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Published in J. Environ. Qual. 34:370-381 (2005).
© ASA, CSSA, SSSA
677 S. Segoe Rd., Madison, WI 53711 USA

TECHNICAL REPORTS

Waste Management

Phosphorus Leaching in Manure-Amended Atlantic Coastal Plain Soils

Jennifer S. Butler and Frank J. Coale*

Department of Natural Resource Sciences and Landscape Architecture, 0214 H.J. Patterson Hall, University of Maryland, College Park, MD 20742

* Corresponding author (fjcoale{at}umd.edu)

Received for publication August 29, 2003.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Targeting the sources of phosphorus (P) and transport pathways of drainage from agricultural land will assist in the reduction of P loading to surface waters. Our research investigated the vertical movement of P from dairy manure and broiler litter through four Atlantic Coastal Plain soils. A randomized split-plot design with two main-plot tillage treatments (no tillage [NT] and chisel tillage [CH]) and five manure P rate split-plot treatments was used at each location. The split-plot P rates were 0, 100, 200, 300, and 400 kg P ha–1 yr–1. Four consecutive years of manure application began at all sites 5 yr before sampling. Soils were sampled to a depth of 150 cm from each split plot in seven depth increments and analyzed for soil test phosphorus (STP), water-extractable soil phosphorus (WSP), and degree of phosphorus saturation (DPS). The DPS of the 0- to 15-cm depths confirmed that at the 100 kg P ha–1 yr–1 application rate, all sites exceeded the threshold for P saturation (30%). At depths greater than 30 cm, DPS was typically below the 30% saturation threshold. The DPS change points ranged from 25 to 34% for the 0- to 90-cm depths. Our research concluded that the risk of P leaching through the matrix of the Atlantic Coastal Plain soils studied was not high; however, P leaching via macropore bypass may contribute to P loss from these soils.

Abbreviations: Alox, oxalate-extractable aluminum • BL, broiler litter • CH, chisel tillage • DM, dairy manure • DPS, degree of phosphorus saturation • Feox, oxalate-extractable iron • NT, no tillage • OM, organic matter • Pox, oxalate-extractable phosphorus • PSC, phosphorus sorption capacity • STP, soil test phosphorus • WSP, water-extractable soil phosphorus


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
EUTROPHICATION of the Chesapeake Bay, its tributaries, and other surface water systems in the Atlantic Coastal Plain continues to be an environmental problem. Agriculture contributes approximately 49% of the total phosphorus (P) load that reaches the Chesapeake Bay, making it the largest source (point and nonpoint) of P (Boesch et al., 2001). Identifying the pathways for P loss from agricultural fields would permit adoption of management techniques to help minimize P losses.

Crops remove a finite amount of P, and any excess applied P accumulates in the soil (Boesch et al., 2001). Researchers have reported soils that were oversaturated with P (Breeuwsma et al., 1995; Kleinman et al., 1999), and P leaching in areas of high animal manure loading (Chalmers and Withers, 1998; Eghball et al., 1996). Livestock and poultry industries generate copious amounts of manure, most of which is applied as fertilizer to cropland. An additional factor contributing to the potential for P loss is nitrogen (N)-based nutrient management systems. Currently recommended rates for manure applications to soil are typically based on the N requirement of the crops to be grown and the plant-available N content of the manure, while the amount of P applied with the manure has not usually been considered when determining recommended application rate (Reddy et al., 1980; Sharpley et al., 1993; Simard et al., 1995).

Ground water flow, a potential pathway for P movement, has been identified as a primary avenue for water movement from fields of the Atlantic Coastal Plain to surface waters (Boesch et al., 2001). Additionally, P leaching has been shown to occur in soils of the Delmarva Peninsula (Mozaffari and Sims, 1994). Calculating the degree of phosphorus saturation (DPS) of a soil provides a measure to determine whether a soil might leach P. The DPS is calculated as: (Pox/PSC) x 100, where phosphorus sorption capacity (PSC) = 0.5(Alox + Feox) and the subscript "ox" indicates oxalate-extractable phosphorus, aluminum, or iron (Van der Zee and van Riemsdijk, 1988; Behrendt and Boekhold, 1993). The PSC measurement quantifies the reactive, amorphous, and microcrystalline Al and Fe in noncalcareous, light-textured soils, common to the Atlantic Coastal Plain.

