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Published in J. Environ. Qual. 33:1431-1439 (2004).
© ASA, CSSA, SSSA
677 S. Segoe Rd., Madison, WI 53711 USA

TECHNICAL REPORTS

Surface Water Quality

Surface Runoff Losses of Phosphorus from Virginia Soils Amended with Turkey Manure Using Phytase and High Available Phosphorus Corn Diets

C. J. Penna,*, G. L. Mullinsa,*, L. W. Zelaznya, J. G. Warrena and J. M. McGrathb

a Department of Crop, Soil, and Environmental Sciences, Virginia Tech, 330 Smyth Hall, Blacksburg, VA 24061
b Department of Plant and Soil Science, University of Delaware, Newark, DE 19717

* Corresponding authors (chpenn{at}vt.edu, gmullins{at}vt.edu).

Received for publication June 17, 2003.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Many states have passed legislation that regulates agricultural P applications based on soil P levels and crop P uptake in an attempt to protect surface waters from nonpoint P inputs. Phytase enzyme and high available phosphorus (HAP) corn supplements to poultry feed are considered potential remedies to this problem because they can reduce total P concentrations in manure. However, less is known about their water solubility of P and potential nonpoint-source P losses when land-applied. This study was conducted to determine the effects of phytase enzyme and HAP corn supplemented diets on runoff P concentrations from pasture soils receiving surface applications of turkey manure. Manure from five poultry diets consisting of various combinations of phytase enzyme, HAP corn, and normal phytic acid (NPA) corn were surface-applied at 60 kg P ha–1 to runoff boxes containing tall fescue (Festuca arundinacea Schreb.) and placed under a rainfall simulator for runoff collection. The alternative diets caused a decrease in manure total P and water soluble phosphorus (WSP) compared with the standard diet. Runoff dissolved reactive phosphorus (DRP) concentrations were significantly higher from HAP manure-amended soils while DRP losses from other manure treatments were not significantly different from each other. The DRP concentrations in runoff were not directly related to manure WSP. Instead, because the mass of manure applied varied for each treatment causing different amounts of manure particles lost in runoff, the runoff DRP concentrations were influenced by a combination of runoff sediment concentrations and manure WSP.

Abbreviations: DRP, dissolved reactive phosphorus • HAP, high available phosphorus • NPA, normal phytic acid • WSP, water soluble phosphorus


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
THE TRANSPORT OF P in runoff from agricultural land to surface waters has been reported to contribute to accelerated eutrophication of receiving surface waters (Schindler, 1977; Edwards and Daniel, 1992; Heathman et al., 1995). This nonpoint source of transported agricultural P is typically either from surface soils with P concentrations considered in excess of agronomic needs (Reddy et al., 1980; Pautler and Sims, 2000; Breeuwsma et al., 1995), or directly from animal manure that has been recently land-applied (Sauer et al., 2000; Edwards and Daniel, 1993). In either situation, a reduction in the amount of total P applied to soils could benefit water quality. In addition, the application of a P source that is less soluble in water could reduce P losses that are occurring directly from land-applied manure that has not had sufficient time to react with soil and convert to more stable forms (Sharpley and Syers, 1979; Edwards and Daniel, 1993; Kleinman et al., 2002). Therefore, the buildup of soil test phosphorus (STP) and the loss of P directly from applied manures could be reduced by either applying less manure or reducing the total P content of manure applied.

In Virginia, the legislative response to water quality problems associated with nonpoint losses of P was the Virginia Poultry Waste Management Act of 1999, which targets the state's poultry industry and mandates that P application rates shall not exceed crop nutrient needs based on soil test recommendations or crop nutrient removal. As a result of this limitation on the amount of poultry manure land-applied, many farms no longer possess sufficient land area to dispose of all their poultry manure and also lack in the capacity to store the waste. Therefore, the ability to decrease manure total P could allow farmers to utilize a greater proportion of their poultry manure in a manner that would also help reduce nonpoint-source P losses to surface waters compared with previous management.

Since naturally occurring organic phytate P found in grain is mostly unavailable for utilization by poultry, supplemental P from inorganic sources (usually Ca phosphate) is often added to conventional corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] diets to meet the nutrient requirements of poultry. Inorganic as well as plant-derived forms of nonphytate P are therefore considered the most digestible forms of P in conventional poultry diets. Although inorganic forms of P are much more digestible than phytate P, they are not 100% available to the bird. As a result, all of the phytate P and a portion of the supplemental inorganic P ingested passes directly through the birds into the manure.

