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NUTRIENT MANAGEMENT & SOIL & PLANT ANALYSIS

Factors Affecting Change in Soil Test Phosphorus Following Manure and Fertilizer Application

Dep. of Crop and Soil Sciences, Cornell Univ., Ithaca, NY 14850

^* Corresponding author(qmk2{at}cornell.edu).

	ABSTRACT

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

 
In many states, Morgan (M) or Mehlich-3 (M3) extractions are the basis for P fertility recommendations and P runoff risk assessment. The impact of timing of sampling, P source, and extractable Al on agronomic soil test P (STP) and P release to the environment are not well understood. Incubation studies were conducted to determine: (i) changes in M-P, M3-P, and 0.01 M CaCl₂–P with time following Ca(H₂PO₄)₂ addition; (ii) the efficiency of liquid dairy manure, NH₄HPO₄, and Ca(H₂PO₄)₂ in raising M-P and M3-P levels; and (iii) the degree to which initial P and extractable Al impact the efficiency of applied P. Twenty-eight noncalcareous soils were incubated at 23°C, in the dark, at field capacity moisture content. Extractable P decreased over 60 d, with a greater proportion of P being lost from the more labile pools (CaCl₂–P > M-P > M3-P). The increase in STP per unit P added (P-M and P-M3) was affected by P source: NH₄HPO₄ raised P levels more efficiently than Ca(H₂PO₄)₂ or liquid dairy manure. There was a positive linear relationship between P and initial STP. Extractable Al was inversely related to P-M and P-M3 and followed exponential decay functions. Extractable Al was positively and linearly related to the amount of P required to raise M-P and M3-P levels 1 mg kg^–1, with Morgan Al as the most accurate predictor of P. Our results suggest that the New York P index should include guidance on sampling time and account for the higher P sorption capacity of soils inherently high in extractable Al.

Abbreviations: CNAL, Cornell Nutrient Analysis Laboratory • LOI, loss on ignition • M, Morgan extraction • M-P, Morgan extractable phosphorus • M3, Mehlich-3 extraction • M3-Al, Mehlich-3 extractable aluminum • M3-P, Mehlich-3 extractable phosphorus • P, change in soil test phosphorus (Morgan or Mehlich-3) per unit P added • STP, soil test P

	INTRODUCTION

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

 
For farmers who are concerned with maintaining both the economic and environmental sustainability of their farms, it is becoming increasingly important to effectively manage P and N inputs. In dairy-based agricultural systems, manure is often applied to fields to meet the N demands of a crop, but given that the N/P ratio in manure is smaller than optimal for crop uptake, P is often overapplied (Laboski and Lamb, 2004). In New York, this has contributed to a steady increase in statewide mean soil test P (STP) levels since 1980 (Ketterings et al., 2005). Currently 47% of soil samples test at or above the agronomic critical soil test level for field crops in New York (Ketterings et al., 2005). Since further increases in soil test P with time can have far-reaching economic and regulatory implications (Czymmek et al., 2003), questions have arisen regarding the impacts of timing of sampling, source of applied P, and inherent soil chemical properties on STP increases following P addition.

There is broad consensus that time-dependent P sorption is biphasic, and thus characterized by an initial rapid phase beginning at P application and lasting up to several days (Van der Zee and Van Riemsdijk, 1988). This is followed by a period of slow sorption, which gradually approaches equilibrium during the subsequent weeks and months (Agbenin and Tiessen, 1995; Griffin et al., 2003). In respect to acidic soils, the dominant view is that P sorbed during the rapid phase is reversibly bound to surface sites of Fe and Al oxide and therefore available for plant uptake and environmental loss (McGechan, 2002). The slow phase is thought to involve the irreversible deposition of P into solid crystalline forms (also Fe and Al oxides) at increasing depths below the surface sorption sites (McGechan, 2002), a process conceptualized by the "unreacted shrinking core" model presented by Van Riemsdijk et al. (1984). Due to the stability of these crystalline structures, P that has undergone slow phase deposition is generally assumed to be unavailable to plants, although opinions differ about the extent to which this process is fully irreversible (Raats et al., 1982; McGechan and Lewis, 2002).