Lookman et al. (1996) and Sharpley et al. (1994) documented positive linear relationships between measured soluble P and the DPS of a soil. The split-line model is a nonlinear regression tool used to describe piecewise polynomials whose function values and first derivatives agree at points where they join; the joining points of the two lines are called knots (Freund and Littell, 1991) or change points (McDowell and Sharpley, 2001; McDowell et al., 2001). Supporting work by Heckrath et al. (1995) conveyed findings of recurring change points in the relationship between adsorbed P and WSP using a split-line model. The WSP points clustered around the lower-slope portion of the linear pairs, which represented P sorbed on clay or oxide sites in the plow layer (0–30 cm) (Heckrath et al., 1995). The second, steeper-sloped portion of the line pair expressed the action of WSP moving through the soil by preferential flow or rapid P transport (Heckrath et al., 1995).

To characterize the potential for P loss from a soil, regression analyses may be used to investigate quantity versus intensity relationships between two measures of soil P. McDowell and Sharpley (2001) documented an informative relationship between WSP in both lysimeter drainage and runoff (intensity) and STP (quantity).

Our research was undertaken to investigate potential P loss through leaching into ground water and subsurface drainage pathways from four Atlantic Coastal Plain soils. The objectives of our research were to (i) determine whether P leaching may significantly contribute to loss of P from Atlantic Coastal Plain soils, (ii) evaluate the effect of tillage (no tillage and chisel tillage) on P loss, and (iii) evaluate the effect of P source and loading rate on DPS and the potential for P leaching. Additionally, the predictive relationship between WSP and STP and between WSP and DPS were investigated.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Location and Experiment Design
Field research plots were established at four locations on the Mid-Atlantic Coastal Plain in Maryland (USA): Beltsville, Upper Marlboro, Queenstown, and Poplar Hill. The sites were selected due to the respective soil types, which represent predominant agricultural soils of Maryland's portion of the Atlantic Coastal Plain (Table 1).


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  		Table 1. Descriptions of the soils and manure P source for the four research locations on Maryland's Atlantic Coastal Plain.

 
A randomized complete block, split-plot experiment design with four replications was used at all four locations. The main-plot treatment was tillage: (i) no tillage (NT) and (ii) chisel tillage and disking (CH) immediately following manure application. Main plots were 4.5-m wide and 61.0-m long. The split-plot treatment was manure P application rate. Split plots were 4.5-m wide and 12.2-m long. Split plots were amended with manure to supply P rates equal 0, 100, 200, 300, and 400 kg P ha–1 yr–1. The Beltsville and Upper Marlboro sites were amended with solid dairy manure (DM) while the Queenstown and Poplar Hill sites received chicken (broiler) litter (BL) (Table 1). The overall experiment was designed to evaluate the potential for P leaching in Atlantic Coastal Plain soils amended with BL within the region of concentrated poultry production (Queenstown and Poplar Hill), while the potential for P leaching in soils amended with DM was evaluated at locations that better represented dairy producing regions. Four annual treatment applications in the spring (April or May) of 1994, 1995, 1996, and 1997 resulted in total plot loads of 0, 400, 800, 1200, and 1600 kg P ha–1 4 yr–1. Nutrient analysis and physical characteristics of the DM and BL sources are presented in Table 2. Moisture content was determined by oven-drying fresh manure samples at 80°C. Organic N and C were determined on dried, ground (<2 mm) samples using a LECO (St. Joseph, MI) CHN 2000 analyzer. Ammonium N was determined using fresh manure samples by distillation with MgO into boric acid and titrated with sulfuric acid. Total N was calculated as the sum of organic N and NH4–N. Total P was determined by digestion of dried, ground samples with nitric and perchloric acids and analyzed on a Spectro (Kleve, Germany) Modula-E inductively coupled plasma analyzer.


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  		Table 2. Nutrient analysis and physical characteristics of dairy manure (DM) and broiler litter (BL) sources applied annually, 1994 through 1997.

