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a Department of Soil Science, North Carolina State University, Raleigh, NC 27695
b Department of Plant and Soil Science, University of Delaware, Newark, DE 19716
c Department of Animal Sciences, Purdue University, West Lafayette, IN 47907
* Corresponding author (rory_maguire{at}ncsu.edu)
Received for publication April 6, 2004.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: DRP, dissolved reactive phosphorus • M3-Al, M3-Ca, M3-Fe, and M3-P, Mehlich 3–extractable aluminum, calcium, iron, and phosphorus, respectively • M3-PSR, Mehlich-3 phosphorus saturation ratio calculated as the molar ratio of M3-P to (M3-Al + M3-Fe) • NPP, non-phytate phosphorus • PAN, plant-available nitrogen • TSP, triple superphosphate • WSP, water-soluble phosphorus
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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One strategy to address the excess manure P produced in areas with intensive animal production is to reduce the P concentration of animal diets and hence manures generated. Feeding closer to nutritional P requirements can reduce dietary P, and feed additives such as phytase (that can increase the digestibility of P fed) and vitamin D3 derivatives can further reduce the P concentration in poultry feeds (Council for Agricultural Science and Technology, 2002). About two-thirds of the P in corn (Zea mays L.) and soybeans [Glycine max (L.) Merr.] is phytate P that is poorly utilized by poultry, yet these two ingredients make up the majority of P in poultry feed (Ravindran, 1996). If this phytate P could be utilized by poultry, it would reduce the need for supplemental P to be added to these diets, thereby reducing the concentration of P in feed and litter produced.
Incorporating current technologies into diet formulations could reduce the amount of P in poultry manures and litters by at least 40%, while it is anticipated that future technologies could lead to decreases of more than 60% (Council for Agricultural Science and Technology, 2002). It has been shown that phytase can increase the availability of phytate P to poultry (Applegate et al., 2003). However, concerns have been raised that although phytase can decrease litter total P, it could increase the WSP in litters produced and hence the potential for P losses to surface waters following land application (Vadas et al., 2004). This is a concern because dissolved P in runoff from manure-amended soils has been linked to WSP in the manures and litters applied (Kleinman et al., 2002; Sauer et al., 2000). For example, Vadas et al. (2004) reported that phytase in broiler diets significantly increased WSP in manure produced, although it did not significantly affect soluble P losses in runoff from surface-amended soils. Miles et al. (2003) also showed that phytase formulation in broiler diets increased WSP in litters following one flock, but after a second and third flock were grown on the same bed of litter there was no evidence for phytase increasing WSP in litters produced. Applegate et al. (2003) reported that the concentration of total P and WSP in fresh broiler litter is dependent on the concentration of P in the broiler feed, but not on phytase supplementation. In a study involving litter sampled following two flocks of turkeys or three flocks of broilers, Maguire et al. (2004) concluded that correctly formulating diets with phytase would decrease litter total P and not affect WSP in litter or amended soils. In a study involving swine, Baxter et al. (2003) reported that phytase in the diet did not increase WSP in the manure produced and even decreased it in one out of two comparisons. In another swine study, Smith et al. (2004a) reported that dietary phytase significantly decreased WSP in manure by 17%, but had no significant effect on dissolved P loss in runoff. In a separate study with broiler litter, Smith et al. (2004b) found that dietary phytase actually decreased both litter WSP (in one of two comparisons) and dissolved P in runoff (in one of two comparisons). Waldroup (2002) also concluded that the balance of research reported in the literature supported the use of phytase for reducing the total P concentration in poultry litters, without increasing litter WSP. The reasons why some studies have shown an increase in P solubility in litters from a diet containing phytase, when the majority of research does not support this conclusion, remains unclear.
The use of diet modifications to decrease the total P concentration in poultry litters, and hence decrease P loading rates on agricultural lands and P transfer to surface waters, is a relatively new area of research. For this approach to be successful at decreasing P losses from agricultural lands, it is essential to understand how litters from modified diets affect P losses in runoff following land application. Also, most studies on litters from modified diets have been conducted using the same rate of total P application, while in many situations litters from reduced P diets are still applied according to PAN. Therefore the aims of this study were to investigate how reducing the P concentration in turkey feed, and the utilization of dietary phytase, affect the P concentrations in litters and P losses in runoff following land application of the litters at the same rate of PAN.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Litters (mixture of feces and wood shavings used as bedding) were collected after the performance trials for analysis and use in the runoff studies. Litter samples were collected from each replicate pen immediately after birds were removed and composited by dietary treatment by thoroughly mixing samples from each pen. Fresh litters were used in the runoff study, but for the analyses, composited litters were spread thinly in forced air ovens, dried at 40°C, and ground to pass a 0.5-mm stainless steel screen.