The principal method for increasing digestibility, and thus the efficiency of phytate P uptake, is the addition of the phytase enzyme to the birds diet. This enzyme cleaves phosphate groups from the phytic acid molecule, thereby making P available to the bird. Phytase-supplemented diets result in greater availability of plant-derived P, less inorganic P needed as a supplement, and therefore less total P content in the manure. The use of phytase supplements along with reduced inorganic P supplements has also been shown to reduce the manure total P content by 25 to 30% (Yi et al., 1996).

A second method for enhancing the digestibility of phytate P in poultry diets involves feeding grains that contain a high amount of digestible P. One example is low phytic acid corn, also known as HAP (high available phosphorus) corn. This corn hybrid produces grain that is similar in total P content to normal corn, but contains about 60% less phytate P (Ertl et al., 1998). Several researchers have shown that more P is available to poultry from diets containing HAP corn than diets containing normal corn (Kersey et al., 1998; Li et al., 1998; Yan et al., 1998). Therefore, the net result of HAP corn use is similar to the effect achieved with the phytase enzyme: since a greater proportion of P is ingested in an available form, the total amount of P in the diet and manure can be reduced without sacrificing animal performance.

Although previous research has shown that the use of HAP corn and phytase enzyme in poultry diets can reduce the amount of total P in manure, a study by Lawrence (2000) showed that an observed decrease in manure total P was coupled with an increase in the percentage of total P that is water soluble. As a result, the authors hypothesized that when applied to soil on an equal total P basis, phytase and HAP corn poultry manure would cause increased concentrations of dissolved P in runoff compared with manure from conventional poultry diets.

Our objective was to determine the effects of phytase enzyme and HAP corn supplemented diets on dissolved and total P losses in runoff under a simulated rainfall when the resulting manure is surface-applied to tall fescue in runoff boxes.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Manure Collection and Characterization
The manure for this experiment was generated from a study conducted at Virginia Tech by the Department of Animal and Poultry Science, which focused on the effect of diet on turkey nutrition, health, and manure properties. Twelve different diets were replicated eight times with nine turkeys per cage (96 cages) ranging in age from 1 to 5 wk. Birds had access to unlimited feed. The pens containing the birds were housed in environmentally controlled rooms with steel mesh acting as the floor of the pens that were further underlain with its own manure tray. These manure trays were removed daily and scraped clean, and the manure was discarded except for three manure collection days during the fourth week of the feeding trial (Lawrence, 2000). No bedding was used in the production of this manure; therefore, we were working with raw turkey manure rather than poultry litter. The manure samples were mixed mechanically to ensure uniformity before being frozen in storage before use in this simulated rainfall study. Dietary treatments used in the study were (i) normal phytic acid corn and 0.135% inorganic P (NPA); (ii) normal phytic acid corn, 600 units phytase enzyme, and 0.135% inorganic P (NPA + 600); (iii) high available phosphorus corn and 0.135% inorganic P (HAP); (iv) HAP corn, 600 units phytase, and 0.135% inorganic P (HAP + 600); and (v) normal phytic acid corn and 0.345% inorganic P (NPA + P). Note that the NPA + P diet is the current industry standard.

Manure was characterized as follows: (i) total P by wet digestion using nitric and perchloric acid (Jones and Case, 1990); (ii) WSP (1:10 wet manure to deionized water, conducted on a dry weight basis, 1 h reaction time, filtration with 0.45-µm Millipore [Billerica, MA] membrane [Kuo, 1996]); (iii) "sediment WSP" extraction on the wet manures using an extraction ratio of manure to deionized water equal to the mass of sediment lost in runoff to the volume of runoff water collected; therefore, each replication had a unique manure to solution ratio, and solutions were then treated the same as previously described for the standard manure WSP tests; (iv) pH (1:1 manure to deionized water); (v) organic matter (OM; by loss on ignition at 360°C [Nelson and Sommers, 1982]); and (vi) moisture content (based on mass of manure before and after drying for 24 h at 65°C). Analysis of P in solution was determined by inductively coupled plasma emission spectroscopy (ICP–AES).

