significant effect on soil B, Cd and Co content under both rainfed and irrigated conditions, but significantly increased total Cu and Zn content under irrigated conditions with Zn in Mi120 and Mi180 reaching the lower maximum concentration (MAC) level set by the European Community. Manure application also significantly increased EDTA-extractable Cd and Zn content in soil. Up to 27% of the total Cd (0.156 mg kg–1) and 21% of total Zn (38 mg kg–1) are found in EDTA-extractable form (Mi180 at 0–15 cm). EDTA-extractable Cd and Zn content was also significantly elevated in the irrigated residual plots due to the higher manure rates used. Thus, the impacts of cattle manure application on trace elements in soil are long lasting. Elevated Cd and Zn are a concern as other studies have linked them with certain types of cancers and human illnesses.


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 
IN the province of Alberta cattle feedlot operations have intensified since the 1980s, reaching about 6.3 million cattle in 2006. This accounts for around 39% of the total Canadian cattle inventory and makes Alberta the largest beef producing province (CANFAX Research Services, 2006). In the County of Lethbridge, southern Alberta, where this study was conducted, there are over 750,000 head of feedlot cattle generating over 1,785,000 t of manure annually, as each cow produces about 2.38 t manure per year (Alberta Agriculture, Food, and Rural Development, 2000). Due to high trucking costs, most cattle manure is applied to nearby cropland under both rainfed and irrigated conditions.

Animal manure contains plant macro and micronutrients (N, P, K, Ca, Mg, B, S, Cu, Fe, Mg, Mo, and Zn), which are essential in improving soil quality and providing nutrients for crop production (Kabata-Pendias and Pendias, 2001; Sistani and Novak, 2006). However, manure is also recognized as a significant source of trace elements (e.g., As, Cu, Mn, Ni, and Zn) (L'Herroux et al., 1997; Nicholson et al., 1999; Bolan et al., 2004; Novak et al., 2004; Sistani and Novak, 2006). Animal feeds are frequently fortified with elements such as Co, Cu, Fe, Mn, and Zn (NRC, 2000) to maintain various physiological processes and prevent animal health disorders. Since not all the trace elements added in the diet are assimilated by the animals, increasing amounts of these elements have been observed in livestock manure (Nicholson et al., 1999). Manure application has also been linked to increasing Cd, Cu, and Zn solubility in soil (Bolan et al., 2004).

The content of trace elements in animal manure is much lower than in sewage sludge and mine spoils (Alloway, 1995; Nicholson et al., 1999; Kabata-Pendias and Pendias, 2001, Bolan et al., 2004). However, continuous long-term application of animal manure may cause accumulation in top soil reaching levels considered toxic to sensitive plants (Brock et al., 2006; Sistani and Novak, 2006). Runoff and leaching may also transport trace elements from manure-applied fields into surface and ground water, posing a threat to the environment (Moore et al., 1998; Brock et al., 2006). Bolan et al. (2004) and Nicholson et al. (2003) reported that animal manure could be a major source of trace elements, especially Cu and Zn, in fields near livestock production facilities.

Numerous studies have been conducted on the effect of long-term cattle manure applications to soil chemical properties, such as total C, N, and P, salinity, and exchangeable cations (Hao and Chang, 2002, 2003; Hao et al., 2003; Nardi et al., 2004; Chang et al., 2005; Blair et al., 2006; Tejada et al., 2006). However, there is little research on the accumulation of trace elements in soils receiving feedlot cattle manure. The objective of this research is to investigate the effect of long-term cattle manure applications on the accumulation (total content) and bioavailability (EDTA-extractable content) of B, Cd, Co, Cu, and Zn in soil under both rainfed and irrigated conditions. The residual effects after manure applications have been discontinued are also examined.


   Materials and Methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results and Discussion
 Conclusions
 REFERENCES
 
Site Description and Experimental Design
The experiment was conducted at the Agricultural and Agri-Food Canada Research Centre in Lethbridge, Alberta. The experimental design was a random-block split plot. The rainfed and irrigated fields were two adjacent blocks of a well-drained Dark Brown Chernozemic clay loam soil (Typic Haploboroll). Details on the soil properties and plot size were provided by Sommerfeldt and Chang (1985).

