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Published online 12 October 2005
Published in J Environ Qual 34:1944-1951 (2005)
DOI: 10.2134/jeq2004.0468
© 2005 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
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TECHNICAL REPORTS

Ecological Risk Assessment

Environmental Index for Estimating the Risk of Phosphorus Loss in Calcareous Soils of Manitoba

D. V. Ige, O. O. Akinremi* and D. N. Flaten

Department of Soil Science, University of Manitoba, Winnipeg, Canada R3T 2N2

* Corresponding author (akinremi{at}ms.umanitoba.ca)

Received for publication December 10, 2004.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The degree of phosphorus saturation (DPS) has been used in evaluating the risk of P loss from soil to runoff. While techniques are available for calculating DPS for acid soils, no widely used technique exists for neutral to calcareous soils that are typical of the Northern Great Plains, including Manitoba (Canada) soils. This study aimed to develop techniques of calculating the DPS of neutral to alkaline soils. Four measures of soil labile P and ten indices of P sorption capacity were used to calculate the DPS of 115 Manitoba soils. The various DPS calculated were evaluated using water-extractable (H2O) P as an index of P susceptibility to runoff loss. The DPS obtained using Olsen-extractable (Ols) P and the Langmuir adsorption maximum (ESmax) ranged from 0.5 to 31.9% while those obtained from POls and the single-point adsorption index (P150) ranged from 0.9 to 73.9%. Of all the DPS evaluated, those that included POls and Mehlich 3–extractable (M3) P as the numerator with either P150 or ESmax as the denominator were fairly well correlated with PH2O (r values ranged between 0.45 and 0.63). Along with ESmax and P150, a new method of calculating DPS was formulated as the ratio of POls or PM3 to CaM3 or (Ca + Mg)M3. We found that the ratio of ammonium oxalate–extractable (ox) P to (Al + Fe)ox, which has been widely used to calculate DPS in acid soils, was not suitable for neutral to alkaline soils of Manitoba. In these neutral to alkaline soils, CaM3 or (Ca + Mg)M3 were better indices of P sorption capacity while POls and PM3 provided better estimates of labile soil P. The DPS calculated using CaM3 or (Ca + Mg)M3 were well correlated with PH2O; however, they were numerically smaller than those obtained from the Langmuir adsorption maximum. As such, a saturation coefficient ({alpha}) with a value of 0.2 was generated to improve the numerical values of the newly estimated DPS. This new approach can be used to estimate the DPS in neutral and calcareous soils without the need to generate a P adsorption maximum.

Abbreviations: DPS, degree of phosphorus saturation • ESmax, estimated Langmuir adsorption maximum • ex (subscript), exchangeable • H2O (subscript), water extractable • M3 (subscript), Mehlich 3 extractable • Ols (subscript), Olsen extractable • ox (subscript), ammonium oxalate extractable • P150, phosphorus sorption index at 150 mg L–1 of added P


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
THE BUILDUP OF SOIL P, beyond the range considered optimum for most agronomic crops, increases the potential of P loss through runoff (Sharpley and Tunney, 2000). Several methods have been devised to assess the risk of P loss from soil to surface water. These include simple measurements such as soil test phosphorus (STP) (Haygarth and Jarvis, 1999; Sharpley and Tunney, 2000), the degree of phosphorus saturation (DPS) (Breeuwsma and Silva, 1992; Sharpley and Tunney, 2000), the phosphorus sorption index (PSI) (Hughes et al., 2000), and the phosphorus index (PI) (Lemunyon and Gilbert, 1993; Bolinder et al., 1998). Of these, PI has been the most robust and widely used index of the risk of P loss from a particular field as it incorporates soil management and transport factors into a single index that has been used to estimate P loss to runoff (Sharpley et al., 2003). An important component of the PI can be the DPS or the PSI, neither of which is available for Manitoba soils.