Characterization of Litter Phosphorus
Before analysis, litters were weighed for moisture content calculations, dried at 40°C, and ground. Total P in litters from each dietary treatment was determined by microwave-assisted digestion of a 0.5-g dried litter sample with 7 mL of concentrated HNO3 and 3 mL of 30% H2O2. Water-soluble P in the litters was analyzed at two ratios in triplicate, first at a ratio of 1:200 (0.2 g litter to 40 mL deionized water) and then at a ratio of 1:10 (2 g litter to 20 mL deionized water). After shaking horizontally at 300 rpm for 1 h, extracts were centrifuged at 1000 x g for 1 h and the supernatant filtered through 0.45-µm Millipore (Billerica, MA) membrane filters. Phosphorus detection in all extracts was by inductively coupled plasma atomic emission spectrometry (ICP–AES). Plant-available nitrogen in the litters was calculated from the following equation, all units in g/kg (University of Delaware, 1992):
 | [1] |
This assumes 50% of organic N will become plant available, while the 80% availability for NH4–N assumes immediate incorporation of litter following application. Total N was determined by combustion in a LECO (St. Joseph, MI) CNS analyzer. Inorganic N was extracted with 2 M KCl and analyzed colorimetrically for NH4–N and NO3–N on a Bran + Lubbe (Elmsford, NY) Auto Analyzer III. Organic N was calculated as the difference between total N and inorganic N, and as NO3–N was negligible, only NH4–N is reported.
Runoff Study
Three soils that varied in chemical and physical properties were selected for the runoff study (Table 2). The three soils were (i) Rumford loamy sand (Coastal Plain of Delaware) (coarse-loamy, siliceous, subactive, thermic Typic Hapludult); (ii) Herndon clay loam (Piedmont of North Carolina) (fine, kaolinitic, thermic Typic Kanhapludult); and (iii) Mattapex silt loam (Coastal Plain of Maryland) (fine-silty, mixed, active, mesic Aquic Hapludults). Soils were collected and sieved through a 7-mm mesh sieve before packing in the boxes for the runoff study.
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Characterization of the Soils
There were two sources of soil samples, those collected (i) before treatment and packing of the boxes (Table 2), and (ii) from the runoff boxes immediately before runoff generation. Soils collected from land under row crops were dried and passed through a 2-mm sieve before characterization in triplicate. Soils were analyzed for WSP at a soil to deionized water ratio of 2 g:20 mL, 1 h of shaking time horizontally at 300 rpm, 15-min centrifugation at 1000 x g, and filtration through a 0.45-µm Millipore membrane. Molybdate-reactive P in the extract was determined by the method of Murphy and Riley (1962). Soil samples were extracted with the Mehlich-3 solution (M3; 0.2 M CH3COOH + 0.25 M NH4NO3 + 0.015 M NH4F + 0.013 M HNO3 + 0.001 M EDTA) for 5 min at a soil to solution ratio of 1:10, filtered through Whatman no. 2 filter paper (Whatman, Maidstone, UK), and analyzed for M3-P, -Al, and -Fe by inductively coupled plasma–atomic emission spectroscopy (ICP–AES) (Mehlich, 1984). All soils were analyzed for pH (1:1 soil to water) and organic matter (OM; loss on ignition) by standard methods of the University of Delaware Soil Testing Program (Sims and Heckendorn, 1991). The Mehlich-3 P saturation ratio (M3-PSR) was calculated by the following equation, with values for P, Al, and Fe in mmol/kg (Maguire and Sims, 2002; Sims et al., 2002):
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Statistical Analyses
Separation of the means was performed using least significant differences calculated with the GLM procedure of Statistical Analysis System, Version 8.2 (SAS Institute, 2003). The probability value used to determine significance was 0.05. All other statistical analyses were performed using the Data Analysis tool pack in Microsoft Excel 2000 (Microsoft, 2000).