Starting in fall 1973, cattle feedlot manure was applied annually at 0, 30, 60, and 90 Mg ha–1yr–1 (wet weight) (treatments Mr0, Mr30, Mr60, and Mr90) on the rainfed field and at 0, 60, 120, and 180 Mg ha–1yr–1 (treatments Mi0, Mi60, Mi120, and Mi180) on the irrigated field for 25 yr. The method of manure incorporation was described by Hao and Chang (2003). The application rates were chosen to simulate on-farm practices and are equivalent to zero, one, two, and three times the local 1973 recommended agronomic rate (based on nitrogen requirements) for all types of feedlot cattle solid manure for this soil type. No chemical fertilization was added to these plots after 1973. Each treatment was initially replicated nine times. In 1987, three replications under rainfed (treatments Rr30, Rr60, and Rr90) and two replications under irrigated conditions (treatments Ri60, Ri120, and Ri180) received no further manure application to investigate the residual effect of 14 annual cattle manure applications followed by 11 yr with no application. The rest of the replications continued to receive annual manure applications.

The manure was obtained from an open, unpaved commercial feedlot near Coaldale, Alberta, over the 25 yr of application. Some characteristics of the Coaldale manure used are shown in Table 1 . There are no data available on the total and EDTA-extractable B, Cd, and Co content in the manure used between 1973 and 1998.


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  		Table 1. Characteristics of cattle feedlot manure used over 25 yr.

 
The study area has a semiarid climate. From 1973 to 1998, the average annual precipitation was 389 mm and the average annual temperature was 5.9°C. The irrigation system used was solid set sprinklers. The amount of irrigation water averaged 148.3 mm yr–1, with a maximum value of 292 mm in 1984 and a minimum value of 0 mm in 1995. Rainfed treatments were seeded to barley (Hordeum vulgare L. ‘Galt’) from 1973 to 1995, switching to canola (Brassica rapa L.) in 1996, barley in 1997, and triticale (Triticosecale L.) in 1998. Irrigated fields were seeded to the same crops, except that two varieties of corn (Zea mays L.) were used in 1997 and 1998. Crops were generally seeded in May and harvested in October. All crop residues were removed with only about 5 to 15 cm of stubble left on the soil surface.

Manure and Soil Analysis
In the fall of 1998, following 25 yr of manure treatments, soil samples were taken after harvesting at 0- to 15-cm, 15- to 30-cm, 30- to 60-cm, 60- to 90-cm, 90- to 120-cm, and 120- to 150-cm depths. Soil samples were air-dried and ground to pass through a 2-mm sieve. Subsamples were further fine ground to pass a 0.150-mm sieve. Total C (TC) was determined by a dry combustion technique using an automated CNS analyzer (Carlo Erba, Milan, Italy) (Smith and Tabatabai, 2004) and the inorganic C was determined according to the method of Amundson et al. (1988). The organic C (OC) content was calculated as the difference between TC and inorganic C. The pH was measured using a pH/conductivity meter (Accumet pH meter 50, Fisher Scientific) directly in the saturated soil paste prepared according to the method of Janzen (1993).

Total B, Cd, Co, Cu, and Zn contents in soil were determined using the method described by Yingming and Corey (1993). After adding 35 mL of 4 mol L–1 HNO3 acid (Fisher Reagent A.C.S. grade A200–225) to 5.0 g of finely ground soil, the mixture was digested at 70°C for 6 h in a water bath. After digestion, the solutions were brought up to 50 mL with distilled deionized water and filtered. The concentration of trace elements in the extracts was determined using a refurbished inductively coupled plasma spectrometer (Questron Technologies Corp., Mississauga, ON).