 
All four locations had been managed for continuous no-till grain corn (Zea mays L.) production for at least three years before experiment initiation. For our experiments, grain corn was planted to all plots each spring following application of manure and tillage treatments. After planting, surface plant residue was estimated to be 80 to 90% surface cover for the NT treatments and 20 to 30% surface cover for the CH treatment. Following corn grain harvest, all plots remained in an undisturbed fallow condition until manure and tillage treatments were again applied the following spring.

Soil Sampling and Analysis
Twelve to eighteen months after the final manure application, soil samples were collected from seven depth increments from each split plot: 0 to 2, 0 to 15, 15 to 30, 30 to 60, 60 to 90, 90 to 120, and 120 to 150 cm (Table 1). The 0- to 2-, 0- to 15-, and 15- to 30-cm depths were collected by hand with a standard tube probe (2-cm diameter). The 0- to 2- and 0-15 cm depths were collected independently within the plot. Ten cores per split-plot comprised the 0- to 2-cm composite sample while five cores per split-plot comprised the 0- to 15- and 15- to 30-cm composite sample. The 30- to 60-, 60- to 90-, 90- to 120-, and 120- to 150-cm samples were collected with a hydraulic auger (5-cm diameter), and five cores per split-plot comprised the composite sample. After sampling, soils were air-dried, crushed, and screened to pass through a 2-mm sieve.

All soils were analyzed for oxalate-extractable amorphous iron (Feox) and aluminum (Alox) oxides and oxalate-extractable phosphorus (Pox) by shaking (2 h) 1.0 g of soil with 40 mL of extracting solution (0.2 M ammonium oxalate, 0.2 M oxalic acid, pH = 3.0), centrifuging (2000 rpm, 13 min), and filtering (Whatman [Maidstone, UK] #42 filter paper) (McKeague and Day, 1966). Analyses of Feox, Alox, and Pox were conducted by inductively coupled plasma atomic emission spectroscopy (ICP–AES) (McKeague and Day, 1966). The results of calculations performed on Feox, Alox, and Pox are presented on a mol kg–1 soil basis.

Water-extractable soil P was determined by shaking (1 h) 10 g of dried soil with 100 mL of distilled water (1:10 ratio) and vacuum-filtering the solution through a 0.45-µm polyethersulfone membrane (Sissingh, 1971; Greenburg et al., 1992; Miller et al., 1993). Colorimetric analysis was conducted on the supernatant with a spectrophotometer (882-nm slit width, 5-cm path length). The detection level for WSP was 0.0104 mg P kg–1 soil.

Particle size analysis was used to determine soil textures for the four sites. A random composite of 20 subsamples (1 g each) was created to represent an average over tillage and P rate treatments, resulting in one particle size analysis value per depth, per site for a total of 28 measures. The pipette method was used for determining the clay fraction in soils while the sands were dried and weighed (Day, 1986).

Soil pH was measured in a 1:1 water to soil ratio with a glass electrode, soil organic matter (OM) was determined by loss-on-ignition, and soil-test nutrients (P, K, Ca, Mg) were determined by Mehlich-1 extraction (0.025 M H2SO4 + 0.05 M HCl) (Northeast Coordinating Committee on Soil Testing, 1995) (Table 3).


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  		Table 3. Pretreatment soil-test nutrients, pH, and organic matter (OM) for the 0- to 15-cm depth at the four research locations before initiation of manure or tillage treatments in 1994.

 
Statistical Analysis
Analysis of variance was used to evaluate tillage and manure rate effects using an F statistic with a probability of p < 0.05 to determine significant differences among the treatments (SAS Institute, 1999). Orthogonal contrasts were used to test the differences between dairy manure and broiler litter P sources (SAS Institute, 1999). Before executing the contrasts, values were normalized by subtracting the control plot value from each value in the block.