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The concentration of WSP in TSP was 148141 mg WSP/kg (1:10 ratio), which translates to 79% of total P. Water-soluble P was always greater in litters from diets high in NPP (A, C) than those low in NPP (B, C), however the effect of phytase depended on the WSP extraction ratio. Kleinman et al. (2002) showed that as the extraction ratio widens the amount of WSP extracted increases, which they attributed to dilution effects promoting dissolution of calcium phosphates. In our study the wider extraction ratio also led to greater extraction of litter WSP. At a ratio of 1:10, WSP followed the same trend as total P, being significantly reduced by both lowering the NPP fed and by formulation of phytase. At a ratio of 1:200, WSP was significantly less in the low (B, F) compared with high (A, C) litters, but WSP was not significantly affected by inclusion of dietary phytase. This agrees with Applegate et al. (2003), who reported that litter WSP concentration was dependent on the concentration of P in broiler feed but not phytase fed. However, in broiler manure rather than litter, Vadas et al. (2004) reported that phytase in diets increased WSP in manures produced, while Maguire et al. (2003) reported no impact of dietary phytase on WSP in turkey manure. Self-Davis and Moore (2000) recommended a litter to water ratio of 1:10 and this extraction ratio has been used successfully by several researchers to relate WSP in litters to solubility of P in soils and/or P losses in runoff following litter amendment (Moore et al., 2000; Sauer et al., 2000; Maguire et al., 2004).
The WSP to total P ratio shows what impact diet modification would have on WSP application rates if all litters were applied at the same rate of total P. When using the 1:10 extraction for WSP, there was very little change in the WSP to total P ratio, with maximum and minimum values of 32 and 28%, respectively (Table 1). As the 1:200 ratio led to greater extraction of WSP, the WSP (1:200) to total P ratios were greater than the WSP (1:10) to total P ratios. There was no consistent trend in WSP (1:200) to total P when comparing litters from high and low diets. However, phytase led to a greater WSP (1:200) to total P ratio in comparisons of high (36%) to high + phytase (46%) and of low (43%) and low + phytase (46%). This agrees with other studies that have shown an increase in the WSP to total P ratio when phytase is used in diets and would lead to greater applications of WSP if litters from phytase diets were land-applied at the same rate of total P (Maguire et al., 2003; Vadas et al., 2004).
Characteristics of Soils Used in Runoff Study before Amendment
The soils used in the box runoff study ranged in texture from a sandy loam to a clay loam and were all moderately acidic (Table 2). Organic matter and M3-Ca were least abundant in the sandy loam (8.7 g/kg and 281 mg/kg, respectively) and greatest in the clay loam (33 g/kg and 891 mg/kg, respectively). Mehlich-3 P increased in the order: silt loam (32 mg/kg) < sandy loam (86 mg/kg) < clay loam (104 mg/kg). However, due to differences in the Al and Fe content of these soils, the M3-PSR increased in the order: silt loam (0.03) < clay loam (0.11) < sandy loam (0.12) (Table 2). To aid interpretation of soil testing with Mehlich 3, Sims et al. (2002) established agri-environmental categories for M3-P (agronomic value for crop production) and the M3-PSR (potential for environmental impact). According to these categories, the sandy loam was optimum in M3-P, but above optimum according to the M3-PSR due to its relatively low Al and Fe content that would increase the potential for P release. The silt loam was below optimum for both M3-P and the M3-PSR due to its relatively low P concentration and high Al and Fe concentration. Meanwhile, the clay loam was above optimum in M3-P, but in the optimum category for M3-PSR due to its relatively high Al and Fe concentration.
Phosphorus Incorporated with the Litter
The litter was incorporated to these soils according to the PAN content of the litters, calculated according to Eq. [1], and the N concentrations in the litter are shown in Table 3. As N contents were the same in all diets, N concentrations were similar for all litters (Table 3). Therefore, reasonably similar amounts of litter were incorporated into each runoff box. The most common agricultural situation is for litters to be applied according to a N-based nutrient management strategy, which leads to an overapplication of P compared with crop uptake of P (Sims et al., 2000). This overapplication of P is true for this experiment, with the total P incorporated ranging from 78 to 126 kg P/ha compared with crop removal of approximately 25 kg P/ha in corn grain (assuming 9.4 Mg/ha [150 bu/acre]; Kellogg et al., 2000). Reducing the NPP concentration fed (high versus low) reduced the P application rate in turkey litter by 30% (from 126 to 88 kg P/ha), which reduced the P surplus 38% (from 101 to 63 kg P/ha). Phytase formulation (with a decrease of 0.08% in dietary NPP) decreased the P application rate on average 16% (126 to 101 kg P/ha and 88 to 78 kg P/ha), which led to an average decrease of 20% in the P surplus. Together, reducing NPP fed and formulating phytase decreased the P applied in turkey litter 38% (126 to 78 kg P/ha) and the P surplus 48% (101 to 53 kg P/ha). Other researchers have reported similar or greater reductions in litter total P when phytase is formulated in broiler diets, for example 23% (Miles et al., 2003), 17 to 24% (Maguire et al., 2004), and 20 to 45% (Vadas et al., 2004). Therefore, reducing the P concentration fed and using feed additives such as phytase in turkey and broiler diets have the potential to greatly reduce the P concentration in litter produced, and an even greater potential to reduce the surplus of manure P being applied to cropland at PAN rates in areas of intensive poultry production. Phytase in diets will reduce the phytate P concentration in litters produced (Maguire et al., 2004), although the long-term impact is unknown as phytate P is strongly bound by soils (Turner et al., 2002).