Agronomic soil tests such as the Morgan (Morgan, 1941) and Mehlich-3 (Mehlich, 1984) tests are used as indicators of P plant availability during a growing season. Since these soil tests aim to predict plant-available P, it stands to reason that, after initial increases following P amendment, the pool of P extracted by agronomic soil tests gradually decreases with time (i.e., via slow phase deposition). A similar decrease in environmental P test results with time also would be expected. An incubation study by Griffin et al. (2003) supports these hypotheses, showing that on a soil with high Mehlich-3 extractable Al levels (M3-Al = 1435 mg Al kg^–1), modified Morgan and Mehlich-3 extractable P (M3-P) as well as 0.01 M CaCl₂ and water-extractable P (common environmental P tests) declined rapidly over the first few days after P application, eventually reaching a steady state during the course of 14 to 21 d. Likewise, Agbenin and Tiessen (1995) reported a decline in extractable P with time using anion exchange membranes on five benchmark soils. The decline was more gradual, however, with equilibrium ultimately being reached at approximately 50 d after P application. Additional studies are needed to assess whether the magnitude of change with time is soil specific and, if so, what the implications might be for nutrient management.

In addition to the temporal component, past research suggests that the source of P applied to the soil can also affect the change in soil test P. Studies by Lucero et al. (1995), Reddy et al. (1999), and Griffin et al. (2003) showed differences in soil test response to applied P from a range of inorganic fertilizer (KH₂PO₄, NH₄HPO₄) and animal manure (dairy, beef, poultry, swine) sources. The variable effects of these amendments on P availability have been linked to their differences in P speciation, solubility, pH, organic matter, and the presence of Al, Fe, and Ca (Zhou et al., 1997; Siddique and Robinson, 2003; Griffin et al., 2003; McDowell and Sharpley, 2004; Sato et al., 2005).

Another factor that could impact STP increase following the addition of fertilizer or manure is the initial amount of P in the soil. Studies by Pote et al. (2003) and Griffin et al. (2003) suggested that changes in STP may increase as initial P levels approach P saturation (P_sat) thresholds. In several studies on naturally acidic soils, P_sat was estimated as the ratio of M3-P over the molar sum of M3-Al and Mehlich-3 extractable Fe (Maguire and Sims, 2002a, 2002b; Kleinman and Sharpley, 2002). Maguire et al. (2001) and Griffin et al. (2003) suggested that when the P_sat of a soil is above a certain "change point" threshold, a greater proportion of applied P remains in the soluble fraction extractable by environmental tests. The impacts, however, of the initial amounts of extractable P, Al, and Fe on changes in agronomic soil test P following fertilizer and manure application are not well understood or quantified (Jokela et al., 1998; Magdoff et al.,1999; Griffin et al., 2003).

Our objectives were to: (i) examine the changes in M-P, M3-P, and 0.01 M CaCl₂–P with time following the application of Ca(H₂PO₄)₂; (ii) evaluate the efficiency by which NH₄HPO₄, Ca(H₂PO₄)₂, and liquid dairy manure raise M-P and M3-P levels; and (iii) assess the degree to which initial P, Al, and Fe alter the efficiency of applied P. The 0.01 M CaCl₂ extraction method was selected as an indicator of biologically and environmentally available P (Self-Davis et al., 2000).

	MATERIALS AND METHODS

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

 
Data Sets and Soil Sampling
Two laboratory experiments were performed between 2003 and 2005. For each experiment, soils were selected using the following criteria: (i) Mehlich-3 Al levels ranging from 500 to 1200 mg kg^–1; (ii) Morgan extractable P values <20 mg kg^–1; and (iii) pH levels <7.5. These criteria were established to obtain the widest range of Al levels, to avoid inclusion of calcareous soils, and to ensure that the soils were not P saturated. Calcareous soils with high pH (>7.5) were avoided as they are known to have distinctive chemical properties that affect P sorption. In New York, the agronomic cutoff between high (starter P only) and very high (no additional P needed) soil test P levels is 20 mg P kg^–1 (Morgan) for most field crops (Ketterings et al., 2003). Individual fields with the potential to meet these qualifications were identified using a database of New York soil samples compiled from recent experiments conducted by the Cornell Nutrient Management Spear Program and the database of the Cornell Nutrient Analysis Laboratory (CNAL).