The DPS is an environmental index to assess the potential for the release of P to runoff and leaching (Zhou and Li, 2001). Research conducted on acid soils has indicated that the degree of P saturation is a useful criterion for predicting P in surface and drainage water (Breeuwsma et al., 1995). Leclerc et al. (2001) reported a good relationship between PH2O and the DPS as estimated by Pox, Alox, and Feox for several Quebec soils (pH 5.4–6.8). Sharpley (1995) and Sims et al. (1998) found a high correlation between runoff dissolved P and DPS obtained by dividing the Pox by the P sorption maximum determined from the Langmuir isotherm for a range of soils in the United States. However, Pox was found to be a poor predictor of the P retention capacity of neutral to calcareous soils (Ige et al., 2005).

A variation of the DPS based on the Olsen-P soil test and the soil PSI was suggested by Hughes et al. (2000). Different regions use different methods for the extractable P depending on soil type. While Pox has been used for acid soils, the use of POls and PM3 had been suggested for neutral and calcareous soils. Nair et al. (2004) suggested the use of Mehlich 3 for estimating DPS in areas where this extractant is routinely used as an agronomic soil test.

Another component of the DPS is the capacity factor as represented by the P sorption capacity. Various methods of estimating P sorption capacity have been used to calculate DPS. These methods range from the P sorbed from single point isotherm, to the use of various soil properties as indices of adsorption capacity (Hughes et al., 2000; Maguire and Sims, 2002). For acid soils, the sum of Alox and Feox is used as an estimate of P sorption capacity (Schoumans, 2000) with Pox as the intensity factor. In Quebec, Canada, AlM3 is currently used as an index of P sorption in evaluating DPS for the soils in this region (Khiari et al., 2000). For Manitoba soils, Ige et al. (2005) have shown that (Ca + Mg)M3 is a better estimate of P sorption capacity than either (Al + Fe)ox or AlM3.

Degree of phosphorus saturation is preferred to STP in assessing the risk of P loss to runoff because DPS takes into account the capacity of soils to retain P and therefore gives a better correlation with drainage dissolved P across different soil types. Sharpley et al. (1996) reported a good correlation between dissolved runoff P and PM3 (R2 = 0.72) but a better correlation (R2 = 0.86) was obtained between dissolved runoff P and the DPS. Degree of phosphorus saturation has been used in the Netherlands to limit the loss of P in surface and ground waters and a threshold DPS (i.e., DPS > 25%) has been identified as contributing to water pollution (Breeuwsma and Silva, 1992).

While techniques are available for calculating the DPS of acid soils, no widely used technique exists for neutral to calcareous soils that are typical of the Northern Great Plains, including Manitoba (Canada). Thus, the objectives of this study were to develop techniques of calculating the DPS for neutral to calcareous soils, obtain values of DPS for representative Manitoba soils, and evaluate these DPS using the PH2O as an index of the soils' potential to release P to runoff.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
One-hundred fifteen archived surface soil samples were collected from five distinct soil groups across the province of Manitoba. These soils are representative Manitoba soils covering a wide range of soil properties including texture (ranging from clay to sand), organic matter, and the carbonate content (Table 1). The physical and chemical properties of the soils such as soil texture, pH, organic matter, and carbonate content were obtained from the Agricultural Resources Section, Manitoba Agriculture, Food and Rural Initiatives.


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  		Table 1. Range of physicochemical properties of 115 Manitoba soils used in the single point adsorption study.

 
For all the samples, water (H2O), Olsen (Ols), Mehlich 3 (M3), and ammonium oxalate (ox)–extractable P, Ca, Mg, Fe, Al, and Mn were determined (Ige et al., 2005). The different soil extracts were analyzed colorimetrically for P using the molybdate blue method (Murphy and Riley, 1962). The extractable Ca, Mg, Fe, Al, and Mn were determined by inductively coupled plasma–atomic emission spectroscopy. The exchangeable cations and the cation exchange capacity of the soil were determined using the ammonium acetate method (McKeague, 1978).

