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ORIGINAL RESEARCH

Soluble Metal Leaching from a Poultry Litter–Amended Udult under Pasture Vegetation

^a Dep. of Crop, Soil, and Environmental Sciences, Univ. of Arkansas, 115 PTSC, Fayetteville, AR 72701
^b Dep. of Biological & Agric. Eng., Univ. of Arkansas, 233 Engineering Hall, Fayetteville, AR 72701
^c Agric. Statistics Lab., 101 Agricultural Annex, Univ. of Arkansas, Fayetteville, AR 72701
^d Dep. of Crop, Soil, and Environ. Sci., Univ. of Arkansas, 1366 W. Altheimer Drive, Fayetteville, AR 72701
^* Corresponding author (kbrye{at}uark.edu)

	ABSTRACT

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

 
More than 1 billion broiler chickens (Gallus gallus domesticus) are produced annually in Arkansas, with nearly 900 Gg of waste (i.e., litter) generated. Poultry litter is typically land applied as a means of disposal to nearby pastures as an organic fertilizer. Aside from essential plant nutrients, poultry litter also contains heavy metals, yet little is known about the potential of these metals to leach from soils with a history of litter application. The objective of this study was to continuously monitor the seasonal and annual effect of poultry litter application rate on soil leachate concentrations and leaching losses of metals (As, Cd, Se, Cr, Mn, Fe, Ni, Cu, and Zn) and dissolved organic carbon (DOC) from tall fescue (Festuca arundinacea Shreb.) pasture using automated equilibrium-tension lysimeters over a 2-yr period. Average annual drainage was 447, 235, and 592 mm in Year 1 (May 2003 through April 2004) and 833, 589, and 827 mm in Year 2 (May 2004 through April 2005) for control, low- (5.6 Mg ha^–1), and high-litter (11.2 Mg ha^–1) treatments, respectively. Drainage was similar among treatments during all time periods except for Winter (November through January) Year 1. Flow-weighted mean concentrations of Mn during Spring Year 1 (February through April 2004) and Ni and Cu for the whole year differed among litter treatments, but there were no consistent trends. In Spring Year 2 (February through April 2005), flow-weighted mean concentrations of Cr and Fe differed among litter treatment, but there were no consistent trends. Metal leaching losses did not differ among litter treatment during Year 1. In Year 2, leaching losses of Zn, Fe, and As differed among litter treatments during Summer, Fall, and Winter, respectively, but there were no consistent trends. Results indicate that poultry litter application rate did not affect metal leaching within two the first 2 yr of altered management; thus simply reducing application rates in areas with a history of litter application may not ensure a short-term reduction of potential further surface and groundwater impairment.

Abbreviations: DOC, dissolved organic carbon • DOY, day of year • EC, electrical conductivity • ETL, equilibrium-tension lysimeter • HDS, heat dissipaton sensor • ICAP, inductively coupled argon plasma • OM, organic matter

	INTRODUCTION

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

 
IN 1997, Arkansas produced more than 1 billion broiler chickens with more than one-third of the total production concentrated in northwest Arkansas (NASS, 2006). The production of this number of birds generates a large amount of waste (i.e., litter). Nearly 900 Gg of poultry litter is generated annually in Arkansas with approximately 300 Gg concentrated within northwest Arkansas (NASS, 2006). Waste disposal is a major challenge in most areas of intensive animal agriculture and can pose a threat to surface and subsurface water quality.

Most of the poultry litter generated in northwest Arkansas is land applied to tall fescue and bermudagrass [Cynodon dactylon (L.) Pers.] pastures as an effective organic soil amendment and fertilizer (Huneycutt et al., 1988). Poultry litter can provide many essential plant micronutrients (i.e., Cu, Fe, Mn, and Zn) needed for proper growth (Stephenson et al., 1990; Edwards and Daniel, 1992).

Due to the large amount of waste generated by the poultry industry, coupled with the economic limitations of transporting waste, pastures in northwest Arkansas often receive repeated applications of litter. A summary of soil tests from more than 2000 soil samples in Arkansas showed an average of 106 mg P kg^–1 in soil that had been amended with poultry litter compared to 59 mg P kg^–1 in soil that had not been treated (Sims and Wolf, 1994). As a result of increased concentrations at the soil surface, P may runoff and cause eutrofication of surface waters. Similar to P, if applied in high enough amounts, other soluble constituents of poultry litter, such as numerous metals (i.e., As, Cd, and Ni), may runoff and impair surface water and/or leach to threaten groundwater quality.

Metals are added to the poultry diet to promote weight gain, increase egg production, and prevent disease; thus notable amounts of metals, such as As, Cu, Fe, Mn, Ni, and Zn, as well as others, are present in manure (van der Watt et al., 1994; Kelley et al., 1996; Kpomblekou-A et al., 2002). Metal concentrations typically increase with the number of flocks raised that contribute to the litter accumulated in a broiler complex. For example, metal concentrations in five-flock litter can be three times higher than that from single-flock litter (Kunkle et al., 1981).