Soil Collection, Preparation, and Characterization
Blocks of sod approximately 20 x 100 cm in size were removed from a Braddock loam (fine, mixed, semiactive, mesic Typic Hapludults) at a depth of 5 cm from the Virginia Tech agronomy research farm (Kentland Farm). The sod consisted of established tall fescue, which had not received any organic amendments or P fertilizer for at least 5 yr before sod removal.

The sod was placed into wooden runoff boxes approximately 20 x 5 x 100 cm with nine drilled holes 3 mm in diameter at the bottom to allow for free drainage. Sod was allowed to equilibrate and establish while being kept moist for 4 wk before use in the rainfall simulation experiment (SERA-17, 2004). One week after the sod was placed in runoff boxes, we applied 17 kg N ha–1 onto the tall fescue in the form of liquid urea-ammonium-nitrate. After two additional weeks of growth, the tall fescue was cut to a height of 10 cm; after an additional week of growth, 24 h before the first rainfall event, the sod was cut to a height of 5 cm. For all trimming, the cuttings were removed from the runoff boxes.

Rainfall Simulation Experiment
Immediately after the tall fescue was trimmed to 5 cm, each turkey manure treatment was uniformly surface-applied to the tall fescue at 60 kg total P ha–1 (dry weight basis). It is important to note that the manure was applied at its original moisture content and nonincorporated; also, the reason for our choice of experimental conditions is because they are representative of agricultural practices found in the Shenandoah Valley of Virginia. In this area, although poultry manure is almost always mixed with bedding, it is surface-applied onto pastures often with slopes greater than 10%.

Three simulated rainfall events were then conducted on the amended soils plus a control (tall fescue receiving no manure amendment) at 1 d (Event 1), 7 d (Event 2), and 21 d (Event 3) after manure application. Soil boxes were kept indoors between rainfall events to prevent runoff from naturally occurring rainfall. Each treatment was replicated three times for a total of 18 runoff samples per event.

Tall fescue was trimmed to a height of 5 cm and runoff boxes were pre-wetted 24 h before each simulated rainfall event to ensure that runoff would occur immediately after rainfall began; however, in regard to Event 1, pre-wetting was done before the manure treatments were surface-applied. Simulated rainfall events were conducted at an intensity of 75 mm h–1 for 30 min with uniformity greater than 95% and all runoff was collected into a single container. Soil runoff boxes were placed at a slope of 5% during rainfall events. The simulator was based on the design of Shelton et al. (1985), with one TeeJet 1/2HH-SS50WSQ nozzle (Spraying Systems, Wheaton, IL) placed in the center of the simulator, 3 m above the surface of the runoff boxes.

Immediately after a rainfall event, total runoff volume was determined by weight. Collected runoff was then subsampled with a pipet while all runoff was constantly mixed on a magnetic stir plate to ensure that all sediment was in suspension. Runoff subsamples were subjected to the following analysis: (i) dissolved reactive phosphorus (DRP, 20 mL of runoff filtered through a 0.45-µm filter and analyzed for P by the molybdate blue method (Murphy and Riley, 1962); (ii) total P (100 mL of nonfiltered runoff digested by the USEPA 3050 total digestion method [USEPA, 1986] with the resulting extract analyzed for P by ICP–AES); and (iii) sediment concentration (40 mL of nonfiltered runoff evaporated at 70oC).

Statistical Analysis
All data were tested for normality by the Shapiro–Wilkes statistic conducted by the PROC UNIVARIATE procedure of the Statistical Analysis System (Version 8.0) and found to be normally distributed (SAS Institute, 1999). All correlation, regression, and analysis of variance procedures were conducted by standard procedures of SAS.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Manure Properties
The addition of phytase enzyme (NPA + 600) and the use of HAP corn (HAP and HAP + 600) in turkey diets decreased the amount of total P and WSP in the resulting manure relative to the standard poultry diet (NPA + P) (Table 1). We observed a 40, 41, and 42% decrease in manure total P for NPA + 600, HAP, and HAP + 600 diet treatments, respectively (relative to NPA + P). Even greater decreases were observed in WSP concentrations in manure from the alternative poultry diets. For example, there was a 52, 47, and 48% decrease in manure WSP for NPA + 600, HAP, and HAP + 600 diet treatments, respectively (relative to NPA + P). Note that there were also decreases in manure total P and WSP for the NPA treatment simply due to the fact that this diet did not receive supplemental P. However, this treatment also resulted in poor bird health whereas the other three diets that decreased manure total P and WSP did not adversely affect bird health and productivity (Lawrence, 2000). One consideration to the manure WSP numbers is that the analysis was done in triplicate on each composited manure sample rather than an analysis conducted on each individual pen replication. Therefore, any measure of variability between replications would be meaningless.