In 2003, 17 fields located across 5 New York counties (Table 1) were sampled. In 2005, to represent the agricultural soil series common to the northern New York region, 11 additional fields were selected and sampled (Table 1). For each of the 28 soils, a composite sample consisting of at least 20 subsamples was taken from the Ap horizon (0–20 cm) using a simple random sampling scheme within a single field. Samples were oven dried (55°C), ground, and passed through a 2-mm sieve before incubation and analysis.

Experiment II: Factors Affecting the Efficiency of Applied P and the Amount of P Required to Raise Soil Test P
The 11 northern New York soils collected in 2005 were subjected to five P treatments, each replicated three times (165 experimental units) according to a randomized complete block design. The treatments consisted of one liquid dairy manure P treatment (12 mg P kg^–1 soil), three inorganic fertilizer P treatments (17, 34, and 52 mg P kg^–1 soil), and an untreated control (0 mg P kg^–1 soil). Three composite samples of the liquid dairy manure (47 g kg^–1 solids) were analyzed for total P (1073 mg kg^–1) and total Ca (3516 mg kg^–1) following a HNO₃–HClO₄ digestion (Peters et al., 2003) and using a TJA 61E ICP–AES (Thermo Electron Corp., Waltham, MA). Oven-dried soil samples (420 g) were added to 0.5-L glass incubation vessels. The liquid dairy manure and inorganic fertilizer P solutions (NH₄HPO₄ dissolved in water) were added to each sample based on the total P content of each at rates expressed in milligrams P per kilogram of soil. The NH₄HPO₄ was applied as monoammonium phosphate (48% P₂O₅). The field capacity of each soil was determined using the methods stated above and ranged from 0.17 to 0.32 kg kg^–1 soil (mean = 0.28 kg kg^–1 soil). After correcting for the moisture content of the manure and fertilizer solutions, distilled water was added to bring the soil mixture to mean field capacity. Soils were mixed thoroughly and packed to a volume of 300 mL to achieve a common bulk density of 1.4 g mL^–1 for all units. Vessels were incubated in the dark at room temperature (23°C) for 28 d. Twice a week, water was added to maintain field capacity. After 28 d, soils were oven dried (55°C), crushed, and passed through a 2-mm sieve. Subsamples were analyzed for pH and Mehlich-3 extractable P, Al, Ca, and Fe at Brookside Laboratories, and Morgan extractable P, Al, Ca, and Fe at CNAL, using the procedures described above and summarized in Table 2. Results from this incubation were pooled with data from Exp. I (Day 14 results) to compare the efficiency of the three P sources [NH₄HPO₄, Ca(H₂PO₄)₂, and liquid dairy manure] in raising STP.

Statistical Analysis
For both experiments, the MIXED procedure in SAS (SAS Institute, 2001) was used to generate multiple linear regressions and estimates of variance and covariance via the restricted maximum likelihood method. This is a standard analytical approach for multilevel data that include repeated measurements with time (Littell et al., 1996). Among the factors influencing CaCl₂–P, M-P, and M3-P levels in Exp. I, soil type and replicate were assumed to be random effects while time and fertilizer rate were regarded as fixed. Changes in M-P and M3-P (P per unit P added) were calculated for each soil type by subtracting the extractable P of control samples (P_control) from the extractable P of samples amended with P (P_amended) from either fertilizer or manure, and dividing by the P application rate. Both the change in soil test P (P) and P application rate are expressed in milligrams P per kilogram soil.