The available B, Cd, Co, Cu, and Zn contents in both manure and soil samples were obtained using an EDTA extraction method (Haq and Miller, 1972). About 20 mL of the EDTA extracting solution (0.01 mol L–1 EDTA and 1 mol L–1 (NH4)2CO3 solution adjusted to 8.6 with NH4OH) was added to 10 g of soil. The mixture was shaken for 30 min in polypropylene bottles, and then filtered. The concentrations of trace elements in the extracting solution was determined as described above for total content. Additionally, EDTA-extractable Cu and Zn in soil for the years 1973 (0 manure annual application) and 1983 (after 10 annual manure applications) were also determined. All results are expressed on a dry weight basis.

Statistical Analysis
Separate analyses were performed for non-irrigated and irrigated treatments using the MIXED procedure in SAS (SAS Institute Inc., 2005) for the analysis of variance (ANOVA) with manure, depth, and their interaction in the model as fixed effects, and the replication by manure interaction as random effects. The UNIVARIATE procedure was used to check the residuals for normality and for potential outliers. The results were statistically evaluated by the LSD test and were considered significant when at P < 0.05.


   Results and Discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
Conclusions
 REFERENCES
 
Total Trace Elements Content in Soil
Under both rainfed and irrigated conditions, soil total B content was not affected by the continuous annual (25 yr) or residual (14 continuous annual + 11 yr no application) manure treatments (Table 2 ). Boron concentration in cattle manure ranges from 0.3 to 24 mg kg–1 (Sistani and Novak, 2006). However, typical cattle diets are not supplemented with B (NRC, 2000), which may explain why manure application had little impact on the total B content in soil under both rainfed and irrigated conditions.


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  		Table 2. Results of analysis of variance for soil total and EDTA-extractable B, Cd, Co, Cu and Zn, OC and pH.

 
Boron toxicity is an important problem in many arid and semiarid regions throughout the world due to addition via irrigation water (Nable et al., 1997). However, our study found that soil B content (0 to 150 cm) was significantly lower under irrigation (range 16.4 to 17.9 mg kg–1, average 16.9 ± 0.2 mg kg–1) than under rainfed conditions (Table 3 ). The irrigation water used was probably low in B content and B leaching could have occurred. Furthermore, B retention by montmorillonite, calcite, and humic acid is low around pH 6 to 7.5 (Goldberg, 1997). The total B content of the surface soil used in our study was similar to the lower range of values (<20–70 mg kg–1) reported at the surface of Chernozemic soils from the U.S. (Kabata-Pendias and Pendias, 2001).


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  		Table 3. Total B, Cd and Co distribution in the soil under rainfed and irrigated conditions.

 
While total Cd content in the 0- to 15-cm depth of the soils studied increased from 0.45 (control) to 0.50 mg kg–1 (Mr90) under rainfed and from 0.49 (control) to 0.58 mg kg–1 (Mi180) under irrigated conditions with manure applications, these increases were not statistically significant (Table 2). Cadmium is found in cattle manure in small amounts. Total Cd content in beef cattle manure in England varies from <0.10 to 0.53 mg kg–1 (Nicholson et al., 1999) and in feedlot manure in southern Alberta varies from 0.30 to 0.65 mg kg–1 (Larney et al., 2008). Cadmium addition through irrigation was probably insignificant because levels in the irrigation water used were below the detection limit. The range of total Cd (0.45 to 0.60 mg kg–1) content in the 0- to 15-cm depth of the soils studied here was within the values (0.20 to 0.83 mg kg–1) obtained in the Ap horizon of Chernozemic soils from Saskatchewan (Mermut et al., 1996; Krishnamurti et al., 1997). These values are below the maximum allowable concentration (MAC) in soils set by both the European Community (E.C.) (1–3 mg kg–1 Cd) and the United States (20 mg kg–1 Cd) (McGrath et al., 1994).

The MAC levels, also called maximum permissible concentrations, of trace metals in soils are concentrations above which the risk of adverse effects in plants, soil microorganisms, grazing animals, and humans (through the food chain) are considered unacceptable (McGrath et al., 1994; Crommentuijn et al., 2000). Despite the diversity of the limits set by different countries, the ongoing research on these guidelines, and the complex nature of soils, they serve as advisory standards for the safe addition of trace metals to land.