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  		Table 1. General manure properties as a function of turkey diet.

 
In addition to a reduction in manure total P and WSP from the use of phytase enzyme and HAP corn, there was a decrease in the percentage of total P that was water soluble (Table 1, % WSP). The percent WSP is greatest for the standard poultry diet (NPA + P) (Table 1), which contrasts with values found by Lawrence (2000) and Sims (personal communication, 2000), although both of those studies did show a reduction in manure total P content from the use of phytase and HAP corn. A potential explanation for the differences in % WSP among the alternative diets in comparison with results from Lawrence (2000) is the fact that their WSP extractions were conducted over a 15-h period (extracted for 15 h using an automatic extractor, not a 15-h shaking period) using 0.01 M CaCl2 as the extracting solution, whereas our study used a 1-h shake period followed by an immediate extraction period. Due to the methods being very different, our results cannot be readily compared with those of Lawrence (2000). In support of our results, McGrath et al. (2002) did find that the use of phytase enzyme supplements to poultry diets caused a 5% decrease in the amount of manure total P that was water soluble. Maguire et al. (unpublished data, 2004) also found that the addition of phytase enzyme to turkey diets receiving decreased inorganic P resulted in lowered total P concentrations in manure, but showed no differences in WSP compared with standard diets.

According to our results, land application based on an equal total P basis of these manures would result in a greater mass of phytase and HAP corn manure amended to the soil (Table 1). With many states limiting P applications based on total P contents of the manure and soil extractable P concentrations, more phytase and HAP corn manure (NPA + 600, HAP + 600, and HAP) could be applied per unit area compared with regular turkey manure (NPA + P), thus helping to prevent potential point-source P pollution (stored manure) and manure storage and disposal problems. In addition, because the percentage of manure total P that is water soluble is lower in the alternative diets compared with the standard diet (Table 1), land application of the phytase and HAP corn manure on a total P basis will also result in a smaller amount of manure WSP applied compared with the standard manure. This is important because previous research has shown that increased additions of WSP applied to soils will cause a corresponding increase in the amount of WSP measured in the soil, which will ultimately result in greater concentrations of DRP in runoff during a rainfall event either from the soil WSP reservoir or directly from the amendment itself (Moore et al., 2000a, 2000b; Withers et al., 2001; Kleinman et al., 2002). Penn and Sims (2002) conducted a simulated rainfall study involving the application of biosolids and poultry litter amendments and found a significant correlation between the amount of WSP applied with the amendment and the corresponding increase (relative to a nonamended soil) in soil WSP and runoff DRP concentrations.

Furthermore, in situations where manure is applied on an equal mass basis instead of an equal total P basis, the phytase and HAP corn treatments would again result in the application of less WSP, and also less total P compared with the standard poultry diet. For example, a hypothetical application rate of 15 Mg dry manure ha–1 (based on an average plant-available nitrogen application rate among the five manure types) would result in the addition of 219, 129, 129, and 127 kg total P ha–1 and 122, 58, 63, and 65 kg WSP ha–1 for the NPA + P, NPA + 600, HAP, and HAP + 600 treatments, respectively. In addition to decreased additions of WSP to the soil, the decrease in the amount (relative to the standard diet) of total P applied is equally important since this could help to slow down the buildup of extractable P in the soil. It has been well documented that increasing concentrations of extractable soil P will cause elevated concentrations of DRP in runoff (Sharpley et al., 1978, 1994; Sharpley, 1995; Daniel et al., 1994; Sims et al., 2002).

