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a Department of Crop, Soil, and Environmental Sciences, University of Arkansas, Fayetteville, AR 72701
b USDA-ARS, Poultry Production and Product Safety Research Unit, Fayetteville, AR 72701
c USDA-NRCS, National Water Management Center, Little Rock, AR 72203
d USDA-ARS, Pasture Systems and Watershed Management Research Unit, University Park, PA 16802
* Corresponding author (pdelaun{at}uark.edu)
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: HAP, high available phosphorus • SRP, soluble reactive phosphorus • TP, total phosphorus • TSP, triple superphosphate
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Annual applications of animal manures to land for several years that exceed plant P removal increase soil test P levels, which leads to greater P losses in surface runoff water (Pote et al., 1996, 1999; Sharpley, 1995; Schreiber, 1988). Most of these researchers have found a positive linear relationship between soil test P and the soluble reactive phosphorus (SRP) concentration in surface runoff water. As a result of these studies, threshold soil test P levels have been established in some regions of the United States above which P application is not recommended or allowed. However, manure applications were generally not made within 1 to 2 yr of the research and runoff concentrations were typically lower than what would be observed from pastures receiving recent or annual manure applications.
Although soil test P is being used to guide P management in some states, factors such as manure application, fertilizer application, crop type, erosion, surface runoff, and residue management may override soil P in determining potential P losses (Sharpley and Tunney, 2000). Sauer et al. (2000) showed that poultry litter applications overwhelmed the soil P influence on SRP concentrations in surface runoff water. Kuykendall et al. (1999) suggested that for pastures receiving routine applications of poultry litter, soil test P should not be the sole criterion for determining the P runoff potential. If soil test P is not a dominant factor controlling P runoff from fertilized pastures, then P management guidelines should reflect the other factors that may contribute to P losses in runoff.
Gburek et al. (2000) suggested that both source and transport factors should be incorporated in agricultural management practices for P. Several management practices have been studied that reduce the risk of P runoff. Precipitation of SRP using alum has been shown to reduce soluble P concentrations in both manure and surface runoff water (Moore and Miller, 1994; Shreve et al., 1995; Moore et al., 2000). Manipulation of animal diets using phytase enzymes and low phytic acid corn also reduces total P in manure (Nelson et al., 1971; Beers and Jongbloed, 1992; Raboy et al., 1994). Transport factors that consider the timing and occurrence of rainfall events have shown that P runoff decreases with an increase in length of time between a manure application and a runoff event (Westerman and Overcash, 1980; Sharpley, 1997). Phosphorus transport has also been shown to occur predominantly from hydrologically active areas of a watershed (Gburek et al., 1996; Gburek and Sharpley, 1998).
A few simulation programs have attempted to calculate edge-of-field P losses. However, none of the existing models provide users with a simple tool to measure the relative potential of P movement from sites based on readily available field parameter values (Lemunyon and Gilbert, 1993). To account for both P transport factors and P sources, the P index was developed to provide conservation planners, watershed planners, and farmers with a simple tool to assess the various landforms and management practices for the potential risk of P movement (Lemunyon and Gilbert, 1993). However, weighting factors for each of the risk variables in the original P index were based on professional judgement. Because the P index was not based on scientific data, a modification of the P index using scientific data should strengthen the ability of the P index concept to evaluate locations and management alternatives for P losses.