Phosphorus Adsorption Study
The single point adsorption experiments were conducted for the entire collection of 115 soil samples, as described by Bache and Williams (1971) using a P concentration of 150 mg P L–1. Two grams of air-dried, sieved soil were weighed into a 50-mL centrifuge tube and 20 mL of solution containing 150 mg P L–1 in 0.01 M KCl was added. Two drops of toluene were added to inhibit microbial activity. The suspension was placed on an end to end shaker and shaken for 24 h at room temperature. The samples were then centrifuged at 10000 rpm (7000 x g) for 10 min and filtered through a 0.45-µm filter. The P in solution was determined colorimetrically by molybdate blue method. The amount of P adsorbed, P150, was determined by the difference between the amount of P added to the soil and the equilibrium P solution concentration.

Twenty-six soils from the entire collection, which included all the soil groups and all the range of adsorption indices, were used to determine the multipoint adsorption isotherm. The Langmuir adsorption maximum was determined for this subset by equilibrating 2 g of soil with 20 mL of 0.01 M KCl solution containing different concentrations of P (0, 1, 5, 15, 50, 150, 250, 400, 600, 800, and 1000 mg L–1). The results obtained were fitted into the linear form of the Langmuir equation to obtain the adsorption maximum (Ige et al., 2005). The relationship generated between the Langmuir adsorption maximum and the P150 for the 26 soils was extrapolated to all 115 soils to estimate the Langmuir adsorption maximum (ESmax) from the measured P150. No significant difference (p = 0.10) was observed between the estimated adsorption maxima and the measured adsorption maxima for the 26-soil subset.

Degree of Phosphorus Saturation
The DPS, defined as the ratio of extractable or labile P to the adsorption capacity of soil, was obtained for all the soils using the relationship:

[1]
where the extractable P was the soil test P as determined from the different extracting agents, expressed in mmol kg–1, and P sorption index was the various estimates of P sorption capacity of the soil, also expressed in mmol kg–1.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
General Soil Properties
The soils, which are representative of the major soils of Manitoba, cover a wide range of soil properties (Table 1). The soils were mostly alkaline, with pH range of 5.3 to 8.1 and an average pH of 7.2. This was expected, as most Manitoba soils have developed from calcareous parent materials. Soil development had resulted in the gradual decrease in the pH and carbonate content in the surface (0–15 cm) of these soils; hence, the carbonate content of the soils varied widely ranging between 0 and 382 g kg–1 with an overall mean of 32 g kg–1. The organic carbon content ranged between 1 and 97 g kg–1. The texture of the soils ranged between heavy clayey (780 g kg–1 clay) and sand (20 g kg–1 clay). The mean values from the texture analysis were 466 g kg–1 sand, 251 g kg–1 silt, and 285 g kg–1 clay.

The Alox ranged between 5.4 and 72 mmol kg–1 while Feox ranged between 4.1 and 92.5 mmol kg–1. These values are small when compared with the results obtained by Samadi and Gilkes (1998) who reported Alox in the range of 45 to 103 mmol kg–1 and Feox in the range of 27 to 586 mmol kg–1 in calcareous soils of Western Australia. Dalal and Hallsworth (1977) reported values between 13 and 16 mmol kg–1 for Feox and values 2.2 to 56 mmol kg–1 for Alox for some alkaline sodic soils.

Phosphorus Sorption Properties of Soil
Wide variation was observed in the adsorption capacities due to wide variation that existed in the physicochemical properties of the soils. The adsorption capacity, P150, ranged between 2.8 and 28.8 mmol kg–1 (Table 2); thus, the adsorption capacity can be classified to range from low to very high according to the classification of Juo and Fox (1977). The estimated adsorption capacity, ESmax, was higher than P150 and ranged from 8.4 to 49.1 mmol kg–1 with a mean of 23.3 mmol kg–1 for the 115 soils (Table 2).