Phosphorus Losses in Runoff
Runoff volumes were not significantly different among treatments, with the result being that runoff loads (mg) are simply 5.3 times runoff concentrations (mg L–1). For the remainder of this paper runoff P "losses" will be expressed as and meant to be taken as concentrations rather than loads. Phosphorus losses from manure-amended soils were significantly greater than losses from the nonamended (control) soil for Event 1 (Fig. 1). For Events 2 and 3, P loss in runoff from manure-amended soils decreased dramatically, so that 80 to 95% of cumulative P loss occurred during the first rainfall. Similarly, 70 to 95% of cumulative sediment loss occurred in the first rainfall event. High loss of P in the first runoff event following manure application is typical (McLeod and Hegg, 1984; Sharpley, 1997). These results show the enormous potential for surface-applied, nonincorporated manures on pasture to contribute to nonpoint-source P pollution. In contrast, Sharpley (1997) showed that DRP concentrations in runoff from soils where poultry litter had been incorporated at a rate of 160 kg P ha–1 ranged from 0.17 to 1.34 mg L–1. In our study the lowest DRP concentration in runoff from manured soils was 61 mg P L–1. Incorporated manures or manures applied to bare soil have a much greater contact with soil than manure applied to pasture. This greater contact increases soil adsorption of the water soluble P in manure, thus decreasing P availability to loss in runoff.



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  		Fig. 1. Mean runoff dissolved reactive phosphorus (DRP) and total P concentrations following surface applications of different poultry manures to tall fescue during (a) Rainfall Event 1, (b) Rainfall Event 2, and (c) Rainfall Event 3. Letters above bars columns identify groupings of means that are not significant at p = 0.05, with uppercase denoting DRP and lowercase denoting total P.

 
Effect of Manure Type on Phosphorus Losses in Runoff
For all simulated rainfall events, total P and DRP concentrations in surface runoff were significantly lowest from the nonamended soils (control) (Fig. 1). Among manure-amended soils, there were no significant differences in total P losses in runoff during Event 1. However, Event 1 DRP concentrations from the HAP manure treatment were significantly higher than all other treatments, while the remaining manure treatments were not significantly different from each other (Fig. 1). For Rainfall Events 2 and 3, DRP and total P concentrations among manure-amended soils were not significantly different from each other, with the exception of HAP manure in Events 2 and 3 being significantly higher and NPA + P significantly lower in total P compared with the other treatments.

In an attempt to explain why the HAP manure caused the largest concentrations of DRP in runoff from Event 1, various water extractions were conducted on the manures and correlated to P losses in runoff. There was no relationship between the WSP concentration in the manure and DRP concentration in runoff. This was not expected since many previous studies have shown that WSP contents of P amendments are usually well-related to DRP losses in runoff (McDowell and Sharpley, 2001; Penn and Sims, 2002; Kleinman et al., 2002). In addition, the percentage of manure total P that is water soluble (% WSP) and the mass of WSP applied with each treatment were also poorly related to runoff DRP concentrations and loads.

Instead, we found that sediment losses from the manure-amended soils appeared to be related to DRP losses with the result being a good relationship between the percentage of manure lost in runoff and runoff DRP concentrations (Fig. 2). Although it is considered typical for sediment losses to be related to total P losses in runoff (Yli-Halla et al., 1995; Sharpley, 1997; Withers et al., 2001), it was unexpected to observe the correlation between the percentage of manure lost and DRP. The calculation of percent manure lost in runoff was based on the fact that sediment losses from nonamended soils (control) represent erosion of only nonmanure particles; one can subtract this amount from the sediment losses of manure-amended soils and estimate how much of the runoff sediment concentration was manure particles. This calculation revealed that 93 to 97% of the sediment load in Rainfall Event 1 from manure-amended soils was manure particles. In this case we hypothesize that the manure particles are desorbing high amounts of P into solution both during transport and after being deposited into the runoff collection container with very few soil particles to act as a P sink resulting from the conditions of the study (wet manure surface-applied to pasture).



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  		Fig. 2. Effect of particulate manure lost in runoff on Event 1 runoff dissolved reactive phosphorus (DRP) and total phosphorus (total P) concentrations.

 
Effect of Sediment Losses on Phosphorus Losses in Runoff
Due to the fact that each manure was amended on an equal total P basis, the total mass of manure applied to each box also varied between treatments (Table 1). The large variation in the mass of wet manure applied to the tall fescue was well related to the amount of sediment (manure particles) collected in runoff during each rainfall event (Fig. 3). Thus, the higher the rates of manure surface-applied onto the boxes, the greater the concentration of manure particles found in the resulting runoff. The increased loss of manure particles then resulted in a corresponding increase of runoff total P and DRP (Fig. 2).



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  		Fig. 3. Effect of manure application rates on sediment concentrations in runoff from manure-amended soils during each rainfall event.