The WSP and DPS relationship was described using a split-line model. The regression line below the knot (Freund and Littell, 1991) was:

[1]
and above the knot:

[2]
where m1 is the slope of the relationship between WSP and DPS below the knot, m2 is the difference in slopes between the two lines above the knot, b is the y intercept, and knot is the change point of WSP. The four parameters of the models (m1, m2, b, knot) were generated by plotting a split-line regression using SAS Version 8.0 (SAS Institute, 1999). The single r2 value describes how closely the split-line regression model fits the entire data set. The data from all four sites were also fit to single linear equations and quadratic equations, but neither equation captured more variance in the data than the split-line model.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
General Soil Properties
The particle size analysis (PSA) detailed soil texture changes with depth for all four sites (Table 4). The soils at the Beltsville location (Keyport fine sandy loam: fine, mixed, semiactive, mesic Aquic Hapludults) exhibited the most variation in texture with depth (Table 4). The texture profile revealed a sandy loam at the surface (0–15 cm) changing to a loam and then a sandy clay loam at 120 cm. The soils at Upper Marlboro (Donlonton fine sandy loam: fine-loamy, glauconitic, mesic Aquic Hapludults) were characterized by a sandy loam surface over a sandy clay loam at 30 cm (Table 4). The soils at the Queenstown location (Matapeake silt loam: fine-silty, mixed, semiactive, mesic Typic Hapludults) also showed variation of soil texture with depth, changing from a silt loam at 60 cm, to a sandy clay loam, and then became a sandy loam at 120 cm (Table 4). The PSA for the soils at Poplar Hill (Mattapex silt loam: fine silty, mixed, active, mesic Aquic Hapludults) described a silt loam from 0 to 90 cm and a sandy loam from 90 to 150 cm (Table 4).


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  		Table 4. Particle size analysis (PSA) and the corresponding soil texture with depth (0–150 cm) of the soils at the four research locations.

 
Soil OM averaged 59, 40, 25, and 29 g kg–1 for the 0- to 15-cm depths; 18, 16, 16, and 19 g kg–1 for the 15- to 30-cm depths; and 12, 16, 12, and 9 g kg–1 for the 30- to 150-cm depths at Beltsville, Upper Marlboro, Queenstown, and Poplar Hill locations, respectively. Averaged across manure application rates, NT management resulted in higher OM contents for the 0- to 2-cm layer than did chisel tillage (CH) for all sites except Poplar Hill. At deeper soil depths, there were no differences in OM between the NT and CH management. Surface soil pH values demonstrated little variability, even across the sites, averaging 6.5, 6.4, 6.7, and 6.4 for 0 to 15 cm at Beltsville, Upper Marlboro, Queenstown, and Poplar Hill locations, respectively.

Water-Extractable Soil Phosphorus
Despite the lack of significant tillage main effects, the five P application rates resulted in significant tillage x P application rate interactions for WSP concentrations among the soil layers. Significant differences in WSP were observed between P application rates in the top four sampled depths at the Beltsville location (Fig. 1) . The WSP values generated by the two highest DM application rates (1200 and 1600 kg P ha–1) were significantly greater than WSP for the two lowest rates (0 and 400 kg P ha–1) in the surface depths. Statistical nonsignificance between the 0 and 400 kg P ha–1 DM rate was observed in the STP results, as well. Under NT conditions, the 0- to 2-cm depth at the Beltsville site had twice the WSP concentration than the 0- to 15-cm agronomic sample depth (Fig. 1). High WSP concentrations at the surface (0–2 cm) suggest that NT management would increase the vulnerability for P loss through surface water runoff. Elevated surface soil (0–2 cm) WSP concentrations were much less prominent under the CH tillage system at Beltsville.



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  		Fig. 1. Water-extractable soil phosphorus (WSP) by depth and tillage method (CH, chisel tillage; NT, no tillage) at Beltsville and Upper Marlboro after four years of dairy manure amendments (kg P ha–1 4 yr–1 load).

 
Five of seven sampled depths at the Upper Marlboro site displayed WSP differences resulting from DM P application rates (Fig. 1). At the 0- to 15-, 15- to 30-, and 30- to 60-cm depths, a tillage x P application rate interaction resulted in higher WSP concentrations with increasing P application rate for the NT tillage treatment. If the soils' P sorption capacity was exceeded, WSP could be lost through leaching or subsurface drainage.