Although land application of heavy metals in sewage sludge is regulated (USEPA, 1993), there are currently no such regulations for metals in animal manures. As a result, accumulation of metals has been observed in soils with a history of poultry litter treatment. Elevated concentrations of As, Cd, Cu, and Mn have been reported in fields to a depth of 20 cm receiving poultry litter applications for up to 20 yr when compared to adjacent sites receiving no litter (Gupta and Charles, 1999). Significant accumulation of Cu, Zn, Mn, Ni, and Pb in the surface soil of bermudagrass pastures receiving 10 to 30 Mg ha^–1 yr^–1 for 25 yr has been reported (Han et al., 2000). Accumulation of Zn in the same study measured at depths up to 180 cm under long-term poultry litter-amended soil is indicative of the mobility of Zn and potential threat to groundwater (Han et al., 2000).

Little research is available documenting concentrations of heavy metals in leachate from poultry litter-amended soil, although limited soil leachate data is available from sewage sludge-amended soil. After a single application of metal-spiked sewage sludge, increased leachate concentrations of Zn, Ni, and Cd were observed at 70 cm under pasture and forest soils (McLaren et al., 2004). Increased leachate concentrations of Ni, Cu, and Cd were reported at 80 cm under sludge-amended loamy sand after receiving 3 to 400 m³ sewage sludge ha^–1 (Keller et al., 2002). Repeated application of litter over time and in areas of intensive animal production may result in significant transport of soluble metals from litter to groundwater.

The soils of northwest Arkansas are generally acidic, which can increase the solubility of metals in long-term poultry litter-amended soil (Renaud, 2000) and allow metals to leach through the soil profile. In addition to low pH, soluble organic complexes may form with organic matter, increasing metal mobility and leaching potential (del Castilho et al., 1993; Li and Shuman, 1996). While soil sampling can be used to assess metal accumulation with depth and provide an indication of metal mobility in poultry litter-amended soil, soil sampling cannot be used to quantify actual leaching losses and leaching potential.

To understand metal leaching potential in areas of intensive poultry production, it is imperative to continuously monitor soil leachate metal concentrations and drainage fluxes in response to multiple litter applications and natural precipitation. Since leaching can occur anytime throughout the year when sufficient water is present, controlled rainfall-simulation experiments are inadequate to address seasonal and inter-annual leaching variability, which is needed to evaluate the long-term effects of repeated poultry litter applications. Therefore, the objective of this study was to continuously monitor and evaluate the effect of poultry litter application rate on soil leachate concentrations and leaching losses of several metals (i.e., As, Cd, Se, Cr, Mn, Fe, Ni, Cu, and Zn) from tall fescue pasture using automated equilibrium-tension lysimeters. We hypothesized that a 50% reduction in litter application rate would result in a proportional decrease in metal leaching losses.

	MATERIALS AND METHODS

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

 
Site Description and Experimental Design
Research was initiated in Summer 2002 at the University of Arkansas Agricultural Research and Extension Center in Fayetteville. Six plots, 1.5-m wide by 6.0-m long with a 5% west–east slope located on a Captina silt loam (fine-silty, siliceous, active, mesic Typic Fragiudult; Harper et al., 1969; USDA-NRCS, 2004), were selected from a series of similar plots that had been amended with poultry litter in previous years. Clay contents at the site increased with depth in the soil profile from 6% in the 0 to 10 cm to 29% in the 65 to 85 cm depth (Pirani, 2005). Since the exact type and amount of litter amendments in prior years was unknown, plots were selected based on general proximity to one another and similar soil pH [6.2 standard error (SE) = 0.5] and Mehlich-3 extractable P [210 (SE = 24) mg kg^–1] in the top 5 cm; thus the study site consisted of three pairs of side-by-side plots within an 18 by 20 m area. Plots had not received any organic soil amendments for at least several years before initiating this study.

The average daily air temperature for Fayetteville is 14.2°C, with mean daily maximum and minimum temperatures of 20.0 and 8.3°C, respectively, and precipitation averages 117 cm annually (NOAA, 2002). Monthly 30-yr precipitation normals are summarized in Table 1 for the duration of this study.

Equilibrium-Tension Lysimeters
Six stainless steel equilibrium-tension lysimeters (ETLs; Fig. 1 ), 76.2-cm long by 25.4-cm wide by 15.2-cm deep, with a 0.1-cm thick, 0.2-µm porous stainless steel plate, similar to those used by Brye et al. (1999), were constructed and installed during Summer 2002. Before installation, lysimeters were cleaned by scrubbing with 70% isopropyl rubbing alcohol followed by a series of nitric acid (HNO₃) washes (i.e., 38 mL of HNO₃ diluted to 200 mL with deionized water; Dolan, 1995; Brye et al., 1999).

View larger version (86K): [in this window] [in a new window]   Fig. 1. (A) Equilibrium-tension lysimeter used in this study and (B) in situ view of a nearly completed lysimeter installation.

Lysimeter Installation
Lysimeters were installed between July and August 2002. Using a backhoe, three trenches approximately 6-m long by 90-cm wide by 1.5-m deep were excavated (i.e., one trench between each pair of plots). (Note: A trench of the above dimensions has the potential to collapse under certain circumstances. Proper precautions, such as bracing the sidewalls with plywood and lumber, should be taken to as to minimize the potential for collapse.) Hand tools were used to excavate cavities 1-m long by 45-cm wide by 55-cm tall into the long sides of the trench underneath each plot. Cavity ceilings were shaved smooth to approximately the 90-cm depth with a shaving implement (Brye et al., 1999). The shaving implement was sloped using shims from back to front and right to left to promote leachate collection from the front left corner of the storage reservoir. An aluminum plate was placed on the floor of the cavity to create a solid, level surface.