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  		Table 2. Variation in the intensity and capacity factors, used to calculate the degree of P saturation, among the different soil groups.{dagger}

 
Degree of Phosphorus Saturation
Degree of P saturation was calculated using Eq. [1]. Two parameters taken into consideration in the calculation were the intensity factor, which represented the labile P pool or the extractable P, and the capacity factor representing the P sorption capacity. In this study, three forms of extractable P (POls, PM3, and Pox) were used as the intensity factor for calculating DPS. The values of the different forms of extractable P varied widely (Table 2) as would be expected from widely varied soil types. While Pox ranged from 3.9 to 26.7 mmol kg–1 with a mean of 12.7 mmol kg–1, POls and PM3 ranged between 0.2 and 6.6 and 0.2 and 5.2 mmol kg–1, respectively (Table 1). Olsen is the commonly used soil test P method in Manitoba due to the calcareous origin of the soils, and Mehlich 3 has been used in few cases. Ammonium oxalate–extractable P has been widely used in the literature for calculating the degree of P saturation in soils (Van der Zee and Van Riemsdijk, 1988; Breeuwsma and Silva, 1992). While Borling et al. (2004) suggested the use of POls for DPS estimation, Khiari et al. (2000) employed PM3 in calculating the DPS for Quebec soils.

For the capacity factor, ten indices of P sorption capacity were evaluated. These include P150, ESmax, CaM3, MgM3, (Ca + Mg)M3, exchangeable (ex) Ca, Mgex, (Ca + Mg)ex, (Al + Fe)ox, and (Al + Fe)M3 (Table 2). While the estimated Langmuir adsorption, ESmax, represents the actual capacity of the soil to retain P, P150 is an index of the soil capacity to retain P and is more easily determined. Mehlich 3–extractable Ca, MgM3, (Ca + Mg)M3, Caex, Mgex, and (Ca + Mg)ex were considered as indices of soil P sorption capacity because of the significant correlation between these parameters and the estimated Langmuir adsorption maximum (Ige et al., 2005). Ammonium oxalate–extractable (Al + Fe) and (Al + Fe)M3 have been used by some researchers (e.g., Van der Zee and Van Riemsdijk, 1988; Breeuwsma and Silva, 1992; Maguire and Sims, 2002) as indices of P retention capacity in calculating DPS for some soils. The values of CaM3 ranged between 14.0 and 549 mmol kg–1, those of MgM3 ranged between 2.9 and 80.6 mmol kg–1, while (Al + Fe)ox ranged from 10.1 to 118 mmol kg–1 (Table 2). The Caex ranged from 7.2 to 197.8 mmol kg–1 and Mgex ranged from 1.7 to 106 mmol kg–1. The values of CaM3 were generally higher than those of Caex indicating that, in addition to Caex, Mehlich 3 dissolved other forms of soil Ca. This may account for the better correlation between CaM3 and P sorption capacity (ESmax) and (Ca + Mg)M3 and ESmax compared to Caex and ESmax and (Ca + Mg)ex and ESmax as reported by Ige et al. (2005).

On the whole, 30 different forms of DPS were generated using the different combinations of the three extractable P and ten indices of P sorption capacity. The DPS obtained from these parameters ranged between 0.04 and 1057% with an overall mean of 18.4%. Table 3 shows the ranges of DPS values obtained from the different methods adopted for DPS determination. The DPS values obtained varied widely due to the wide variations in the parameters used in generating the values.


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  		Table 3. Range of the degree of phosphorus saturation (DPS) calculated for Manitoba soils using different parameter combinations and the correlation coefficient (r) with water-extractable P.

 
Evaluation of the Calculated Degree of Phosphorus Saturation of Manitoba Soils
For the DPS to be an effective environmental indicator of P loss potential, a good correlation with the form of soil P most susceptible to runoff and leaching losses is imperative. Several authors (e.g., Yli-Halla et al., 1995; Pote et al., 1996, 1999) have reported highly significant correlation between dissolved reactive P concentration in surface runoff and PH2O. Thus for the study, PH2O was used as an index of soil P form that is readily lost to runoff. Good correlations (p < 0.05) were observed between PH2O and DPS estimated using either POls or PM3 as the intensity factor and CaM3, (Ca + Mg)M3, and P150 or ESmax as the capacity factor (r values ranged between 0.45 and 0.63; Table 3). However, the correlation between PH2O and DPS derived from (Ca + Mg)ex was not as good as those derived from (Ca + Mg)M3. This may be due to the acid nature of Mehlich-3 extracting agent, which is able to extract other reactive forms of Ca and Mg, which could be important to soil P retention in addition to Ca and Mg at the exchange sites.