As litter applications were similar for all treatments, the WSP incorporated into the runoff boxes in the litter application followed a similar trend to that of WSP in the litters (Tables 1 and 3). Water-soluble P applied to the runoff boxes was greater in the high versus low diets and the 1:200 litter to water extraction ratio resulted in higher calculated WSP application rates than the 1:10 extraction ratio.
Dissolved Reactive Phosphorus Lost in Runoff
Previous research has shown that DRP concentrations in runoff from boxes are comparable with larger field plots, which are considered to represent P transfer processes (Kleinman et al., 2004). Dissolved reactive P lost in runoff decreased greatly between Days 1 and 7, probably due to a combination of loss of DRP during the first runoff event and a longer reaction time with the soil (Fig. 1)
. Loss of DRP in runoff being much greater immediately after manure application than a week or more later has been reported by several other researchers. Penn and Sims (2002) reported that biosolids incorporation into soils initially increased DRP losses in runoff but that DRP losses decreased with time, which they attributed to P in the biosolids reacting with the soil. Therefore, Day 1 DRP loss in runoff reflects mostly the influence of the litter WSP applied while Day 7 DRP loss reflects more of the soil's influence on runoff.
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On Day 1, grouping of treatments for statistical analysis showed that DRP concentrations in runoff followed the order: (high = high + phytase = TSP) > (low = low + phytase) > control (Fig. 1a; P < 0.05). Therefore, phytase formulation in the diet, with a concurrent reduction in NPP fed, did not significantly affect Day 1 DRP concentrations in runoff. Out of six individual comparisons for the influence of phytase formulation on Day 1 DRP concentrations (high versus high + phytase and low versus low + phytase for each soil), litters from phytase diets led to greater DRP concentrations on one occasion, lower concentrations on one occasion, and there was no significant effect in the remaining four comparisons (Fig. 1a). Even though Vadas et al. (2004) showed that phytase could increase the WSP in broiler manures, they also reported no significant effect of phytase in diets on DRP in runoff from soils they surface-amended with these litters. The Day 1 DRP concentrations from the TSP treatments were among the greatest and this was probably associated with the high WSP concentration in TSP (148141 mg/kg or 79% of total P). It is interesting that TSP produced similar Day 1 DRP to litters high in dietary P, even though the application rate of WSP was greater for TSP (79 kg WSP/ha) than the high P litters (33–35 kg WSP/ha). Overall, converting to diets that combined feeding lower concentrations of NPP and phytase, and incorporating these litters in soils according to a N-based nutrient management plan, would be expected to not only reduce total P applied, but also DRP losses in runoff (Table 3; Fig. 1a).
Influence of Water-Soluble Phosphorus Applied on Dissolved Reactive Phosphorus in Runoff
Dissolved reactive P concentrations in runoff on Day 1 increased with the rate of WSP incorporated in either turkey litter or TSP (Table 3; Fig. 2)
and this was significant for all soils using both the 1:10 and 1:200 litter to water extraction ratios. This agrees with previous reports of the importance of WSP applications (broadcast) on DRP losses in runoff (Kleinman et al., 2002). Coefficients of determination were similar for the 1:10 and 1:200 litter to water extraction ratios, suggesting both methods are suitable for predicting P losses in runoff following litter amendment. Day 1 DRP in runoff was used rather than Day 7, due to there being few differences between treatments in DRP in runoff by Day 7 (Fig. 1b). The Day 1 DRP in runoff was greater for the amended sandy loam and clay loam soils (Fig. 1a), and this is shown by steeper slopes on the regression lines between DRP in runoff and WSP application rate for these two soils (Fig. 2). Exponential regressions produced substantially greater coefficients of determination than linear fits. For example, in Fig. 2a the exponential and linear coefficients of determination for the sandy loam, silt loam, and clay loams were 0.89 (0.70 linear), 0.80 (0.46 linear), and 0.94 (0.77 linear), respectively. This suggests more rapid increases in DRP in runoff per unit WSP application rate, at higher WSP application rates. Sharpley (1995) showed a close relationship between soil saturation with P and DRP in runoff. The M3-PSR in the amended soils immediately before runoff was related to DRP in runoff, but was not as good at predicting DRP in runoff as WSP added in litter. For example, on Day 1 the relationships between M3-PSR and DRP in runoff for litter-amended soils produced exponential fits with r2 values of 0.41, 0.51, and 0.82 for the sandy loam, silt loam, and clay loams, respectively. For the clay loam and the sandy loam soil, it took an application rate of approximately 27 kg WSP/ha (1:10 extraction ratio) or 40 kg WSP/ha (1:200 extraction ratio) to raise Day 1 runoff DRP by 1 mg/L, while DRP in runoff from the silt loam never reached 1 mg/L. As discussed earlier, the greater increases in Day 1 DRP in runoff from the clay loam and sandy loam, compared with the silt loam, are probably due to the higher M3-PSR values for these soils (Table 2). These results show not only the importance of accounting for WSP application rates when predicting DRP in runoff, but also the significance of extraction ratio used and the influence of soil saturation with P.