The Queenstown soil exhibited similar results as were observed for the soils at Upper Marlboro. The WSP concentration of the 0- to 2- and 0- to 15-cm depths were dependent on P loading rate (Fig. 2) . Below the 15-cm depth, differences in WSP concentration resulting from P loading rates were very small, particularly under CH tillage management.



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  		Fig. 2. Water-extractable soil phosphorus (WSP) by depth and tillage method (CH, chisel tillage; NT, no tillage) at Queenstown after four years of broiler litter amendments (kg P ha–1 4 yr–1 load).

 
At Poplar Hill, there was no tillage x P rate interaction, but the level of P applied significantly affected WSP measured at five sampling depths from 0–2 to 60–90 cm (Fig. 3) . At the surface depths, 0 to 2 and 0 to 15 cm, WSP concentration paralleled the increasing levels of applied P. Below 15 cm, differences in WSP concentrations were very small and only significantly different between the 1600 kg P ha–1 treatment and all lower treatment rates.



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  		Fig. 3. Water-extractable soil phosphorus (WSP) by depth at Poplar Hill after four years of broiler litter amendments (kg P ha–1 4 yr–1 load).

 
The contrasts comparing the dairy manure and broiler litter P source effects were significant for the 0- to 2-, 0- to 15-, 15- to 30-, and 30- to 60-cm depths. Overall, at equivalent rates of applied P, WSP means for DM-treated soils were greater than the WSP of soils treated with BL. Comparable results were found in water-extractable P from manure–soil incubations with DM- and poultry litter–amended surface soils (Cooperband and Good, 2003). The results of the contrast imply that DM application promotes relatively more P leaching, whereas P originating from BL tends to remain at the surface of the soil. Due to the composition of dairy cows' feed, DM contains more organic matter than BL. The excess OM in the soil can block P sorption sites, thereby encouraging P leaching (Anderson and Wu, 2001).

Relationship between Water-Extractable Soil Phosphorus and Soil Test Phosphorus
Both WSP and STP measure available P in the soil; however, WSP analyses quantify the soluble P immediately available to plants and most susceptible to loss in runoff, while STP quantifies plant-available P over the growing season. Predicting WSP values of a soil using standard STP data could help identify soils vulnerable to WSP loss. Recent research on Maryland soils found a positive and predictive relationship between WSP and STP at the 0- to 2-cm depth when BL, DM, and granular fertilizer were examined as P sources (Olear, 1996). In a supporting study involving poultry litter, Sharpley (1995) examined the relationship between dissolved P from simulated runoff and STP for 10 surface soils that were mixed with poultry litter to supply six levels of P from 0 to 500 mg P kg–1 soil. Positive significant linear relationships were described between the dissolved P and STP samples for all 10 soils (Sharpley, 1995).

In our studies, significant (p < 0.05) relationships between WSP and STP were observed at the 0- to 2-cm depth (Fig. 4) for all four locations. Data from Beltsville, Upper Marlboro, Queenstown, and Poplar Hill sites exhibited close relationships between WSP and STP, r2 = 0.80, 0.74, 0.81, and 0.85, respectively. The regression analyses indicated similar slopes, 0.14, for the two soils that received DM as a P source, Beltsville and Upper Marlboro, and different but similar slopes for the relationship on the soils from the two sites that received BL as a P source, Queenstown and Poplar Hill. The Beltsville and Upper Marlboro regressions described a relationship of greater intensity between STP and WSP than was observed between the Queenstown and Poplar Hill regressions. The regressions imply that soils receiving DM could have a greater chance of losing P as WSP as compared with soils amended with BL. The increased loss of P following DM amendments probably results from fewer absorption sites available for P on the clay surfaces. The increase in organic matter from the DM increases organic acids in the soil, which compete with P for binding sites in the soil (Evans, 1992). Alternatively, the soils that received DM treatments in this study generally contained more sand and less clay than the two soils that received BL amendments (Table 2). The difference in soil texture also could support greater WSP concentration at a given STP from the Beltsville and Upper Marlboro soils.