Before lysimeter installation, one heat dissipation sensor (HDS; 229-L, Campbell Scientific Inc., Logan UT; Reece, 1996) was placed in the bulk soil at the back of the excavated cavity behind and at the same depth where each lysimeter was to be placed to monitor the bulk soil matric potential. A second HDS was placed in the soil directly above each lysimeter to ensure that the lysimeter was maintaining suction equivalent to the bulk-soil matric potential.

To prevent soil particles from clogging the 0.2-µm porous plate, two sheets of glass fiber filter paper (i.e., 0.5 µm overlain by 1.0 µm) were cut to size and placed on top of the porous plate. Immediately before lysimeter installation, a thick slurry of soil from the depth the lysimeters were to be installed was poured on top of the glass fiber filter paper to ensure proper capillary contact between the lysimeter and the undisturbed soil above.

The lysimeter was placed atop an aluminum spring plate (Fig. 1; Brye et al., 1999) and pressed into the above soil using scissor jacks. Once the front left corner of the top of the lip surrounding the porous plate was within approximately 5 mm of the shaved cavity ceiling, a scissor jack was used to press the right end up into the soil until the slurry leaked out from the left front corner. Treated wood blocks were used to hold the lysimeter in place while the jacks were removed. Stainless steel tube extensions were attached to the sampling and vacuum tubes and cut to extend approximately 15 cm above the soil surface. Treated plywood sheets, with holes for the tube extensions, were used to protect the cavity while the trench was being refilled. Trenches were refilled manually in opposite order in which the soil was removed with approximately every 15 cm of soil replaced being tamped down to approximate the original soil bulk density.

Heat Dissipation Sensors
Each HDS was individually calibrated to obtain the most accurate measurement of soil matric potential. Using a small-scale prototype of the ETL, HDSs were placed in a silty-clay-loam slurry on top of the porous plate, while a range of applied tensions (i.e., 0, –10, –20, –30, –40, and –50 kPa) were maintained with an external vacuum system until equilibrium was reached and temperature measurements were obtained. A measurement taken under air-dry conditions was assumed equivalent to –100 MPa. A linear regression calibration equation was determined by plotting the natural logarithm of the tension in each ceramic sensor against the measured temperature difference before and after a 10 s period of electrical excitation. Linear regression analysis resulted in R² values ranging from 0.953 to 0.993.

Vacuum System
An automated vacuum system (Masarik et al., 2004) was used to vary the lysimeter suction and collect leachate in response to naturally fluctuating soil matric potentials. Vacuum systems were controlled by a CR10X datalogger (Campbell Scientific Inc.). Each datalogger maintained two lysimeters. The two HDSs per lysimeter were connected to an excitation module (CE8, Campbell Scientific Inc.) and helped vary lysimeter suction to maintain equilibrium between the soil matric potential and lysimeter suction.

Suction in the lysimeter was measured with a differential pressure transducer (PX170–014GV; Omega Engineering, Stamford, CT) and suction was either applied with a vacuum pump (TD-2N; Brailsford and Company, Rye, NY) or relieved by bleeding vacuum from the closed system. Four, three-way solenoid valves (ETO-3–12; Clippard Instrument Laboratory, Cincinnati, OH) connected the differential pressure transducer and vacuum pump. The vacuum pump and solenoid valves were controlled with a six-channel relay driver (A6REL-12; Campbell Scientific Inc.). All equipment was housed aboveground in a weatherproof fiberglass enclosure (ENC 16/18; Campbell Scientific Inc.). Vacuum pumps were connected to each lysimeter vacuum tube using copper refrigerator tubing (6.3 mm o.d.).

Since suction applied to each porous plate was maintained independently, each lysimeter was allowed 2.5 min after every 5 min bulk-soil matric potential measurement to establish equilibrium. Masarik et al. (2004) used minimum and maximum tension limits of –2 and –35 kPa, respectively, for a Plano silt loam (fine-silty, mixed, superactive, mesic Typic Argiudolls) in south-central Wisconsin. A minimum tension limit was set to avoid unnatural ponding above the lysimeter due to the soil-to-porous-plate discontinuity in the event that the soil approached saturation. A maximum tension limit was set to minimize the potential for inducing lysimeter cavitation when the soil dried out. For this study, the maximum suction was changed to –45 kPa due to the finer subsoil texture of the Captina silt loam. Similar to Brye et al. (1999) and Masarik et al. (2004), applied suction was maintained 2 kPa lower than the actual measured bulk-soil matric potential so as to avoid potential unnatural ponding above the lysimeter caused by the soil-to-porous-plate discontinuity.

Soil Characterization
During lysimeter installation, soil samples were collected from each lysimeter pit at depth intervals of 0 to 10, 10 to 25, 25 to 45, 45 to 65, and 65 to 85 cm. Soil samples were dried at 70°C for 48 h, crushed, and sieved through a 2-mm mesh screen for particle-size analysis using the hydrometer method of Arshad et al. (1996).