A poor correlation (r = 0.16; p > 0.1) was obtained between DPS, calculated using Pox and (Al + Fe)ox, and PH2O (Table 3). This confirms the results reported by Ige et al. (2005) that Alox and Feox are poor indicators of the P adsorption capacity of these neutral to alkaline soils. Although the ratio of Pox to (Al + Fe)ox has been used as an index of soil P saturation in acid soils (Breeuwsma and Silva, 1992) our study shows that this method is not appropriate for neutral to alkaline soils of Manitoba. Estimation of DPS using PM3 as the extractable P and (Al + Fe)M3 as the index of P sorption (denominator), as suggested by Maguire and Sims (2002), was poorly correlated with PH2O (r = –0.04; p > 0.10), further indicating that the sum of extractable Al and Fe was not an effective index of P sorption for the alkaline soils in our study.

Of all the DPS methods evaluated for the 115 soils, those that included POls and PM3 as the numerator with either P150 or ESmax as the denominator gave fairly good correlation with PH2O (r = 0.45–0.63; p < 0.05). This may be because these extractants (Olsen and Mehlich 3) are good indices of labile soil P for agronomic purposes. Olsen P is normally recommended for the type of soil in our study. However, a better correlation (p < 0.05) was obtained when PM3 was used as the index of labile P than when POls was used (r = 0.59 for PM3/P150 versus r = 0.45 for POls/P150; Table 3).

The regression plot of various measures of DPS against PH2O showed that the parameters were better related nonlinearly in this study (Fig. 1) . Such nonlinear relationships were reported by Nair et al. (2004). However, a change point in the plot of PH2O against DPS as report by Nair et al. (2004) was not observed in our study. This could be due to the lower DPS values obtained in this study compared to what was reported by Nair et al. (2004). In our study few of the soils studied have DPS values above 40% while in the research work reported by Nair et al. (2004) about 50% of the soils studied had DPS values above 40%.



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  		Fig. 1. Relationship between water-extractable phosphorus (PH2O) and degree of phosphorus saturation estimated as the ratio of either Olsen (POls) or Mehlich 3 (PM3)–extractable P to P sorption indices. CaM3, Mehlich 3–extractable Ca; (Ca + Mg)M3, the sum of Mehlich 3–extractable Ca and Mg; P150, P sorption index at 150 mg L–1 of added P; ESmax, estimated Langmuir adsorption. All relationships are significant at p < 0.01 (n = 112). Three points with PH2O values of 0.9, 1.1, and 2.3 mmol kg–1 were statistically shown to be outliers and were eliminated from the regression plot.

 
Generating Degree of Phosphorus Saturation with a Saturation Factor ({alpha})
Since a good correlation (p < 0.01) was observed between the estimated Langmuir adsorption maximum (ESmax) and (Ca + Mg)M3 as reported by Ige et al. (2005), the DPS was calculated using these parameters as indices of P sorption capacity, just as Alox and Feox have been used for acid soils. The DPS obtained using CaM3 and (Ca + Mg)M3 as indices of P sorption capacity and either POls or PM3 as the intensity factor correlated well (p < 0.05) with PH2O [r = 0.56 for POls/CaM3 and r = 0.46 for PM3/(Ca + Mg)M3] but the absolute values of the DPS obtained were consistently lower than the DPS obtained from the Langmuir adsorption maximum (Table 3). Therefore, as suggested by Van der Zee et al. (1987) for Alox and Feox in acid soils, a saturation factor, {alpha}, was estimated for CaM3 and (Ca + Mg)M3.

The effect of a saturation factor will be to increase the absolute values of the DPS, making them comparable to those generated using the Langmuir sorption maximum. Van der Zee et al. (1987) proposed estimating the maximum saturation factor, {alpha}, as three times the value of the slope of the relationship between the adsorption capacity and (Al + Fe)ox. Using the same approach for (Ca + Mg)M3 and CaM3 in this study, the {alpha} factor was estimated to be 0.2 in both cases.