[1]

In Exp. II (combined data set), soil type and replicate were considered random effects, while the fixed effects were P source and the initial P, Al, and Fe extracted by the Morgan and Mehlich 3 tests. For each soil test, the effect of Morgan or Mehlich-3 extractable Al (mg kg^–1) on the efficiency of the P source was further evaluated using nonlinear regression analysis and was fit to the following modified three-parameter exponential decay function using SigmaPlot 8.0 (SYSTAT, Richmond, CA):

[2]

The amount of P required to raise agronomic soil test P levels by 1 mg kg^–1 was calculated as the reciprocal of the efficiency of applied P (i.e., 1/P per unit P applied). This dependant variable was evaluated for the effect of extractable Al using regression analysis and plotted according to the linear model using SigmaPlot 8.0.

	RESULTS AND DISCUSSION

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

 
Effect of Time and Three Rates of Fertilizer P on Soil Test P
Extractable P from the 0.01 M CaCl₂, Morgan, and Mehlich-3 STP pools were each significantly affected by time and P application rate, as well as their interaction effect (Table 3). For all soil tests, P levels increased with fertilizer application rate but decreased throughout the 60-d incubation (Table 4). The magnitude by which time affected each STP pool and the time point at which significant reductions were observed differed among soil tests. The CaCl₂–P fraction was reduced by the greatest proportion (36–44%), with the majority of those losses observed in the second half of the incubation (Days 28–60). The M-P levels declined more steadily throughout the incubation, with a mean loss of 24% across all fertilizer P levels. While there was an effect of time on the M3-P pool, the magnitude of that effect was minimal, reducing extractable P only 8 to 10% between Days 14 and 60. The most labile pools of P appeared to be more affected by slow-phase P sorption (CaCl₂–P > M-P > M3-P) when viewed in relative terms. Our results are in agreement with the work of Agbenin and Tiessen (1995), who showed significant declines in CaCl₂–P, M-P, and M3-P during a 50-d period. The gradual downward trend in extractable P for all soil test P pools is consistent with the kinetics of slow-phase P sorption, which approaches a stable equilibrium P concentration during an extended time period, rather than the rapid adsorption reaction that takes place immediately following fertilizer application and dissolution. From these results, we conclude that soils sampled within 2 mo of the most recent P application are likely to have not yet reached equilibrium and may thus overestimate baseline agronomic P for the next cropping cycle or planning year.

It is well documented that the risks of P loss are elevated immediately following manure or fertilizer application and subsequently decrease with time and with exposure to repeated rainfall events (Sharpley, 1995; Sharpley and Moyer, 2000). It is also important, however, to establish sampling protocols for the assessment of baseline P levels when sorption is at or near equilibrium. Both the New York P runoff index (Czymmek et al., 2003) and the New York P fertilizer guidelines for field crops (Ketterings et al., 2003) assume that agronomic soil test P levels are at a steady state.

The use of the Morgan and Mehlich-3 tests as the basis for P runoff indices in various states is often justified by their strong relationship with P extracted using standard environmental tests, usually via either a dilute salt solution (0.01 M CaCl₂) or a distilled water extraction (McDowell and Sharpley, 2001a, 2001b, 2002; Sims et al., 2002). A number of researchers have taken this idea a step further and drawn a connection between agronomic soil tests and the dissolved residual P in surface runoff and leachate samples (Sharpley, 1995; Sharpley and Moyer, 2000; Sharpley et al., 2001; McDowell and Sharpley, 2001a, 2001b). While the validity of these predictive methods is not in question, our results suggest that if these pools lose P at different rates following fertilizer application, the accuracy of M-P and M3-P based predictions of CaCl₂–P may also change with time.

Past studies have shown poor correlation between Morgan and Mehlich-3 P (Klausner and Reid, 1996). Ketterings et al. (2002) showed that the inclusion of additional soil properties (pH, Ca, and Al) can result in more accurate Mehlich-3 to Morgan P conversion models. Our analysis suggests that since the M-P and M3-P pools lose P at different rates following P application, the timing of sampling may be yet another source of variation in such Mehlich-3 to Morgan P conversion models.