In Spring 2003 and 2004, before poultry litter applications, four soil cores were collected from each plot in 10-cm increments to a depth of 90 cm and composited into one sample per depth per plot. To minimize preferential flow, soil cores removed from the plots were replaced with cores of similar size taken from soil adjacent to the plot from which it was removed. Soil profile samples were dried at 70°C for 48 h, crushed, and sieved through a 2-mm screen for determination of organic matter, pH, electrical conductivity (EC), and total recoverable metals. Soil organic matter (OM) concentration was determined on sieved soil by weight-loss-on-ignition for 2 h at 360°C (Schulte and Hopkins, 1996). Soil pH and EC were determined potentiometrically using a 1:2 (w/v) soil-to-water mixture. Sieved soil was digested in a concentrated HCl and HNO₃ solution, treated with hydrogen peroxide (H₂O₂; USEPA, 1996), and total recoverable Fe, Mn, Zn, Cu, As, Cd, Cr, Ni, and Se were determined by inductively coupled argon-plasma spectrometry (ICAP; CIROS CCD model, Spectro Analytical Instruments, MA).

Leachate Collection and Analyses
Vacuum was first applied to all lysimeters in October 2002. However, leachate was not collected until late February 2003. Leachate was collected from the lysimeter reservoirs using a portable vacuum pump approximately every 14 d during wet months (January through June) and as needed during drier months (July through December). Precipitation was continuously monitored on-site between February 2003 and April 2005 using a funnel-type collector (Likens et al., 1977).

Approximately 1 L of leachate was kept for chemical analyses, while the remaining leachate volume was measured and discarded in the field. Approximately 150 mL of leachate were acidified to pH 2 with concentrated hydrochloric acid (HCl), while approximately 150 mL remained unacidified. Before acidification, leachate pH and redox potential were measured using a combination electrode (Model 313, Corning Inc., Corning, NY). Leachate EC was measured using a conductivity meter (Model 441, Corning Inc., Corning, NY).

Soluble As, Cd, Se, Ni, Cu, Zn, Cr, Mn, and Fe concentrations were measured on filtered, acidified leachate samples using a Spectro Modula ICAP spectrometer (Spectro Analytical Instruments, Fitchburg, MA). Dissolved organic carbon was measured on unacidified samples by combustion/catalytic oxidation using a Shimadzu Total Organic Carbon Analyzer (Model TOC-V CSH, Shimadzu Scientific Instruments, Columbia, MD). Leachate samples were stored no longer than 5 mo at roughly 4°C before chemical analysis.

Poultry Litter Characterization and Application
Poultry litter was obtained from a local source in Spring 2003 and 2004. The litter applied in Spring 2003 was 18-mo old and a result of eight flocks, while the litter applied in Spring 2004 was 12-mo old and a result of six flocks. Bedding material was comprised of an approximately even mixture of sawdust and rice (Oryza sativa L.) hulls. Chemical characterization of the poultry litter was conducted before application according to established procedures for animal manure analyses (Peters, 2003). Litter pH and EC were determined potentiometrically on a 1:2 (w/v) litter-to-water mixture. Total N and C were determined by high-temperature combustion using a LECO CN-2000 analyzer (LECO Corp., St. Joseph, MI). Litter subsamples were digested in HNO₃, treated with H₂O₂, and analyzed for total P, K, Ca, Mg, S, Na, Fe, Mn, Zn, and Cu using ICAP spectrometry. Litter subsamples were extracted with potassium chloride and NO₃–N and NH₄–N were determined using a Skalar San Plus automated wet chemistry analyzer (Skalar Analytical B.V., The Netherlands). Litter subsamples were also digested in a concentrated HNO₃/HCl solution, treated with H₂O₂, heated (USEPA, 1996), and analyzed for total recoverable Al, B, Cd, Cr, Ni, As, and Se by ICAP spectrometry.

Litter treatments were applied by hand on 30 Apr. 2003 and 6 May 2004. Litter chemical compositions for both years are summarized in Table 2.

Data Manipulations and Statistical Analyses
Flow-weighted mean concentrations (mg L^–1) were calculated on a seasonal and annual basis from elemental concentrations determined on individual sample dates. Similarly, drainage (mm) and nutrient-leaching losses (kg ha^–1) were summed by season and annual period. Seasons were defined as follows: Spring (February, March, and April), Summer (May, June, and July), Fall (August, September, and October), and Winter (November, December, and January). The annual period was defined as the time from one litter application to the next (i.e., May through the following April).

Since exact previous soil amendments and application amounts were unknown before initiation of this experiment, an analysis of variance (ANOVA) based on a randomized complete block design was conducted to assess uniformity of initial soil properties (i.e., soil pH, EC, OM, total As, Cd, Cr, Ni, Cu, Zn, Mn, and Fe) at 10-cm intervals to a depth of 90 cm, flow-weighted mean leachate concentrations and leaching losses of DOC and metals (i.e., As, Cd, Cr, Ni, Cu, Zn, Fe, and Mn), and dry matter production using pre-assigned treatments (SAS Version 9.1, SAS Institute, Inc., Cary, NC). After establishing uniformity among treatments before initial poultry litter application, a similar ANOVA was conducted to determine the effect of poultry litter application rate [i.e., control (0 Mg ha^–1), low (5.6 Mg ha^–1), and high (11.2 Mg ha^–1)] on seasonal and annual drainage, leachate pH, EC, redox potential, flow-weighted mean leachate concentrations and cumulative DOC and metal leaching losses, and cumulative dry matter production. In addition, an ANOVA was conducted to determine the effect of poultry litter application rate on the 2-yr cumulative leaching losses of DOC and metals. Due to variability in precipitation amounts in similar seasons across years, seasons and annual periods were not formally compared. Treatment means were separated by least significant difference (LSD) at the 0.05 level.