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Effect of Treatments on Soil Water-Soluble Phosphorus on Days 1 and 7, Immediately before Runoff
Water-soluble P was always greater in the treated soils than the unamended soils on Day 1, although this difference was not always significant (Table 4). Also, when WSP was grouped by soil on Day 1, mean WSP was not significantly different in the three soils, ranging from 1.39 to 1.53 mg/kg. By Day 7, mean WSP was significantly (P < 0.05) lower in the silt loam soil (0.47 mg/kg) than in the sandy loam (0.87 mg/kg) and clay loam (0.95 mg/kg) soils. The lack of significant difference between soils on Day 1 suggests that WSP in the soils was controlled by the P amendment. However, the significant differences on Day 7 suggest that the influence of the soil on controlling WSP is returning, despite WSP being consistently greater in the treated soils compared with the unamended control.
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control, while on Day 7 the pattern was TSP > (high = high + phytase = low) low + phytase > control (P < 0.05). If we look in more detail at the influence of phytase on WSP, in all 12 comparisons (high versus high + phytase and low versus low + phytase for each soil) on Days 1 and 7, there was never a significant effect of phytase on WSP in the soils. In fact, in 6 of these 12 comparisons of WSP in soils treated with litters with and without phytase, WSP was slightly lower (not significant) in the litters from phytase diets than non-phytase diets, while the reverse was true on six occasions. The lack of impact of phytase on soil WSP agrees with the lack of impact of phytase on DRP loss in runoff, and would be expected as WSP was developed as an environmental soil test to predict P losses from land to surface waters in runoff and leaching (Maguire et al., 2005). This also agrees with the results of Maguire et al. (2003)(2004), who showed that there was no influence of phytase on WSP in soils amended with broiler litter, turkey litter, and turkey manure. For the influence of reduced dietary P on soil WSP, of the 12 comparisons (high versus low and high + phytase versus low + phytase) on Days 1 and 7, there were only two occasions where there was a significant difference (P < 0.05), with the WSP being greater in the soils amended with the high P litters than the low P litters in the clay loam on Day 7. However, there was a strong trend for greater WSP (10 of 12 comparisons, not significant) in the soils amended with high P litters than low P litters.
Influence of Litter and Triple Super Phosphate Additions on Soil Mehlich-3 Phosphorus
When the M3-P concentration in the unamended control soil was subtracted from the M3-P of an amended soil, the impact of the amendment on M3-P could be seen (Table 5). This was calculated to show how dietary amendment and land application of litters produced would affect future agronomic soil tests. All amendments substantially increased M3-P in the soils at Day 1 (34–148 mg/kg) and Day 7 (37–117 mg/kg) relative to the unamended control, although the mean increase in M3-P for Day 1 of 77 mg/kg decreased to 63 mg/kg on Day 7. These reductions in M3-P with time were almost certainly due to the stabilization of P in the soils by processes such as the diffusion of P further into Al and Fe hydroxides and precipitation reactions that will make P less extractable (Bolan et al., 1985). Such reactions would likely continue with time beyond the seven days of this experiment, as soils have been shown to have the ability to continue taking P out of solution well beyond 200 d (Bolan et al., 1985; Maguire et al., 2001a; van der Zee and van Riemsdijk, 1988). There were frequently no significant differences between treatments for individual soils (Table 5). However, when grouped by treatments for all soils, the increase in M3-P was significantly (P < 0.05) greater in the TSP treatment than any of the litter treatments at Days 1 and 7 (except for Day 1 when TSP was not greater than the high NPP litter treatment). When all soils were grouped by treatment, there were almost no significant differences between litters for impact on soil M3-P, but the same trend in change in M3-P was apparent at Days 1 and 7: high > high + phytase > low > low + phytase. This trend in change in M3-P was the same as the trend in total P application rate (Table 3), indicating that the main controlling factor on M3-P when litters are applied to soil is the P load.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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