Several states have modified the original P index to better assess the risk of P movement in localized regions. Most of the modified P indices contain weighting factors that were assigned by regional and state specialists in consultation with local field conservationists. Information from actual runoff studies were not incorporated into many of the currently used P indices. The objective of this series of studies was to determine the effect of (i) soil test P, (ii) soluble P in poultry litter, (iii) P in poultry diets, (iv) fertilizer type, and (v) poultry litter application rates on P concentrations in surface runoff water. The results from these runoff studies were then used to develop a P source component for a P index for pastures fertilized with poultry litter.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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After the initial construction of the plots was completed, the concentration of P in the soil was augmented on 24 plots with commercial P fertilizer. Triple superphosphate (TSP, 200 g P kg–1) was incorporated to about 15 cm at rates of 0, 168, 336, 672, 1008, and 1344 kg P ha–1. Triple superphosphate was used to increase soil test P since it would not result in the input of other nutrients or contaminants, such as arsenic, copper, zinc, and 17ß-estradiol, that may have been added using organic fertilizers. The addition of organic fertilizer could have also increased infiltration rates of the plots.
Effect of Soil Test Phosphorus
The first study was conducted on the P-augmented plots to determine the effect of soil P on P concentrations in runoff. The experimental design was a randomized complete block design with four replications of six treatments consisting of six levels of P augmentation. Two rainfall simulations were conducted 1 wk apart to determine the relationship between soil P and P runoff. A third rainfall simulation was conducted to determine the relationship between soil P and P runoff after poultry litter application. One day before the third rainfall simulation, poultry litter was applied to each plot at a rate equivalent to 5.60 Mg ha–1.
Effect of Diet
The effect of dietary reductions in P was evaluated in a second runoff study. There were four treatments replicated three times in a randomized complete block design. Treatments included litter from poultry that were fed diets consisting of a (i) normal diet, (ii) phytase diet, (iii) high available phosphorus (HAP) corn diet, and (iv) HAP + phytase diet. Litter samples from poultry that were fed these diets were collected from a study conducted in Delaware and complete details of formulated diets were provided by Saylor et al. (2001). The litter had been deep-stacked for 6 to 8 mo before collection and shipping. Litter was applied at rates equivalent to 4.48 Mg ha–1 (dry wt.) 1 d before the first rainfall simulation. Rainfall simulations were conducted 1, 7, and 14 d after litter application.
Effect of Alum Amendment
A third study was conducted to evaluate the effect of soluble P in manure on P runoff. The experimental design consisted of a randomized complete block design with six replications of four treatments consisting of (i) normal poultry litter, (ii) poultry litter treated with 5% alum, (iii) poultry litter treated with 10% alum, and (iv) poultry litter treated with 20% alum. Each treatment was applied at rates equivalent to 5.60 Mg ha–1. These plots were the same as those used in the first study (see Effect of Soil Test Phosphorus, above). Each treatment was applied to each of the six levels of soil test P; therefore, there were six replications of each treatment. Rainfall simulations occurred 1, 16, and 21 d after litter application. Litter used for the study was collected from a poultry (broiler) farm in northwestern Arkansas that had six broiler houses, three of which were treated with alum at 10% rates after each flock was removed. The 5% alum treatment was made by mixing one-half alum-treated litter and one-half untreated litter. The 20% alum treatment was made by adding the appropriate amount of alum to the 10% alum-treated litter and mixing.
Effect of Fertilizer Type
A fourth study was conducted to determine the effect of fertilizer type on P runoff. Treatments included (i) an unfertilized control, (ii) normal poultry litter, (iii) alum-treated poultry litter, and (iv) triple superphosphate. All fertilizer types were applied at a similar total P rate (78 kg P ha–1). Poultry litter was applied at rates equivalent to 5.60 Mg ha–1. Treatments were replicated three times in a randomized complete block design. Rainfall simulations were conducted 1, 7, and 14 d after fertilizer application. Both alum-treated and untreated poultry litter were collected from a poultry farm in northwestern Arkansas. Alum was applied at a 10% rate in the broiler house.
Effect of Litter Application Rate
The effect of litter application rates on P runoff was determined in a fifth study. Treatments included poultry litter application rates equivalent to (i) 2.24, (ii) 4.48, (iii) 6.72, and (iv) 8.96 Mg ha–1. Simulated rainfall was applied 1, 7, and 14 d after litter application. Litter was collected from a poultry farm in northwestern Arkansas. Treatments were replicated three times in a randomized complete block design.