	RESULTS AND DISCUSSION

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

 
Pre-Litter Characteristics
In the 3 mo before initial litter application, 179 mm of precipitation fell on the study site, which was 98 mm below normal for this time period (Table 1). Cumulative drainage averaged 46 (SE = 45), 35 (SE = 34), and 187 (SE = 140) mm in the pre-assigned control, low-, and high-litter treatments, respectively (Fig. 2 ). Due to a large amount of plot-to-plot variability, drainage did not differ among pre-assigned litter treatments (P > 0.05).

View larger version (37K): [in this window] [in a new window]   Fig. 2. Measured and cumulative drainage since previous sampling and precipitation for 3 mo before initial litter application and for two annual periods (Year 1: May 2003 through April 2004 and Year 2: May 2004 through April 2005) following each litter application. Litter was applied on day of year 120 and 127 in 2003 and 2004, respectively. Thirty-year normal precipitation data was obtained from NOAA (2002). Vertical bars represent standard error from two replications per treatment mean.

Few soil metal concentration differences existed among plots before initial litter application (Table 3). Iron differed in the 20- to 30-cm depth, Ni in the 30- to 40-cm depth, As in the 50- to 60-cm depth, Mn in the 60- to 70-cm depth, and Cr in the 80- to 90-cm depth. Soil pH, EC, and OM were similar and showed little variability among plots (Table 3). Iron was the predominant metal throughout the profile (14.8–26.3 g kg^–1) and Cd was the least concentrated (0.33–1.02 mg kg^–1; Table 3). The soil was acidic with pH values ranging from 6.0 to 6.4 (Table 3).

View larger version (46K): [in this window] [in a new window]   Fig. 3. Measured leachate pH, electrical conductivity (EC), and redox potential for 3 mo before initial litter application and for two annual periods (Year 1: May 2003 through April 2004 and Year 2: May 2004 through April 2005) following each litter application. Litter was applied on day of year 120 and 127 of 2003 and 2004, respectively. Vertical bars represent standard error from two replications per treatment mean.

View larger version (35K): [in this window] [in a new window]   Fig. 4. Measured (i.e., raw) leachate dissolved organic carbon (DOC) concentrations and leaching losses for 3 mo before initial litter application and for two annual periods (Year 1: May 2003 through April 2004 and Year 2: May 2004 through April 2005) following each litter application. Litter was applied on day of year 120 and 127 of 2003 and 2004, respectively. Vertical bars represent standard error from two replications per treatment mean.

View larger version (34K): [in this window] [in a new window]   Fig. 5. Measured (i.e., raw) leachate As, Cd, and Se concentrations for 3 mo before initial litter application and for two annual periods (Year 1: May 2003 through April 2004 and Year 2: May 2004 through April 2005) following each litter application. Litter was applied on day of year 120 and 127 of 2003 and 2004, respectively. Vertical bars represent standard error from two replications per treatment mean.

View larger version (39K): [in this window] [in a new window]   Fig. 6. Measured (i.e., raw) leachate Cr, Mn, and Fe concentrations for 3 mo before initial litter application and for two annual periods (Year 1: May 2003 through April 2004 and Year 2: May 2004 through April 2005) following each litter application. Litter was applied on day of year 120 and 127 of 2003 and 2004, respectively. Vertical bars represent standard error from two replications per treatment mean.

View larger version (37K): [in this window] [in a new window]   Fig. 7. Measured (i.e., raw) leachate Ni, Cu, and Zn concentrations for 3 mo before initial litter application and for two annual periods (Year 1: May 2003 through April 2004 and Year 2: May 2004 through April 2005) following each litter application. Litter was applied on day of year 120 and 127 of 2003 and 2004, respectively. Vertical bars represent standard error from two replications per treatment mean.

View larger version (30K): [in this window] [in a new window]   Fig. 8. Measured As, Cd, and Se leaching losses for 3 mo before initial litter application and for two annual periods (Year 1: May 2003 through April 2004 and Year 2: May 2004 through April 2005) following each litter application. Litter was applied on day of year 120 and 127 of 2003 and 2004, respectively. Vertical bars represent standard error from two replications per treatment mean.

View larger version (36K): [in this window] [in a new window]   Fig. 9. Measured Cr, Mn, and Fe leaching losses for 3 mo before initial litter application and for two annual periods (Year 1: May 2003 through April 2004 and Year 2: May 2004 through April 2005) following each litter application. Litter was applied on day of year 120 and 127 of 2003 and 2004, respectively. Vertical bars represent standard error from two replications per treatment mean.

View larger version (37K): [in this window] [in a new window]   Fig. 10. Measured Ni, Cu, and Zn leaching losses for 3 mo before initial litter application and for two annual periods (Year 1: May 2003 through April 2004 and Year 2: May 2004 through April 2005) following each litter application. Litter was applied on day of year 120 and 127 of 2003 and 2004, respectively. Vertical bars represent standard error from two replications per treatment mean.