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  		Fig. 4. The relationship between water-extractable soil phosphorus (WSP) and Mehlich-1 soil test phosphorus (STP) for 0 to 2 cm at Beltsville and Upper Marlboro after four years of dairy manure amendments, and at Queenstown and Poplar Hill after four years of broiler litter amendments.

 
Degree of Phosphorus Saturation
Phosphorus may be sorbed onto a multitude of soil constituents including soil OM, amorphous Al and Fe, Ca, and clay particles. The DPS describes the degree to which the potential P sorption sites are saturated with P. Studies in Germany and Belgium have identified soils with DPS greater than 100% (Lookman et al., 1995; De Smet et al., 1996; Leinweber et al., 1997). In fact, DPS only accounts for the contribution of amorphous Fe and Al to the total PSC of the soil, leaving the sorption capacity associated with clay, Ca, and soil OM unaccounted for. In our work, we expected that high levels of surface soil organic P would result in DPS values close to and greater than 100%. Additionally, DPS describes P desorption as well as P sorption (Hooda et al., 2000), thus allowing researchers to use DPS when predicting possible P loss from soils due to desorption over time.

Tillage Effects
The DPS values of soils from Beltsville were unaffected by tillage method at all depths (Fig. 5) . Additionally, there were no tillage x P application rate interactions for DPS for the soils from Beltsville. Phosphorus application rate only affected the DPS for the 0- to 2-cm soil depth (Fig. 5). The DPS for the 0- to 2-cm depth soils exceeded the 30% saturation threshold (Breeuwsma et al., 1995; De Smet et al., 1996; Pautler and Sims, 2000) where DPS ranged from 76% (0 kg P ha–1) to 124% (1600 kg P ha–1).



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  		Fig. 5. Degree of phosphorus saturation (DPS) by depth at Beltsville after four years of dairy manure amendments (kg P ha–1 4 yr–1 load).

 
At the Upper Marlboro site, tillage main plot treatment effects were observed at the 15- to 30- and 30- to 60-cm depths, where the DPS for NT management exceeded the DPS for CH plots (Fig. 6) .



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  		Fig. 6. Degree of phosphorus saturation (DPS) by depth and tillage method (CH, chisel tillage; NT, no tillage) at Upper Marlboro after four years of dairy manure amendments (kg P ha–1 4 yr–1 load).

 
At both Queenstown and Poplar Hill (Fig. 7) , DPS was affected by tillage only at the 0- to 2-cm depth, where NT treatments resulted in greater P saturation than did the CH treatments. In a study on a sandy loam soil in Michigan, significantly greater Fe-P and Al-P concentrations were observed in surface soil (0–2 cm) of NT plots than moldboard-plowed plots (Daroub et al., 2000). Even though there may be somewhat greater PSC at the surface under NT conditions, greater quantities of OM left on the surface of the NT soil would result in P mineralization and a net increase in DPS than would be observed in the CH treatments.



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  		Fig. 7. Degree of phosphorus saturation (DPS) by depth and tillage method (CH, chisel tillage; NT, no tillage) at Queenstown and Poplar Hill after four years of broiler litter amendments (kg P ha–1 4 yr–1 load).

 
Manure Rate Effects
In addition to no response to tillage, DPS of the Beltsville soils was relatively unresponsive to the different rates of P applied (Fig. 5). The one depth, 0 to 2 cm, at which DPS responded to the P application rate revealed the 1200 and 1600 kg P ha–1 rates were significantly larger than the 0 and 400 kg P ha–1 rates. The two lower rates were slightly below 100% DPS (75 and 90%), while the three highest rates exceeded 100% DPS. The large amount of OM in the 0- to 2-cm depth contributed to the excessive degree of the P saturation of the soil because PSC was determined by only the Alox and Feox concentrations.

At Upper Marlboro, P application rate affected DPS from 0 to 90 cm (Fig. 6). The trend of DPS analyses showed significantly different DPS when comparing the highest and lowest P application rates. The soil texture shift from a sandy loam to a sandy clay loam occurred at the 30- to 60-cm depth (Table 2). The increase in soil clay content appeared to have resulted in lower DPS in the subsoil.