Chemical Analysis
A subsample of manure applied to each plot for each study described above was taken for analysis of SRP and total phosphorus (TP). Upon return to the laboratory, 20 g of poultry litter from each sample was placed into a 250-mL polycarbonate centrifuge tube and extracted with 200 mL of double deionized water for 2 h on a mechanical shaker (Self-Davis and Moore, 2000). After each sample was centrifuged, aliquots were filtered through a 0.45-µm membrane and acidified to pH 2 with HCl for SRP analysis. Subsamples of TSP were also taken and extracted with double deionized water. Soluble reactive P was determined colorimetrically using the automated ascorbic reduction method (American Public Health Association, 1998). Total P was determined by digesting oven-dried litter (60°C) with HNO3 and 30% H2O2 and analyzing the digested sample using an inductively coupled plasma spectrometer (ICP) (Zarcinas et al., 1987).
After fertilizer application to each plot, simulated rainfall was applied to each plot using a large rainfall simulator with eight TeeJet 1/2HH-SS30WSQ nozzles (Spraying Systems Co., Wheaton, IL) approximately 3 m above the soil (Miller, 1987). To provide the best coefficient of uniformity and to produce drops with size, velocity, and impact energies characterizing natural rainfall, a water pressure of 28 kPa was regulated at the nozzle to establish a water flow rate of 126 mL s–1 (Shelton et al., 1985). Using solenoid valves, a rainfall intensity of 50 mm h–1 was used. Runoff samples were collected at 2.5, 7.5, 12.5, 17.5, 22.5, and 27.5 min after continuous runoff was achieved from each plot and composited based on flow rates at the time of sampling. Composited runoff samples were filtered through a 0.45-µm membrane and acidified to pH 2 with concentrated HCl. Soluble reactive P concentrations in the runoff water were determined colorimetrically on filtered, acidified samples using the automated ascorbic acid reduction method (American Public Health Association, 1998).
Soil cores were taken from each plot immediately before each rainfall simulation. Composite soil samples consisting of five random cores were taken each for Mehlich-III (0–15 cm) and water-soluble P (0–5 cm). Soil was collected immediately adjacent to each plot to replace the soil within each plot removed via sampling. Cores were dried in an oven for 48 h at 60°C and ground to pass a 2-mm sieve. Mehlich-III P extracts were analyzed using ICP after extracting 2 g of soil with 14 mL of Mehlich-III solution for 5 min and filtering through a Whatman (Maidstone, UK) no. 1 filter (Mehlich, 1984). A 1:7 ratio was used since this is the method used by the University of Arkansas Soil Test Laboratory. Water-soluble P was determined by measuring 2.5 g soil into a 40-mL polycarbonate centrifuge tube and extracting with 25 mL of double deionized water for 2 h on a mechanical shaker (Self-Davis et al., 2000). After each sample was centrifuged, aliquots were filtered through a 0.45-µm membrane and acidified to pH 2 with HCl. Water-soluble P was determined colorimetrically using the automated ascorbic acid reduction method (American Public Health Association, 1998).
Statistical Analysis and Phosphorus Index Development
Analysis of variance was used to determine significant treatment effects on P loss from each runoff event (SAS Institute, 1990). When significance was indicated, means were separated using Fisher's protected LSD (P < 0.05). For linear regression data, an F test was performed using JMPIN to test if the slope was significant (SAS Institute, 1996).