First-Year Response to Litter Treatment
In the first-year period (May 2003 through Apr. 2004), there was 1130 mm of precipitation at the study site, which was 40 mm less than the 30-yr normal for Fayetteville (i.e., 1170 mm; Table 1, Fig. 2). Cumulative mean annual drainage was 447, 235, and 592 mm for the control, low-, and high-litter treatments, respectively (Table 4). The majority of drainage occurred in response to increased precipitation during early Summer (May through July) and Spring (February through April; Fig. 2, Table 4). Although 242 mm of precipitation fell during Fall (August through October), no measurable drainage occurred at the 90-cm depth. Except for Winter (November 2003 through January 2004), in which the control and high-litter treatments had more than twice the drainage (P = 0.03) than the low-litter treatment, drainage did not differ among treatments during any other season or cumulatively for the year (Table 4).

Of all the metals, Zn consistently had the highest flow-weighted mean concentration throughout the year (Table 6, Fig. 7), but stayed well within the suggested range for safe drinking water (i.e., <5 mg L^–1; USEPA, 2002). Though Zn was not the most abundant metal in the soil (Table 3), nor was it very concentrated in poultry litter (Table 2), the solubility of Zn is greatly increased in acidic conditions and poorly aerated (i.e., Eh <100 mV) soils (Kabata-Pendias, 2001). Soil pH in this study ranged from 6.1 to 6.3 throughout the profile (Table 3) and although we have no direct measurement of soil redox potential, the redox potential of leachate was generally <100 mV (Fig. 3). These results are supported by the findings of Gupta and Charles (1999) who measured no accumulation of Zn in soil with a 15- to 20-yr history of poultry litter application; thus indicating some mechanism of Zn removal from the system. Han et al. (2000) also documented Zn mobility to a depth of 180 cm in soils with a 25-yr history of poultry litter treatment.

The flow-weighted mean concentrations of Cd, Se, Cr, and Cu remained relatively low throughout the year (Table 6, Fig. 5, 6, and 7). It was expected that Cd, Se, and Cr would be in low concentrations in leachate since these elements have relatively low background soil concentrations and were not present in large quantities in the poultry litter applied (Table 2). Though Cu was in relatively larger concentrations in the poultry litter than Cd, Se, and Cr (Table 2), Cu is strongly chelated by OM and has been documented in significant concentrations in surface soils with a history of litter application (Gupta and Charles, 1999; Han et al., 2000). In addition to OM, phosphates exhibit strong affinity for Cu (Kabata-Pendias, 2001). With increased levels of P at the soil surface resulting from litter application (Pirani, 2005), Cu may bind with P and remain relatively immobile at the surface.

The flow-weighted mean concentrations of As, Cd, Se, Cr, Fe, and Zn were similar across treatments throughout Year 1 following initial litter application, while Mn, Ni, and Cu differed among litter treatments during one season or annually (Table 6). Flow-weighted mean concentrations of Mn ranged from 0.02 to 0.04 mg L^–1 and differed significantly (P = 0.04) between the high- and low-litter treatments, while each were similar to the control in Spring Year 1. The cause for the Mn difference is not clear, however the solubility of Mn is sensitive to changes in pH and redox potential (Kabata-Pendias, 2001). While Spring Year 1 (February through April) leachate pH and redox potential were similar among litter treatments (Table 5), small fluctuations during these 3 mo could have temporarily promoted the solubility of Mn and increased its concentration in leachate. Though flow-weighted mean concentrations of Ni were similar among litter treatments throughout the seasons, they were significantly greater (P = 0.05) in the control than in the high-litter treatment for the cumulative 1-yr period. This observation likely resulted from consistently, though not significantly, higher Ni concentrations in the control compared to the high-litter treatment (Fig. 7) despite the high-litter treatment having nearly 150 mm more cumulative drainage for the year than the control (Table 5). Similar to Ni, Cu concentrations differed significantly among litter treatments for the annual period of Year 1, where the high-litter treatment had the highest Cu concentration compared to the control and low-litter treatments, which did not differ. These results suggest that significant leaching loss may occur from soils with a long history of litter application.

As a result of rather low leachate concentrations of Cd, Se, Cr, and Cu, leaching losses of these metals were also low throughout Year 1 (Fig. 8, 9, and 10; Table 7). As a result of having the highest leachate concentrations, Fe and Zn had the highest leaching losses of all the metals evaluated (Fig. 9 and 10; Table 7). Iron is ubiquitous in soil and was present in the Captina soil in very high concentrations relative to other metals (Table 3), therefore Fe would be expected to leach in larger quantities compared to other metals. Although Mn was more prevalent in the soil than Zn (Table 3), more Zn leached from the soil (Table 7). This indicates that although the solubility of both Mn and Zn are increased in an acid and low-oxidation environment (Kabata-Pendias, 2001), Zn was more mobile than Mn in these conditions.

In the first year following litter application, cumulative dry matter production ranged from 4.95 Mg ha^–1 in the control to 12.0 Mg ha^–1 in the high-litter treatment. However, though greater than the control, cumulative dry matter did not differ among the low- and high-litter treatments. The effectiveness of poultry litter as a fertilizer for tall fescue has been demonstrated by others (Huneycutt et al., 1988) and is supported by the results of this study.