Factors Affecting the Efficiency of Applied P and the Amount of P Required to Raise Soil Test P
Effect of P Source on Efficiency of Applied P
The rate of P application had no effect on P-M or P-M3 per unit P added across all 28 soils (Table 5), so for further assessments of the impact of P source, initial P, and Al on P, we pooled all the application rates within the fertilizer P sources. Changes in STP were significantly affected by P source (Table 6), with NH₄HPO₄ more efficiently raising P levels than either Ca(H₂PO₄)₂ or liquid dairy manure. There were no differences observed between Ca(H₂PO₄)₂ or liquid dairy manure in their ability to raise M-P or M3-P. On average, NH₄HPO₄ was approximately two times more effective at raising M-P and M3-P levels than either Ca(H₂PO₄)₂ or liquid dairy manure on a per-unit-P-added basis (Table 5).

View this table: [in this window] [in a new window]   Table 5. Efficiency of fertilizer and dairy manure P sources in raising P levels in the Morgan (P-M) and Mehlich-3 (P-M3) soil test pools.

View this table: [in this window] [in a new window]   Table 6. Summary results evaluating the effects of P source, initial P level, and extractable Al and Fe on changes in Morgan (P-M) and Mehlich-3 (P-M3) P per unit P added. Analysis combines data from Exp. I and II.

A number of researchers have observed that P from various fertilizer and manure sources can have differential effects on the chemistry of P sorption and availability (McDowell and Sharpley, 2004). Griffin et al. (2003) found significant differences in the efficiency by which KH₂PO₄, beef, dairy, poultry, and swine manure raised modified M-P and M3-P, and argued that a significantly higher fraction of P from KH₂PO₄ remains soluble during a 90-d period relative to P from manure sources. Zhou et al. (1997) suggested that P sorption may increase when sufficient amounts of organic matter are present and in association with Al, Fe, and Ca compounds. In particular, the substantial amount of CaCO₃ and organic carboxylate functional groups contained in poultry litter are thought to increase P sorption and reduce P availability by both increasing soil pH and providing additional sorption sites (Brock et al., 2007; Siddique and Robinson, 2003, 2004; Pote et al., 2003; Sato et al., 2005). Other studies, however, have suggested that the modest amounts of CaCO₃ contained in liquid dairy manure are insufficient to cause significant reductions in P availability (Siddique and Robinson, 2003). The liquid dairy manure and Ca(H₂PO₄)₂ treatments each contained Ca compounds with the potential to bind P, but further research is needed to determine what caused NH₄HPO₄ to be twice as effective in raising M-P and M3-P levels.

Effects of Initial P and Extractable Al on the Efficiency of Applied P
The initial P status of individual soils had a significant effect on both P-M and P-M3 (Table 6), suggesting that soils already high in initial P may exhibit greater changes in STP following P application. This result supports those of Pote et al. (2003) and Griffin et al. (2003), who found that increases in water-extractable P, CaCl₂–P, anion exchange membrane extraction P, modified M-P, and M3-P induced through P application were positively related to initial background P levels. This is not surprising, because soils with greater P saturation are likely to release more P into plant-available pools. The soil evaluated in the Griffin et al. (2003) study was amended with high rates of P fertilizer (resulting in M3-P levels of 150, 471, 732 mg P kg^–1) before the incubation with manure and KH₂PO₄ (the actual treatments in the study) to establish baseline P levels that span the range above and below the expected P saturation change point. The significant effect of initial P in our experiment was more surprising, given that the soils were selected to have P levels below New York's "very high" soil test category (<20 mg M-P kg^–1), at which, for corn (Zea mays L.), additional P would still be recommended (Ketterings et al., 2003). These results suggest that the initial P level should be taken into account when estimating soil test increase following the application of inorganic fertilizer or manure, even for soils within the agronomic response range.

We observed a relationship between P and extractable Al for both the Morgan and Mehlich-3 tests (Table 6). Extractable Fe, which if present in sufficient amounts can also bind P, had no effect on either P-M or P-M3 after accounting for the effects of initial P level and Al. The inverse relationship observed between extractable Al and P-M was best described by a modified three-parameter exponential decay function and the nonlinear regressions generated for both NH₄HPO₄ and liquid dairy manure were significant (Fig. 1A). While the relationship between extractable Al and P-M3 followed the same nonlinear function in respect to NH₄HPO₄ (albeit with lower predictive accuracy than Morgan results), nonlinear regressions for liquid dairy manure were not significant (Fig. 1B). The effect of extractable Al on P-M and P-M3 following the addition of granular Ca(H₂PO₄)₂ also displayed similar trends (data not shown).