To develop an index based on the data collected from runoff studies, multiple regression was used (SAS Proc Reg, stepwise option). Annual P loads were calculated for each of the 72 plots used in the runoff study. Calculations were made using the following assumptions: (i) six runoff events per year, (ii) 1 cm of runoff per event, and (iii) three runoff events occur before litter application and three occur after litter application. An assumption of six runoff events per year and 1 cm runoff per event was used based on the observation of several studies in Arkansas. Bartholomew et al. (1954) measured 0.75 cm runoff yr–1 for 19 yr from a Newtonia silt loam on ungrazed bermudagrass plots in the Ozark Highlands. Hood and Bartholomew (1956) measured 3.5 cm runoff yr–1 from bermudagrass on a Baxter silt loam over a 10-yr period in central Arkansas. These studies have reported the amount of runoff to be 0.7 and 3.9% of the total precipitation, respectively. Ma et al. (1998) measured 1.35 cm runoff yr–1 from tall fescue on a Captina silt loam in northwestern Arkansas. Sauer et al. (2002) recently reported that runoff from one-acre watersheds occurred in response to three precipitation events and totaled 3.6 cm of water or 2.6% of the total precipitation that fell at the site during the 14-mo study period (3.1 cm yr–1). Edwards et al. (1994) measured 19.3 cm runoff yr–1 from a Captina silt loam soil cropped to tall fescue. The average annual runoff volume from published reports on silt loam soils in Arkansas is 5.6 cm yr–1, which is similar to the 6 cm runoff yr–1 assumed in the loading calculations. An assumption of 6 cm of runoff per year in the Ozark Highlands means that it is assumed that approximately 5% of the annual precipitation contributes to runoff.
Load calculations were made using the runoff concentrations from the three runoff events that occurred from each of the fertilized plots after litter application. The load for each runoff event from each plot was calculated and summed. Load calculations for runoff events that occurred before litter application were calculated from unfertilized control plots. The P load resulting from three runoff events from control plots was added to the calculated load from fertilized plots. The calculated P load was set as the dependent variable whereas water-soluble soil P, Mehlich-III P, amount of total P applied, and amount of soluble P applied were set as the independent variables. Parameter estimates were used as the weighting factor for respective variables.
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Initial rainfall simulations showed a positive linear relationship (r2 = 0.86) between SRP concentrations in runoff water and Mehlich-III P from the P-augmented plots (Fig. 1a) . A strong positive linear relationship between SRP in runoff and soil P has also been shown in past studies (Pote et al., 1996, 1999; Sharpley, 1995). Concentrations of P in runoff ranged from 2.32 to 3.2 mg P L–1 from plots receiving 1344 kg P ha–1. This may be attributed to the addition of a highly soluble P source such as TSP to augment soil P levels. Most studies relating soil test P to soluble P concentrations in runoff report SRP concentrations less than 2.0 mg P L–1. Burch et al. (2001) reported soluble P concentrations in runoff of 1.6 mg P L–1 at a Mehlich III–extractable P concentration of 1500 mg P kg–1 in the top 5 cm of soil. On heavily manured soils, a nonlinear diminishing relationship, rather than a strong positive linear relationship, has been shown between Mehlich III–extractable P and soluble P concentrations in runoff water (Burch et al., 2001; Lory et al., 2001; Humphry, 2000). Since poultry litter is a liming agent, the pH of manured soils increases. As calcium (Ca) is added and the pH increases, P appears to be partitioned into less-soluble Ca-bound phases. Thus, precipitation of Ca-phosphate minerals results in a diminishing nonlinear relationship between soil test P and P concentrations in runoff water. Burch et al. (2001) noted this trend once the soil pH increased to greater than 6.4.
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Effect of Soluble Phosphorus on Runoff
In contrast, a significant linear relationship was found between soluble P in the litter and SRP concentrations in runoff water (r2 = 0.75; Fig. 1c). Although SRP concentrations in runoff water were positively correlated to TP in the litter, they were more strongly correlated to the soluble P in the litter (data not shown). A poor relationship also existed between TP in the manure and SRP concentrations in runoff water. Sharpley et al. (2001) also showed that Mehlich III–extractable soil P had little or no effect on P concentration in runoff from soils fertilized with dairy manure. Therefore, it may be concluded that soluble P in manure is one of the most important factors contributing to SRP concentrations in runoff water following litter application.