Second-Year Response to Litter Treatment
In the second-year period (May 2004 through April 2005), there was 1202 mm of precipitation at the study site, which was 32 mm greater than the 30-yr normal for Fayetteville (Table 1, Fig. 2). Cumulative mean annual drainage was 833, 589, and 827 mm for the control, low-, and high-litter treatments, respectively (Table 4, Fig. 2). The majority of drainage occurred in Summer (May through July) and Winter (November 2004 through January 2005) in response to increased precipitation during these months.

The cumulative mean annual drainage measured in the second year of this study represents between 49 and 69% of total annual measured precipitation (Table 1), which is somewhat high for a fine-textured soil. However, the Captina silt loam has rather well-developed subsoil structure, considering subsoil clay contents, that promotes vertical water transport.

Drainage did not differ among litter treatments during any season or for the whole year. However, in contrast to Year 1, which had a large degree of drainage variability among treatments (CV = 74%, Table 4), the amount of drainage variability among treatments was much smaller (CV = 26%; Table 4) in Year 2 indicating that any residual effects of lysimeter installation on soil hydraulic properties and potential effects on drainage response had greatly diminished or had been eliminated altogether. Any decrease in annual drainage from the low- and high-litter treatments due to increased evapotranspiration losses from greater dry matter production was likely offset by increased infiltration capacity and preferential flow paths caused by channels left by decaying roots.

Averaged across all sample dates in Year 2, after two litter applications, mean leachate pH was 6.1 across all treatments (Table 5). Although no significant differences in leachate pH among treatments resulted as an effect of poultry litter treatment in this study, significant soil pH differences have been reported by others conducting long-term litter studies. Kingery et al. (1994) reported an increase of 0.5 pH units in soil to a depth of 60 cm under tall fescue pastures that had received up to 22 Mg litter ha^–1 yr^–1 for up to 28 yr. Such a slight change in soil pH for a long-term study could indicate the potential for pH in leachate to change very slowly, if at all, over the course of this study. In contrast, after a single application of sewage sludge to forest and pasture soils, McLaren et al. (2004) reported a decrease in leachate pH of 1.0 to 1.5 units at a depth of 70 cm compared to control soils after 600 mm of drainage occurred.

Annual leachate redox potentials averaged 60, 66, and 67 mV for the control, low-, and high-litter treatments during Year 2 of this study (Table 5). Leachate redox potentials reported in this study may not be truly representative of actual soil leachate solution redox potentials. Leachate redox potentials measured on leachate that had remained in lysimeter reservoirs, which were part of a closed system, for up to 14 d before collection. After collection reservoirs were emptied, leachate subsamples were placed in air-tight bottles for up to 5 h before the redox potential could be measured. However, due to limited exposure to air, reported redox potentials are likely reasonably similar to those of actual soil leachate solution.

Average annual leachate EC was 0.19, 0.19, and 0.22 dS m^–1 for the control, low-, and high-litter treatments, respectively (Table 5). These values are similar to the 0.18 dS m^–1 reported by Gupta and Charles (1999) at a depth of 60 cm under agricultural fields that had been treated with 8.97 Mg ha^–1 poultry litter in alternate years for 15 to 20 yr. Beginning in Fall Year 2, leachate EC trends among treatments appeared to change somewhat. Before litter application and throughout Year 1, leachate EC in the control treatment appeared to be greater than in low- and high-litter treatments. However, in Fall Year 2, leachate EC in the high-litter treatments appeared to begin to increase numerically over the control and low-litter treatments (Fig. 3), perhaps indicating the increased leaching of soluble salts as a result of litter treatment.

Flow-weighted mean DOC concentrations (Table 8, Fig. 4) and leaching losses (Table 9, Fig. 4) did not differ among litter treatments in Year 2 after two litter applications. Concentrations were similar to those measured in Year 1 of this study, however leaching losses were greater in Year 2 than Year 1 due to increased drainage during this time.

In Year 2, Fe surpassed Zn as having the highest annual flow-weighted mean concentration across treatments (Table 8). Even with increased values, Fe concentrations, which have a non-enforceable secondary water standard, averaged across litter treatments, still fell within the acceptable range (i.e., <0.3 mg L^–1; USEPA, 2002) for drinking water throughout the four seasons. The amount of soluble Fe in soils is extremely low when compared to total Fe (Table 3). Iron solubility is increased in acidic and intermittently saturated soils (Kabata-Pendias, 2001). These characteristics were evident not only in Year 2, but throughout the course of this 2-yr study. When comparing leachate amounts (Fig. 2) to Fe concentrations over time (Fig. 6), the correlation between drainage amount and leachate Fe concentration becomes apparent. A leachate amount of 115 mm is equivalent to a full lysimeter, meaning that very wet conditions existed in the soil at that time, therefore, mobilizing Fe and increasing Fe concentrations in leachate.

In Spring 2005, the flow-weighted mean concentration of Cr in the low-litter treatment was significantly greater than that in the control and high-litter treatment (Table 8). The reason for this is not clear, but perhaps the relatively low drainage from the low-litter treatment compared to the other treatments during this time period (Table 9) caused Cr to be more concentrated in leachate at the 90-cm depth.