View larger version (17K): [in this window] [in a new window]   Fig. 1. Effect of extractable Al on the change in Morgan P (P-M) and Mehlich-3 P (P-M3) per unit P added following the addition of P from NH4HPO4 and liquid dairy manure.

Effects of Extractable Al on the Amount of Applied P Required to Raise Soil Test P
It is recognized that in some situations, manure disposal needs may result in applications that exceed both agronomic P requirements and crop removal rates. In these situations, producers often ask how quickly they should expect soil test levels to rise with repeated overapplication of P. Our results suggest that soils with high extractable Al levels may have a greater capacity to bind this additional P. The amount of P needed (as fertilizer or manure) to raise Morgan and Mehlich-3 STP levels is shown in Fig. 2. For both NH₄HPO₄ and liquid dairy manure, the amount of P required to raise M-P levels by 1 mg kg^–1 increased linearly as Morgan extractable Al levels increased (Fig. 2A). As not all P in liquid dairy manure is readily plant available, the comparatively higher amounts of manure P required to raise soil test levels were expected. The amount of NH₄HPO₄ required to raise M3-P levels by 1 mg kg^–1 was also positively and linearly related to M3-Al, but while significant, the high levels of variance yielded weaker predictive accuracy than the Morgan results (Fig. 2B). These results are similar to those reported by Jokela et al. (1998), who used this relationship to determine the amount of fertilizer needed to raise modified Morgan P levels by 1 mg kg^–1 so that modifications based on soil Al content could be made to Vermont's fertilizer P guidelines. Again, our work shows that this approach may also be used in tandem with the Morgan test in New York and further extended to P applied via dairy manure. There was no significant relationship between M3-Al and the amount of P from dairy manure required to raise M3-P levels by 1 mg kg^–1. This result is attributed to both the higher variability in the liquid dairy manure data and the lower statistical power inherent in the fewer points analyzed in this treatment.

View larger version (20K): [in this window] [in a new window]   Fig. 2. The relationship between extractable Al and the amount of applied P from NH4HPO4 and liquid dairy manure required to raise Morgan P (M-P) and Mehlich-3 P (M3-P) soil test levels 1 mg kg–1.

	CONCLUSIONS

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

 
Our results demonstrate that the level of P, as measured by the CaCl₂, Morgan, and Mehlich-3 soil tests, can change dramatically during the first 2 mo following fertilizer application. Of particular relevance to states that base their nutrient management policies on the Morgan soil test, we recorded a mean decrease in Morgan extractable P of 24% between 14 and 60 d in closed containers and in the absence of plants. While field experiments are needed to establish the role of plant uptake and environmental losses on soil test P, the present study suggests that producers and nutrient management planners should be able to limit some of the year-to-year variation by adopting a sampling protocol that allows soil test levels to stabilize so that they become representative of P status for the next cropping cycle or planning year. Our data suggest waiting 2 mo following the most recent fertilizer or manure application.

Phosphorus applied in the form of liquid NH₄HPO₄ was nearly twice as effective as P from either granular Ca(H₂PO₄)₂ or liquid dairy manure in raising M-P and M3-P levels per unit P added. Understanding the efficiency of these P sources, as well as others not included in this study, may help in the refinement of best management practices that address crop nutrient demands and environmental risks.

Across a wide range of noncalcareous New York soils, soils higher in initial P showed a larger increase in STP per unit P added, while soils with higher Al content exhibited less dramatic changes, presumably due to greater P sorption. The results of this study suggest that producers and planners in New York who are aiming to avoid excessively high New York P runoff index values, should consider the Al content of soils when deciding where to apply excess P. To aid in making manure management decisions, modifications to the New York P runoff index based on Morgan Al levels might be needed for noncalcareous New York soils. Additional research is needed to conclude if similar trends appear for calcareous soils.

	NOTES

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

 
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Received for publication June 14, 2006.

	REFERENCES

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

 

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