Effect of Diet on Phosphorus Runoff
Total P levels in the manure from broiler chickens fed low-P diets were significantly lower than that from birds fed normal diets (Table 1). These data support the findings of past studies (Nelson et al., 1971; Jongbloed and Kemme, 1990; Raboy et al., 1994). Poultry and swine lack the phytase enzyme necessary to utilize the P from phytic acid, thereby making the P unavailable to these animals. As a result, only 10 to 20% of the P in corn is available to swine and poultry (Cromwell et al., 1993; Jongbloed and Kemme, 1990). Because nonruminant animals, such as poultry and swine, lack the phytase enzyme, animal producers must add inorganic phosphate, such as defluorinated phosphate, to animal diets to provide sufficient levels of P. Addition of phytase to feed results in increased availability of phytate P by converting unavailable phytic acid to a more readily available form of P (Cromwell et al., 1993). An alternative approach to adding phytase enzyme is the use of a low-phytate corn, thus increasing the availability of P in corn grain by genetically manipulating the form of P. Raboy et al. (1990) first noted a concomitant increase in kernel inorganic P with a reduction in phytic acid in corn. Two nonlethal corn mutants, lpa-1 and lpa-2, showed reductions in phytic acid P ranging from 50 to 60% with no decrease in total P in the seed (Raboy and Gerbasi, 1996). As a result of phytase additions, as well as the use of low-phytate corn, inorganic phosphate additions to feed can be dramatically reduced. Therefore, total P concentrations in manure are reduced.
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Although not significant, Moore et al. (1998) found that SRP concentrations in runoff from plots amended with litter from broilers fed a phytase diet were numerically higher (12%) than a standard diet. Smith et al. (2001) found a 25% increase in SRP concentrations in runoff water from plots amended with manure from swine fed a phytase diet compared with a normal diet. While studies have shown that total P concentrations are decreased in litter from modified diets, more research is needed to define how diet modification and litter storage affect the SRP of these manures. Phosphorus is more readily available using phytase as it releases inorganic orthophosphate from phytate. The activity of the phytase enzyme could potentially continue in stored manure and increase the soluble P content of the manure. Controlling SRP concentrations in litter is particularly important when litter is surface-applied to pastures, since P runoff from pastures is predominantly of the soluble form. As phytase decreases the total P content of manures, the addition of chemical amendments to reduce soluble P levels in the manure could result in reduced P concentrations in runoff water. Smith et al. (2001) found a 41% reduction in P concentrations in runoff when adding AlCl3 to manure from swine fed a phytase diet compared with standard diets with an equal AlCl3 rate. Reducing total P concentrations in manure could also reduce increases in soil P with continuing litter application.
Effect of Alum Amendment on Phosphorus Runoff
Mean runoff concentrations for the first runoff event after litter application were 26.0 mg P L–1 for normal litter, 15.1 mg P L–1 for 5% alum, 13.4 mg P L–1 for 10% alum, and 0.88 mg P L–1 for 20% alum. A significant linear relationship was found between the amount of alum in the litter and SRP concentrations in the runoff water (Fig. 3a)
. Soluble reactive P concentrations significantly decreased as the alum rate increased. Soluble reactive P concentrations in runoff were reduced by 42% with 5% alum, 49% with 10% alum, and 97% with 20% alum. These data support the findings of Shreve et al. (1995), who found that alum additions to poultry litter reduced P runoff by 87% from small plots of tall fescue. Moore et al. (2000) showed a 73% reduction in SRP concentrations in runoff from field-scale watersheds fertilized with alum-treated litter receiving natural rainfall compared with those amended with normal litter for a 3-yr period. Alum additions to litter in broiler houses also improve bird performance parameters, such as weight gain and feed conversion and lower energy use, enhancing economic returns for both integrators and growers (Moore et al., 1999). Moore and Miller (1994) showed that alum additions to poultry litter greatly decreased water-soluble P, with virtually 100% of the soluble P being removed from solution in some instances. The exact mechanism in which P solubility is reduced by alum is still unclear.