In contrast to Year 1, in which no significant differences in seasonal metal leaching losses occurred, several differences were observed during Year 2. In Summer 2004, cumulative Zn leaching in the control and high-litter treatments was greater (P = 0.02) than in the low-litter treatment (Table 9). Though not significant, the amount of drainage and the flow-weighted mean concentration of Zn in the control and high-litter treatments were numerically greater than those in the low-litter treatment (Tables 8 and 9). As a result of the increased drainage, a low oxidation environment was created and the solubility of Zn was likely increased in these two treatments; thus resulting in a greater concentration and leaching loss. Iron leaching differed among litter treatments in Fall 2004, in which Fe leaching losses were greater (P = 0.04) in low-litter than the control and high-litter treatments (Table 9). Drainage in the low-litter treatment was 2- to 2.5-times greater than drainage in the control and high-litter treatments and the flow-weighted-mean concentration of Fe was also numerically greater in Fall (Tables 4 and 8), resulting in significantly greater Fe leaching loss during this time period. Arsenic leaching during Winter (November 2004 through January 2005) also differed among litter treatments (Table 9). The control had significantly (P = 0.001) greater As leaching than the low- and high-litter treatments, which also differed significantly. Since the flow-weighted-mean concentrations of As were almost identical during Winter, ranging from 0.12 to 0.13 mg L^–1 (Table 8), As leaching differences were the result of variable drainage for this season, which totaled 557, 440, and 542 mm for the control, low- and high-litter treatments, respectively (Table 4). Similar to Year 1, Cd, Se, Cr, and Cu leaching losses in Year 2 remained low likely due to low soil and litter concentrations.

Cumulative dry matter production during Year 2 averaged 5.61 Mg ha^–1 in the control, 9.18 Mg ha^–1 in the low-, and 12.2 Mg ha^–1 in the high-litter treatment and, in contrast to Year 1, differed significantly among all treatments. Preliminary data indicate that annual plant uptake and dry matter removal is not a significant mechanism of metal export from the system (Brye and Pirani, 2005).

Cumulative Two-Year Leaching Losses
After 2 yr of annual poultry litter additions, cumulative 2-yr leaching losses of As, Cd, Se, Cr, Mn, Fe, Ni, Cu, Zn, and DOC did not differ significantly among litter treatments (Table 10). Cumulative 2-yr leaching losses of Mn, Cu, and DOC represented <6% of that applied in the low- and high-litter treatments (Table 2), while cumulative 2-yr leaching losses of Cd, Se, Cr, Fe, Ni, and Zn represented between 24 and 172% of that applied in litter. Cumulative 2-yr leaching losses of As represented between 378 and 757% of that applied in litter. These data indicate that leaching losses of certain soluble metals may be significant from poultry litter-amended pasture soil that has had a history of litter applications. Since metals are added to the soil when poultry litter is added and due to the fact that adding litter also adds a large amount of organic material that when it decomposes produces low molecular weight, soluble, and mobile organic compounds that are highly efficient at complexing metals, it stands to reason that a soil that has never received metal-ladened poultry litter will likely have much less metal leaching than a soil that has a history of poultry litter additions.

With similar leachate pH, EC, and redox potential (Table 5) among litter treatments, it is likely that soil biogeochemical properties were not altered enough by either litter treatment to significantly affect metal adsorption and ion exchange processes to result in elevated leaching of soluble metals. In addition, similar DOC leaching (Tables 7, 8, and 10) each year suggests that metal chelation with additional organic matter has also not occurred to a significant extent to enhance metal leaching in the littered treatments. Therefore, litter treatment will likely not result in elevated metal leaching until soil biogeochemical properties, namely soil and/or leachate pH, EC, and redox potential or soil metal concentrations, have been sufficiently altered.

	CONCLUSIONS

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

 
Based on 2 yr of monitoring metal leaching losses from poultry litter-amended pasture soil with a history of litter applications using state-of-the-art lysimetry techniques, and despite a few significant, though inconsistent, differences in flow-weighted mean concentrations and leaching losses among seasons and annual periods, poultry litter application rate had no effect on the 2-yr cumulative leaching losses of soluble metals in the Captina silt-loam soil.

It is reasonable to conclude that few treatment effects occurred in this study for several reasons: (i) this study only represented the initial 2 yr and it may take continued repeated annual applications of poultry litter for soil concentrations of metals to increase such that flow-weighted mean concentrations and leaching losses will begin to differ among litter rates; (ii) litter was applied at practical rates, therefore the adsorption capacity of the soil would likely not have been overloaded to force leaching; (iii) leachate was collected in response to natural precipitation rather than in response to rainfall simulations; and (iv) limited replication and large variability in drainage amounts between replications, especially in Year 1. As a result, metals may have diffused slowly enough through the soil that they adsorbed to soil particles before leaching to and beyond the 90-cm depth. Observations from this study are similar to others in that soil variability tends to mask treatment differences, but that does not necessarily mean that imposed treatments, particularly organic soil amendments, do not promote important physical and/or chemical changes in the soil matrix.

Results indicate that poultry litter application rate did not affect leaching losses of soluble metals within the first 2 yr after litter application to a pasture soil that has had a prior history of litter applications. Although few treatment effects were observed, apparent trends observed during the latter stages of the study period suggest that the leaching of metals and other soluble litter-contained constituents may be a significant mechanism of export from the system after several years of repeated litter application in areas with a history of litter applications. Preliminary data also indicate that metal export through plant uptake and removal is likely not a significant loss mechanism from the system. Further in situ monitoring is needed to better ascertain the effect of poultry litter application rates on the biogeochemical cycling of metals contained in poultry litter once land-applied.

	REFERENCES

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

 

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