The five BL P application rates applied at the Queenstown site affected DPS at three depths (0–2, 0–15, and 30–60 cm) (Fig. 7). The DPS values in these surface soils for the lower two application rates (0 and 400 kg P ha–1) were significantly less than for the three higher rates (800, 1200, and 1600 kg P ha–1). In the soils from Poplar Hill, DPS was significantly affected at four depths by the rate of surface BL P applied to the plots (Fig. 7). Both the 0- to 2- and 0- to 15-cm depths showed DPS was relative to the rate of P applied, with significant differences among all levels of applied P.

The DPS data from all four locations revealed that for all P application rates greater than zero, the 0- to 15-cm DPS was greater than 25% (Fig. 5, 6, and 7). As reported in other research, soils with DPS > 25% (Sims et al., 1998; Breeuwsma and Silva, 1992) and 30% (De Smet et al., 1996; Leinweber et al., 1997) have shown a marked increase in P desorption. Additionally, the DPS for Beltsville soils was >30% at the 15- to 30-cm depth for the higher P application rate treatments (Fig. 5). Soils saturated (DPS > 30%) with P will tend to lose desorbed P through leaching or runoff. In our study, the high DPS levels in the surface soils could pose an enhanced risk of P loss through either leaching or runoff.

Contrasts examining DPS differences between DM and BL amendments across all four locations did not disclose a significant effect of P source; however, DPS resulting from DM amendments tended to be higher than DPS from BL.

Relationship between Water-Extractable Soil Phosphorus and Degree of Phosphorus Saturation
In the split-line relationships developed from this project, the relationship of WSP and DPS above the change point showed the loss of WSP increased at a greater rate per unit increase in DPS concentration than below the change point (Freund and Littell, 1991).

The split-line model was used to describe the WSP and DPS data for all four locations to a depth of 90 cm (Fig. 8) . The 0- to 2-cm depth was omitted so as not to be partially redundant with the 0- to 15-cm depth data. At Beltsville, the presence of outlying points with low DPS values and corresponding high WSP values resulted in a relatively high change point with 36% (±6.6) DPS (r2 = 0.69). The outliers were examined to determine the origin of the differences, but no unifying reasons were discovered except that all were from the 0- to 15-cm depth. Removing the four apparent outliers caused the change point to drop to 34% (±3.8) DPS (r2 = 0.87) (Fig. 8). The data pattern followed a logical sequence with the most shallow depth (0–15 cm) showing the highest levels of WSP and DPS followed by the other three depths, in succession. The regressions used to model the Upper Marlboro soils captured most of the variance in the data set (r2 = 0.87) (Fig. 8). The Upper Marlboro split-line model disclosed a change point at 28% (±2.6) DPS. Again, the data maintains a logical succession with 0 to 15 cm exhibiting the highest levels of WSP and DPS, followed by 15 to 30, 30 to 60, and 60 to 90 cm. The Queenstown DPS and WSP data were almost perfectly described by the split-line model (r2 = 0.96), with a change point of 30% (±1.0) DPS (Fig. 8).



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  		Fig. 8. The relationship between water-extractable soil phosphorus (WSP) and degree of phosphorus saturation (DPS) for 0 to 90 cm at Beltsville and Upper Marlboro after four years of dairy manure amendments and Queenstown and Poplar Hill after four years of broiler litter amendments. The arrow indicates the change point.

 
Initially, the Poplar Hill model did not fit the data well and had a change point of 14% (±4.6) DPS (r2 = 0.47). After removing five apparent outlying points (noted on Fig. 8 by a circle) from the Poplar Hill data, the change point was 25% (±1.0) DPS (r2 = 0.71) (Fig. 8).

The WSP vs. DPS data from the four locations corroborate the work of Lookman (1995)(via McDowell et al., 2001), who found 30% DPS to be the change point of soil P saturation above which WSP rapidly increased for a varied set of soils from Belgium. In similar work on Delaware soils, Sims et al. (1998) reported the trend of rapidly increasing WSP when DPS exceeded the 25% saturation.