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Effect of Poultry Litter Application Rate on Phosphorus Runoff
Phosphorus concentrations in runoff increased linearly as the P application rate increased for each rainfall event. Phosphorus concentrations in runoff were 8.8, 16.6, 27.7, and 33.0 mg P L–1 for applications of 2.24, 4.48, 6.72, and 8.96 Mg ha–1, respectively, for the first runoff event (Fig. 3c). A positive linear relationship was seen for each simulated rainfall event, although the magnitude of SRP concentration in runoff decreased with each rainfall event (Fig. 3c). This would be expected if the soluble P concentration in poultry litter is a major contributor to SRP concentrations in runoff. The mean SRP concentration in runoff water from plots fertilized with litter at rates equivalent to 2.24 Mg ha–1 was 4.75 mg P L–1 after the third runoff event. The Mehlich-III P concentration in the soil for the fertilized plots was not significantly different from the unfertilized plots. Mean Mehlich-III P concentrations (0–15 cm) were 103 and 102 mg P kg–1 for unfertilized control plots and plots receiving 2.24 Mg ha–1 litter application, respectively. These data suggest that even after three runoff events, soluble P applied in manure was still overwhelming the effect of soil P with respect to P concentrations in runoff water.
Determination of Weighting Factors for the Phosphorus Source Components
Both water-soluble soil P and Mehlich-III P have been shown to be positively correlated to P concentrations in runoff water when no manure has been applied (Pote et al., 1996, 1999). However, a weak correlation was found between soil test P and P concentrations in runoff water in this study (Fig. 1b) as well as previous studies soon after manure has been applied (Pierson et al., 2001; Sauer et al., 2000; Sharpley and Moyer, 2000). Both water-soluble soil P (r = 0.24) and Mehlich-III soil P (r = 0.24) were poorly correlated with annual P loading in runoff water from plots receiving P amendments (Table 2).
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To determine the weighting factors of the P source component, P loads in runoff were initially modeled using Mehlich-III soil P, water-soluble soil P, TP application rates, and soluble P application rates. Results showed that both water-soluble soil P and Mehlich-III P concentrations did not make a significant contribution to the model, whereas both soluble P and TP application rates were highly significant (P < 0.0001). Water-soluble soil P and Mehlich-III soil P were highly correlated (y = 4.54x + 115; r = 0.93). The Mehlich-III extraction procedure is used by many laboratories that perform routine soil analysis. Therefore, Mehlich-III rather than water-soluble soil P was used in the model. Although total P applications were highly correlated to resulting runoff P loads, SRP in the litter is more highly correlated to runoff P loads. The dominant form of P found in runoff from pastures is SRP. Thus, SRP was used in the model.
When the multiple regression analysis was performed, with water-soluble P in the soil and TP in litter omitted, results showed that the weighting factors for the source factors were 0.404 for soluble P application rate (P < 0.0001) and 0.000666 for soil test P (P > 0.2; Mehlich III) (r2 = 0.74, RMSE = 1.11) (Eq. [1]). Although Mehlich-III soil P was not significant, it was included in the index to account for background levels of P in runoff or for fields in which poultry litter has not been applied, thereby providing an estimate of P loss for both fertilized and unfertilized fields. The vast difference in weighting factors is attributed to the much larger values of Mehlich-III P in soil compared with soluble P applied in the manure. Whereas Mehlich-III values for soils are often on the order of 100 to 1000 lb P acre–1 (112 to 1120 kg ha–1), soluble P applied in manure is typically 2 to 3 lb P acre–1 (2.24 to 3.36 kg ha–1). These weighting factors were assigned to P source factors in the P index for pastures (Eq. [1]):
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CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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