Cobalt is an essential mineral for cattle growth added as a supplement in the feed in small quantities. About 1.5 mg Co kg–1 feed is normally used as a supplement in cattle diets in southern Alberta (D. Gibb, personal communication, 2007). Total Co content in cattle manure is reported to be about 2.2 to 3.6 mg kg–1 (Bolan et al., 2004; Larney et al., 2008). However, no significant increase in soil total Co was observed with continuous annual manure application for 25 yr. This could be attributed to the possibility of plant absorption (which may be higher in manure-applied areas) and the fact that the amount of Co applied over the years was probably low compared to the natural content of Co in these soils. L'Herroux et al. (1997) also reported no change in soil Co concentrations after intense pig slurry addition equivalent to 100 yr of applications.

Twenty-five yr of manure applications under rainfed conditions resulted in a small increase in the total Cu content at the 0- to 15-cm depth, but this increase was not statistically significant (Table 2). However, under irrigated conditions, the increase in total Cu content with manure applications was statistically significant (Table 2 and Fig. 1 ). This could be attributed to the higher manure rates under irrigated than rainfed conditions. Supplementation of Cu at 20 mg kg–1 feed in beef cattle diets is normally used in southern Alberta (D. Gibb, personal communication, 2007). Copper assimilation by the animal's digestive tract is not very efficient and some Cu added in the animal's diet is ultimately released into feces (NRC, 2000). Total Cu content in the manure used in our study varied from 27.4 to 71.1 mg kg–1 (Table 1). This is typical of manure from commercial feedlots in southern Alberta (Larney et al., 2008).


Figure 1
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  		Fig. 1. Total Cu and Zn distribution in the soil under the irrigated conditions (treatments Mi0, Mi60, Mi120, and Mi180 refer to 25 annual manure applications at 0, 60, 120, and 180 Mg ha–1 yr–1 and residual treatments Ri60, Ri120, and Ri180 refer to 14 annual manure applications at 60, 120, and 180 Mg ha–1 yr–1 followed by 11 yr with no manure applications).

 
The majority of the Cu added by manure application under irrigated conditions accumulated in the 0- to 15-cm depth with values in the Mi180 treatment (34.0 mg kg–1) significantly higher than values (20.6 mg kg–1) in the control (Fig. 1). A significantly higher Cu content in Mi180 (22.7, 20.0, and 20.9 mg kg–1 in the 15- to 30-, 30- to 60-, and 60- to 90-cm depths, respectively) than in the control (17.3, 17.5, and 18.0 mg kg–1) was also observed. This suggests a downward movement of Cu in the soil profile. Kuo (1981), Kingery et al. (1994), and Novak et al. (2004) also reported high Cu concentration in subsoil horizons after long-term animal manure applications.

The total Cu content in most residual plots under both rainfed (Rr30, Rr60, and Rr90) and irrigated (Ri60 and Ri180) conditions was similar to the total Cu values in the control (Mr0 and Mi0) in all soil depths studied (Fig. 1). This suggests that plant Cu uptake during the 11 yr with no manure applications returned total Cu levels back to the levels found in the control plots.

Total Cu contents in the surface soil ranged from 20.2 to 34.0 mg kg–1 in our study, which are at the lower end of values (19.3 to 68.1 mg kg–1) reported for 13 cultivated Dark and Dark Brown Chernozemic surface soils in Saskatchewan (Mermut et al., 1996). The MAC for Cu in soil set by the E.C. and the U.S. is 50 to 140 and 750 mg kg–1, respectively (McGrath et al., 1994). The 25 annual applications of cattle feedlot manure increased total Cu content in this calcareous soil, but the highest level (34 mg kg–1) was still below the limits set by both the E.C. and the U.S.