With the inclusion of an {alpha} value, the absolute DPS values became comparable to those evaluated using the Langmuir adsorption maximum (Table 3). For example, while the value of the DPS obtained for POls/{alpha}(CaM3) ranged from 0.24 to 32%, that based on POls/{alpha}[(Ca + Mg)M3] ranged from 0.21 to 27%, which is comparable with the range of POls/ESmax (0.51–31.9%).

Following the evaluation of DPS by correlating it with the PH2O, the following equations were established for calculating the degree of P saturation for Manitoba soils:

[2]

[3]

[4]

[5]

[6]

[7]

[8]

[9]
where {alpha} = 0.2.

The various estimates of the degree of P saturation were highly correlated with each other as evidenced by the plot of these estimates, with coefficients of determination, R2, ranging from 0.47 to 0.81 (Fig. 2) . The DPS calculated using the same extractable P method as the numerator were highly linearly correlated (e.g., DPSOls 1 and DPSOls 3, R2 = 0.81 or DPSM3 1 and DPSM3 2, R2 = 0.67) while those between the DPS from different extractable P methods (e.g., DPSM3 1 and DPSOls 1) were poorly related. Using the nonlinear approach, the coefficient of determination of the regression between DPSOls 3 and DPSM3 3 was improved from 0.47 to 0.77 (Fig. 2c and 2d).



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  		Fig. 2. Relationship between different measures of degree of phosphorus saturation (DPS). The DPS is estimated as the ratio of either Olsen (POls) or Mehlich 3 (PM3)–extractable P to P sorption indices. CaM3, Mehlich 3–extractable Ca; (Ca + Mg)M3, the sum of Mehlich 3–extractable Ca and Mg; P150, P sorption index at 150 mg L–1 of added P; ESmax, estimated Langmuir adsorption.

 
The DPS obtained using P150 as the P sorption capacity were generally higher than those obtained from other sorption parameters. This was due to the lower magnitude of P150 compared to the Langmuir adsorption maxima, ESmax. While the values of DPSOls 1 (Eq. [2]) ranged between 0.9 and 74%, the values for DPSOls 2 (Eq. [3]) ranged between 0.5 and 32% and the values of DPSOls 3 (Eq. [4]) ranged from 0.2 to 27%. Although the single isotherm, P150, is a good index of P retention capacity of different soils on a comparative basis, it may overestimate the DPS relative to the multipoint isotherm, ESmax.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
The ratio of Pox to (Al + Fe)ox has been widely used as an index of the degree of phosphorus saturation (DPS) in acid soils. Our study showed that this method is not appropriate for neutral to alkaline soils of Manitoba. Of all the DPS evaluated for the 115 soils, those that included POls and PM3 as the numerator with either P150 or ESmax as the denominator gave fairly good correlation with PH2O. Along with ESmax and P150, a new method of calculating the DPS was formulated as the ratio of POls or PM3 to CaM3 or (Ca + Mg)M3. Mehlich 3 appeared to extract the labile portion of Ca and Mg in the soil that is responsible for P retention in neutral to calcareous soils typically found in Manitoba. This requires the use of a saturation factor ({alpha} = 0.2) to make the magnitude comparable to that obtained from the P adsorption capacity. Thus, rather than going through the tedious, time consuming procedure for estimating the adsorption maximum for DPS calculation, the use of CaM3 or (Ca + Mg)M3 proved to be a simpler and faster alternative.


   ACKNOWLEDGMENTS
 
The authors acknowledge the Manitoba Livestock Manure Management Inc. (MLMMI), the Sustainable Development Innovation Fund (SDIF), and the Manitoba Rural Adaptation Council (MRAC) for making funds available for this study. The contributions of Bob Eilers of Agriculture and Agri-Food Canada and Peter Haluschak of Manitoba Agriculture, Food and Rural Initiatives in the selection and provision of archived soil samples are gratefully acknowledged. Sola Ajiboye and Abdul Kashem helped in carrying out initial sorption studies.


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


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