Lastly, we evaluated the relationship between WSP and DPS for the 0- to 2-cm depth. This depth typifies the soil that most strongly influences P in surface runoff water from fields (Sharpley et al., 1979). The 0- to 2-cm depth data from Beltsville exhibited a significant relationship (r2 = 0.68) between WSP and DPS (Fig. 9) . Almost all the samples had DPS ratios greater than the 30% DPS saturation point. Tillage method did not affect the relationship between WSP and DPS. The Upper Marlboro WSP and DPS regression with the 0- to 2-cm depth data was not significant and is presented for informational purposes only (Fig. 9). Soils from Queenstown expressed at good fit (r2 = 0.73) for the 0- to 2-cm WSP and DPS relationship (Fig. 9). All the data points were above the critical 30% DPS, and, again, tillage practice did not affect the WSP vs. DPS relationship. The WSP vs. DPS relationship for the 0- to 2-cm depth at Poplar Hill exhibited a strong linear relationship (r2 = 0.94) (Fig. 9). Except for four points, all the data points were above the critical 30% DPS point, which denotes P saturation of the soil, and, again, there were no differences between tillage practices.



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  		Fig. 9. The relationship between water-extractable soil phosphorus (WSP) and degree of phosphorus saturation (DPS) for 0 to 2 cm at Beltsville and Upper Marlboro after four years of dairy manure amendments and Queenstown and Poplar Hill after four years of broiler litter amendments.

 

   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
The strong regression relationships between WSP and STP indicated that conventional STP of the 0- to 2-cm depth may be useful for predicting WSP concentrations at the surface of Atlantic Coastal Plain soils. Additionally, differences among the slopes of the WSP vs. STP relationships suggest that as STP concentrations increased, WSP increased more rapidly in soils receiving DM amendments than in soils receiving BL.

Generally, the DPS for the BL-amended plots increased with increasing P application rate only at the 0- to 2-cm depth while the DPS for the DM-amended plots tended to be elevated at deeper depths. We suggest that the increase in DPS for the DM-amended plots could be due to the increased organic acids present from the applied DM. The organic acids would compete with P for bonding sites causing potential P loss through leaching.

The WSP versus DPS regressions indicated that repeated applications of P could overwhelm the PSCs of soils, leading to WSP desorption and P loss with percolating water. The descriptive WSP versus DPS relationship could be used when calculating potential for P loss from soils with site assessment tools such as the P site index (Coale et al., 2002).

The WSP data reflected soil changes in response to P application rates, but we lacked further descriptors that could have been used to explain the movement of P in the soils. The DPS ratios encompassed numerous characteristics of the soils, which allowed us to describe and speculate about the status of leaching P. Our DPS change point results (between 25 and 34% DPS) followed trends described by other researchers and indicated that the soils are susceptible to P saturation (DPS > 30%) and potential P loss through surface runoff or leaching (Kleinman et al., 1999; Leinweber et al., 1997; De Smet et al., 1996). Furthermore, the WSP versus DPS relationships support the idea that P loss through surface runoff would be a greater concern in manured soils because most surface soil (0–2 cm) exceeded the 30% DPS saturation threshold.

The two tillage treatments failed to affect the measures of P loss in the study significantly. The contrasts between soils treated with DM and those treated with BL seemed to indicate that DM P may be more downwardly mobile in the soil than BL P. Additionally, we found that the split-line regression proved to be the most powerful tool to describe the predictive relationship between WSP and DPS.

Overall, the results intimate that the Atlantic Coastal Plain soils studied were not prone to leaching P after four years of manure application. However, DPS results comparing BL and DM amendments imply that preferential flow through macropore bypass could contribute to P loss to shallow ground water as suggested by Brye et al. (2002) and Kleinman et al. (2003). Our data only permitted speculation regarding the significance of macropore bypass flow and, hopefully, will encourage further investigation into this potentially important P loss pathway. Furthermore, the WSP and DPS relationship for the surface soils, 0 to 2 cm, suggests that the Chesapeake Bay and its tributaries are more vulnerable to receiving excess P from surface runoff originating from manure applications than from P leaching. Indeed, to remedy the eutrophication problems resulting from excessive P in waterways, we suggest that P loss through surface runoff pathways should be the primary focus coupled with monitoring the DPS of surface soils.


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


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