The impact on soil total Zn content from the annual manure applications was in some ways similar to that of total Cu. As observed for total Cu content, total Zn content in soil increases with the manure application rate under irrigated conditions. However, the effect was only significant in soils up to 30-cm depth (Fig. 1). Twenty-five continuous annual manure applications under irrigated conditions resulted in a higher increase in total Zn content compared with total Cu. Total Zn content in the manure used was about 4 to 11 times higher than total Cu content (Table 1). Around 60 mg kg–1 of Zn is added as a supplement to beef cattle diets in southern Alberta (D. Gibb, personal communication, 2007). Other sources of Zn in cattle manure are animal and vegetable feedstuff and contaminated soil (Bolan et al., 2004).

At 0- to 15-cm depth, total Zn increased from 72.2 mg kg–1 in the control treatment (Mi0) to 115.5 mg kg–1 (Mi60), 159.5 mg kg–1 (Mi120), and 187.5 mg kg–1 (Mi180). Compared to the control plots, applications of manure at 120 and 180 Mg ha–1yr–1 for 25 yr doubled the total Zn content of the soil surface. In the 15- to 30-cm depth, total Zn also increased significantly from 54.5 mg kg–1 (Mi0) to 77.1 mg kg–1 (Mi60), 82.6 mg kg–1 (Mi120), and 95.3 mg kg–1 (Mi180). Brock et al. (2006) also observed an increase in total Zn content in areas treated with liquid dairy manure and solid poultry during 40 yr of applications. Most of the Zn in the soils studied by these authors was concentrated in the 0- to 15-cm depth although one field that received solid poultry had an elevated Zn level at 30-cm depth.

Unlike Cu the total Zn contents in the residual treatments under irrigated conditions (Ri60, Ri120, and Ri180) were significantly higher than in the control treatment (Mi0) for the surface soil (0–15 cm). At all the other depths studied the total Zn content in the residual treatments did not differ significantly from the control treatment. The slower decrease in total Zn than total Cu content in the residual treatments surface soil could be explained by the higher content of total Zn in the manure-treated soils.

The total Zn levels of 159.5 and 187.5 mg kg–1, observed in the surface of soils that received 120 and 180 Mg ha–1yr–1 of manure for 25 yr, were higher than the range of 41 to 137 mg kg–1 observed in the surface of Chernozemic soils from Saskatchewan (Mermut et al., 1996). These levels reached the lower end of the MAC (150–300 mg kg–1) set by the E.C., but were much lower than the MAC set by the U.S. (1400 mg kg–1) (McGrath et al., 1994). The lower limits set by the E.C. are based on the evidence of Zn toxicity to Rhizobium and soil microorganisms (Giller et al., 1998). These authors pointed out that recent research has shown soil microorganisms to be far more sensitive to trace metal stress than plants or grazing animals. This suggests that manure should not be applied to soils at these higher rates to avoid Zn toxicity in plants and soil microbes and its accumulation in the food chain.

EDTA-Extractable Trace Elements Content in Soil
The levels of both B and Co in the EDTA extracting solution were below the detection limit (<0.017 mg L–1) of the equipment used, preventing proper assessment of available B and Co in this study.

Even though the application of manure did not seem to influence the total Cd content of these soils, it did increase EDTA-extractable Cd at surface and subsurface depths under both rainfed and irrigated conditions (Fig. 2 and 3 ). However, this increase was only significant (Table 2) under irrigated conditions that received manure at 120 Mg ha–1yr–1 (Mi120) and 180 Mg ha–1yr–1 (Mi180) at the 0- to 30-cm soil depth. In the Mi180 treatment, 27, 21, and 10.5% of the total Cd in the 0- to 15-, 15- to 30-, and 30- to 60-cm depths was in EDTA-extractable form. Downward movement of soluble Cd after cattle manure application to cropped soils was also observed by del Castilho et al. (1993).


Figure 2
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  		Fig. 2. Distribution of EDTA-extractable Cd, Cu, and Zn in soil under rainfed conditions (treatments Mr0, Mr30, Mr60, and Mr90 refer to 25 annual manure applications at 0, 30, 60, and 90 Mg ha–1 yr–1 and residual treatments Rr30, Rr60, and Ri90 refer to 14 annual manure applications at 30, 60, and 90 Mg ha–1 yr–1 followed by 11 yr with no manure applications).