 
We hypothesized that under these conditions (wet manure surface-applied onto pasture, without incorporation), the runoff DRP concentration is strongly influenced by manure particles lost in runoff that desorb high amounts of P in solution both during and after transport into the water body (collection container). However, because the WSP content of each manure is different from one another (Table 1) and since each manure type had a different concentration of sediment in runoff (Fig. 3), it seems logical that each manure would have a unique "sediment WSP concentration," where sediment WSP concentration is defined as the concentration of dissolved P in runoff resulting from the desorption of P from sediment (manure particles in runoff). This is based on the knowledge that more than 90% of sediment lost in this study was actually particles of manure. This hypothesis was tested by correlating measured "sediment WSP" concentrations (WSP extraction on the wet manures using an extraction ratio of manure to deionized water equal to the mass of sediment lost in runoff to the volume of runoff water collected) to runoff DRP concentrations. It has been shown that soil and manure particles may be preferentially eroded due to their physical and chemical characteristics (Sharpley and Kleinman, 2003; Sharpley, 1980); therefore, an extraction of the entire manure would not exactly represent the eroded particles. Additionally, the chemical composition of the runoff water was probably quite different from the deionized water used to extract the manure. Nonetheless, we hypothesized such an extraction would provide a good index of how the manure properties and sediment concentrations in runoff interacted to produce the DRP concentrations determined in this study. Figure 4 shows that both the sediment concentration in runoff (i.e., eroded manure particles) and the WSP concentration of the manure controlled runoff DRP concentrations from each manure-amended soil.



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  		Fig. 4. Relationship between measured sediment water soluble phosphorus (WSP) and dissolved reactive phosphorus (DRP) concentrations in runoff from manure-amended soils. Measured sediment WSP concentration was determined from WSP extracted from manure using a manure to solution ratio equal to measured sediment concentration to measured runoff volume.

 
Figure 5 shows that DRP in runoff is probably a function of sediment loss and P solubility in manure. In Fig. 5, sediment and manure WSP concentrations are calculated on a relative 1 to 10 scale (i.e., for manure WSP, the treatment with the highest manure WSP concentration is assigned a level of 10, and all other treatments are calculated relative to this) and added to each other (20 is the highest possible level for any treatment). One can see that the sum of relative sediment and manure WSP levels is well-related to actual DRP concentrations measured in runoff. As previously discussed, eroded manure particles comprised most of the sediment lost in runoff from pastures receiving surface-applied manure and manure application rate determines the amount of manure eroded in runoff (Fig. 3). Therefore, because losses of manure particles were well-related to runoff DRP (Fig. 2) and runoff DRP under these conditions was shown to be a function of both manure lost and manure WSP (Fig. 4), we can illustrate the extent to which each of the two controlling factors influenced runoff DRP concentrations for each treatment in Fig. 5. The relationship observed in Fig. 5 clearly demonstrates that DRP concentrations in runoff from pastures receiving surface applications of poultry manures can be dominated by high manure application rates (thus, high concentrations of manure particles in runoff; i.e., HAP), high P solubility in manure (NPA + P), or a combination of both (NPA + 600, HAP + 600, and NPA).



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  		Fig. 5. Relative contribution of manure sediment losses and manure water soluble phosphorus (WSP) concentrations to measured Event 1 runoff dissolved reactive phosphorus (DRP) concentrations. Relative manure WSP and sediment levels were calculated on a 1 to 10 scale with the highest sediment and manure WSP concentration set equal to 10 and all others calculated relative to it.

 
Under these conditions where manure was surface-applied onto pasture, the amount of manure particles lost in runoff (which was a function of manure application rate) appeared to have at least as much influence on DRP concentrations in runoff compared with the manure WSP content (Fig. 3 and 4). Therefore, when manure application rates are based on total P, manures with lower total P contents resulting from alternative poultry diets such as NPA + 600 and HAP (Table 1) can result in a greater mass of poultry litter land-applied (Table 1) and thus higher concentrations of DRP and total P in runoff even though these manures contain the lowest amounts of WSP and % WSP compared with the other treatments (Table 1). However, we speculate that if the tall fescue was amended at equivalent rates of manure it is likely that we would have observed that manure types such as NPA + P containing high concentrations of total P and WSP would cause the greatest concentrations of DRP and total P in runoff. Another consideration is that if the tall fescue was amended in a manner that left a short length of nonamended buffer area at the end of the runoff box, we may have found that the concentration of WSP in the manure would have a greater effect on runoff DRP than sediment losses since this would prevent much of the eroded manure particles from entering the water body (collection container). Finally, because the simulated rainfall intensity (75 mm h–1) is much higher than most natural rainfall events, the loss of particulate manure in runoff would probably be less under normal circumstances. This would probably cause DRP and total P concentrations to be significantly lower than what we observed and may also result in manure WSP concentrations to be the dominating factor controlling DRP losses in runoff.