 

Figure 3
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  		Fig. 3. Distribution of EDTA-extractable Cd, Cu, and Zn in soil under irrigated conditions (treatments Mi0, Mi60, Mi120, and Mi180 refer to 25 annual manure applications at 0, 60, 120, and 180 Mg ha–1 yr–1 and residual treatments Ri60, Ri120, and Ri180 refer to 14 annual manure applications at 60, 120, and 180 Mg ha–1 yr–1 followed by 11 yr with no manure applications).

 
The increase in Cd solubility could be explained by the fact that manure applications significantly lowered pH (Table 4 ) and increased OC (Table 5 ) and Cl contents (Table 6 ) in the 0- to 60-cm depth raising Cd levels in the soil solution and leading to the formation of Cd-organic and inorganic complexes. del Castilho et al. (1993) reported that all Cd in leachate solution collected from soils amended with cattle manure slurry was in the form of organic-metal complexes. These authors found that Cd concentration in the soil solution was positively correlated with dissolved organic carbon (DOC) and negatively correlated with pH. They also observed that high EC levels in the soil surface increased Cd solubility.


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  		Table 4. Distribution of pH in the soil in continuous (25 annual manure applications) and residual (14 annual manure applications followed by 11 yr of no application) manure applications under rainfed and irrigated conditions.

 

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  		Table 5. Distribution of soil organic carbon (OC) in the soil from continuous (25 annual manure applications) and residual (14 annual manure applications followed by 11 yr of no application) manure applications under rainfed and irrigated conditions.

 

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  		Table 6. Distribution of water soluble Cl– in soil from continuous 25 annual manure applications under rainfed and irrigated conditions.{dagger}

 
Solubility studies indicate that below pH 7.5, Cd in soils becomes increasingly mobile and may form organic complexes (Kabata-Pendias and Pendias, 2001). Continuous manure application for 25 yr under irrigated conditions significantly decreased soil pH from 7.4 (control) to 7.0 (Mi180) in the 0- to 15-cm depth and from 7.5 (control) to 7.1 (Mi180) in the 15- to 30-cm depth (Table 4). Regression analysis indicated that the EDTA-extractable Cd levels in soil are negatively related to soil pH (r2 = 0.76, P < 0.0001, n = 24).

Manure application to soils has been directly related to an increase in DOC-bound metal complexes (Hesterberg et al., 1993). The concentration of DOC in most soils is around 2 to 30 mg L–1 (Thurman, 1985) compared to manure by-products at about 1050 mg L–1 (Zmora-Nahum et al., 2007). No measurement of DOC levels either in the manure applied or in the soil solution is available in our study. However, long-term application of manure under the irrigated conditions significantly increased OC from 18.8 (control) to 72.5 g kg–1 (Mi180) in the 0- to 15-cm depth and from 9.4 (control) to 29.6 g kg–1 (Mi180) in the 15- to 30-cm depth (Table 5) suggesting increasing DOC content and its downward movement in the soil profile. The EDTA-extractable Cd is positively related to the soil OC content (r2 = 0.88, P < 0.0001, n = 24) based on data collected in our experiment.

Formation of strong complex ions, such as CdCl+, CdCl3–, and CdCl42–, is also expected below pH 7.5 in soils (Kabata-Pendias and Pendias, 2001). According to Antoniadis et al. (2006), formation of Cd-chloride complex ions enhances Cd solubility and therefore its phytoavailability. Chloride is the most dominant anion in the manure used in our study (Table 1). Based on the same study, Hao and Chang (2003) reported that Cl– content in the soil solution had significantly increased with the rate of manure application at all subsurface (below 15 cm) depths under irrigated conditions (Table 6).

The elevated available Cd content should be a concern as this is the most mobile and bio-available fraction of Cd in soil. Chaudri et al. (2001) has reported a stronger linear correlation between wheat grain Cd content and soil soluble Cd than with soil total Cd. Protecting the food chain from being contaminated with Cd is of particular concern because of its high toxicity and long body retention time (Senesi et al., 1999). High levels of Cd in food and water has been related to bone disease and renal dysfunctions (Senesi et al., 1999). Because of this concern new legislation limiting grain Cd content has been introduced in many countries (Chaudri et al., 2001). High levels of EDTA-extractable Cd in the Canadian prairie soils would not only represent a concern for human health, but also to the Canadian economy since it is one of the world's biggest grain producers.

Under irrigation, the EDTA-extractable Cd content in the residual treatments (Ri60 and Ri180) was significantly higher compared to the respective values in both the control and the continuous manure treatments (Mi60 and Mi180) in the 0- to 15-cm depth (Fig. 3). The pH of the surface soil (0–15 cm depth) in all residual treatments was significantly lower than the control plot (Table 4) and the OC content in the surface (0–15 cm) in all residual treatments was significantly higher than the control plot (Table 5). Several studies have shown that application of sewage sludge increases Cd solubility in soils and those high levels of soluble Cd persist in the soil several years after application has ceased (McGrath and Cegarra, 1992; Hyun et al., 1998; Walter and Cuevas, 1999). For example, McGrath and Cegarra (1992) observed very little change in the elevated levels of Cd-EDTA 20 yr after discontinuing sewage sludge application. This residue effect has been explained by the fact that the DOC concentration in sewage sludge–applied soils remains high even after application has ended (Bergkvist et al., 2003).

Despite the small significant increase in soil total Cu as a result of manure applications, soil EDTA-extractable Cu has been declining, even in the manure-applied plots, under both rainfed and irrigated conditions over the years (Tables 7 and 8 ). Results from previous years (after 0, 10, and 25 yr manure applications) indicate a statistically significant decrease in soil EDTA-extractable Cu in the 0- to 15-cm depth for all manure rates studied. The rate of decrease is greater under rainfed conditions because of the lower manure rates used compared to the irrigated treatments. This decline could be related to the fact that plant demand was probably higher than that supplied through manure applications and that a portion of the soil EDTA-extractable Cu reverted to less soluble forms throughout the soil profile. Mullins et al. (1982) and Payne et al. (1988) also reported transformation of soil DTPA-extractable Cu to less soluble forms in soils that received long-term Cu-enriched swine manure, as Cu in soil is strongly held by both inorganic (silicate clay minerals) and organic surfaces at pH 7 to 8 (Kabata-Pendias and Pendias, 2001).


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  		Table 7. EDTA-extractable Cu and Zn distribution in the soil profile in 1973 (0 manure applications), 1983 (10 annual manure applications), and 1998 (25 annual manure applications) under rainfed conditions.

 

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  		Table 8. EDTA-extractable Cu and Zn distribution in the soil profile in 1973 (0 manure applications), 1983 (10 annual manure applications) and 1998 (25 annual manure applications) under irrigated conditions.

 
Contrary to what was observed for the soil EDTA-extractable Cu content, the content of soil EDTA-extractable Zn significantly increased with long-term manure applications over the years at surface and subsurface depths under both rainfed and irrigated conditions (Tables 7 and 8). The content of EDTA-extractable Zn in the manure samples used was also higher than the manure EDTA-extractable Cu (Table 1). Under rainfed conditions, 25 annual manure applications at 90 Mg ha–1 yr–1 resulted in a 16.6-fold increase in EDTA-extractable Zn compared to the original surface soil level. Under irrigated conditions, a 32.3-fold increase was observed with manure treatment Mi180 in the same depth. The high level of 38.45 mg kg–1 EDTA-extractable Zn found in the surface soil in treatment Mi180 could be detrimental to soil microorganisms. Christie and Beattie (1989) observed that long-term applications of high rates of pig slurry to soils in Ireland elevated surface EDTA-extractable Zn to 50 mg kg–1 which was found to be associated with a significant decrease in soil biomass.