Nussbaum-Wagler et al. (2002) conducted a rainfall simulation study on Cecil (fine, kaolinitic, thermic Typic Kanhapludults), Sleeth (fine-loamy, mixed, active, mesic Aeric Endoaqualfs), and Creldon (fine, mixed, active, mesic Oxyaquic Fragiudalfs) soils, in which swine and poultry manure from standard diets and a low phytic acid corn plus phytase enzyme diet were surface-applied at equivalent N rates. The authors found that there was less DRP, bioavailable P, and total P in runoff from the alternative diet compared with the standard diet for both animal species.

Finally, in regard to P loss assessment tools such as the P index (Lemunyon and Gilbert, 1993), our results suggest that there is a need to account for P losses from actual manure particles that are lost in runoff. For example, Kleinman and Sharpley (2003) found that the influence of manure WSP on runoff P grows with application rate in a study that involved surface applications of poultry, swine, and dairy manure to soil at six different rates. Many P loss models include soil erosion as a transport factor, but do not directly account for manure losses (Weld et al., 2000).


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
With the Virginia Poultry Waste Management Act in effect as of October, 2001, the maximum amount of P that poultry producers can apply onto "high" P soils (>55 mg kg–1 Mehlich-1 P) is equal to a 3-yr P crop removal rate. As a result, there is a need to reduce the total P content of poultry manure and litters so that producers can continue to utilize their manure by land application in a manner that will not adversely affect water quality. Results from this study show that the addition of phytase and HAP corn to poultry diets can reduce the amount of total P and WSP in manure compared with standard poultry diets. In addition, the percent of manure total P that is water soluble was also reduced by using phytase and HAP corn supplemented poultry diets. Therefore, when applied on an equal total P basis, a greater rate of HAP corn and phytase manure could be land-applied compared with manure from standard diets without applying greater amounts of WSP that can potentially be lost in runoff. When applied on an equal mass basis, manure from phytase and HAP corn diets will add less total P and WSP to the soil compared with manure from standard poultry diets, which will help to prevent soils from becoming "very high" in soil test P (assuming that the amount of total P applied is not in excess of crop requirements).

With the exception of HAP manure, the alternative diets caused no significant increases in Event 1 runoff DRP and total P concentrations compared with the standard diet (NPA + P). Under the conditions in this study, which consisted of surface-applied wet manure applied at an equivalent total P basis onto tall fescue, we found that runoff DRP and total P concentrations were poorly related to the WSP contents of the manures alone, instead P losses were also influenced by the sediment concentrations in the resulting runoff due to the fact that >90% of the eroded sediment was made up of manure particles. As a result, when applied on a total P basis, manure types that contained lower concentrations of total P (i.e., HAP) had higher application rates of manure resulting in higher sediment concentrations, which translated into higher total P and DRP losses. We showed that eroded manure particles were strongly contributing to the observed runoff DRP concentrations by conducting manure WSP extractions using a manure to solution ratio equal to the measured sediment concentration in runoff and correlating the resulting WSP concentrations to DRP losses in runoff.

Overall, runoff DRP concentrations were influenced by the combination of application rates and manure WSP concentrations. As the total amount of surface-applied manure increased (i.e., manure from the alternative diets), manure particles were more easily lost in runoff. These eroded manure particles desorbed P into solution based on the WSP concentration of the manure itself. In determining what manure types posses the greatest potential for causing P losses in runoff, the application conditions will greatly affect the outcome (type of surface, incorporation vs. nonincorporation, type of application rate), not just the WSP content of the manure itself. Although the alternative diets can reduce the WSP contents of the manure, potentially resulting in a decrease in runoff DRP losses, the management of the manures is equally influential on DRP runoff losses.


   ACKNOWLEDGMENTS
 
The authors are grateful to the United States Poultry and Egg Association for funding of this project.


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


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