Although most of the EDTA-extractable Zn in the manured plots was found in the 0- to 15-cm depth under rainfed conditions, small but significant increases were also observed in the 15- to 60-cm depth in the Mr60 and Mr90 treatments. Under irrigated conditions, large amounts of EDTA-Zn were found in both the 0- to 15- and 15- to 30-cm depths in all manure treatments. Small but significant increases were also observed in the 30- to 150-cm depth with the highest manure rate (Mi180). This suggests Zn leached down the soil profile over the years. The lower pH and higher OC content due to manure applications may have increased Zn solubility and its downward movement. The EDTA-extractable Zn is positively related to soil OC content (r2 = 0.96, P < 0.0001, n = 24) and negatively to soil pH (r2 = 0.69, P < 0.0001, n = 24) under irrigated conditions. High soil and water Zn contents have been associated with unusually high rates of multiple sclerosis in some communities in both Canada and the U.S. (Irvine et al., 1988, 1989; Schiffer et al., 2001).

In our study, application of high rates of cattle manure over the years probably favored the formation of soluble DOC-Zn complexes. Richards and Webster (1999) reported increasing downward Zn mobility after long-term applications of farmyard manure to soils (pH around neutral) which was attributed to the formation of DOC-Zn complexes. High DOC concentration due to sewage sludge applications in soils with pH around 7 has been shown to be effective in increasing Zn solubility (Antoniadis and Alloway, 2002). Reddy et al. (1995) observed that Zn in water extracts was dominantly present in DOC-Zn complex form in soil with pH near 7.

As observed in the case of EDTA-extractable Cd, EDTA-extractable Zn in the residual plots did not return back to the control levels. EDTA-extractable Zn content in the residual treatments Rr60 and Rr90 was statistically higher than the control at 0- to 15-cm depth (Fig. 2). Significantly higher amounts of OC in the residual treatments compared to the control were also observed in the 0- to 15-cm depth under rainfed conditions (Table 5). Under irrigated conditions, EDTA-extractable Zn content in all residual treatments was statistically higher than the control from the 0- to 15- to the 30- to 60-cm depth (Fig. 3).


   Conclusions
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
REFERENCES
 
Livestock production has rapidly increased in most developed and developing countries throughout the world. Associated with this growth is the increased application of manure to agricultural land. Our results suggest that long-term application of cattle manure at recommended agronomic rates (30 Mg ha–1 under rainfed and 60 Mg ha–1 under irrigated conditions) to soils in southern Alberta does not pose an immediate threat to the environment. Although 25 yr of continuous annual applications at the recommended rates significantly increased total Cu and Zn levels under irrigated conditions, both total Cu and Zn levels remained below the MAC values set by the E.C. and the U.S. However, a threefold increase in the manure application rate (to 180 Mg ha–1 wet basis) under irrigated conditions elevated total Zn levels to 187.5 mg kg–1 in the surface soil. This is above the lower end of the MAC recommended by the E.C. No significant increases in total B, Cd, and Co contents were observed after 25 annual manure applications at all levels studied.

Cattle manure applications at the higher rates (120 and 180 Mg ha–1 yr–1 wet weight) under irrigated conditions significantly increased EDTA-extractable Cd in the 0- to 30-cm depth. Elevated and significantly higher levels of EDTA-extractable Cu in the 0- to 30-cm depth and EDTA-extractable Zn in the 0- to 150-cm depth were observed with all manure rates under both rainfed and irrigated conditions.

Discontinuing manure applications for 11 yr under irrigation resulted in small decreases in EDTA-extractable Cd and Zn. However, the residual levels were still higher than the values for the non-manured control. Thus, the impacts of cattle manure application on soil trace elements in soil, especially Cd and Zn, are long lasting. The elevated Cd and Zn levels in manured soil are a concern as other studies have linked them with certain types of cancers and human illnesses.


   ACKNOWLEDGMENTS
 
We thank Greg Travis, Brett Hill, and Pam Caffyn for field and laboratory work and Toby Entz for helping with the statistical analysis. This is Agricultural and Agri-Food Canada Lethbridge Research Centre Contribution number 38706047.


   NOTES
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 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
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