Comparison of three persulfate digestion methods for total phosphorus analysis and estimation of suspended sediments

Applied Geochemistry 78 (2017) 357e362

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Comparison of three persulfate digestion methods for total phosphorus analysis and estimation of suspended sediments Elizabeth Ann Dayton a, *, Shane Whitacre a, Christopher Holloman b a b

School of Environment and Natural Resources, The Ohio State University, Columbus, OH, United States Information Control Company, Columbus, OH, United States

a r t i c l e i n f o

a b s t r a c t

Article history: Received 20 October 2016 Received in revised form 11 January 2017 Accepted 12 January 2017 Available online 20 January 2017

As a result of impairments to fresh surface water quality due to phosphorus enrichment, substantial research effort has been put forth to quantify agricultural runoff phosphorus as related to on-field practices. While the analysis of runoff dissolved phosphorus is well prescribed and leaves little room for variability in methodology, there are several methods and variations of sample preparation reagents as well as analysis procedures for determining runoff total phosphorus. Due to the variation in methodology for determination of total phosphorus and an additional laboratory procedure required to measure suspended solids, the objectives of the current study are to i. compare the performance of three persulfate digestion methods (Acid Persulfate, USGS, and Alkaline Persulfate) for total phosphorus percent recovery across a wide range of suspended sediments (SS), and ii. evaluate the ability of using Al and/or Fe in digestion solution to predict SS as a surrogate to the traditional gravimetric method. Percent recovery of total phosphorus was determined using suspensions prepared from soils collected from 21 agricultural fields in Ohio. The Acid Persulfate method was most effective, with an average total phosphorus percent recovery of 96.6%. The second most effective method was the USGS with an average total phosphorus recovery of 76.1%. However, the Alkaline Persulfate method performed poorly with an average 24.5% total phosphorus recovery. As a result application of Alkaline Persulfate digestion to edge of field monitoring may drastically underestimated runoff total phosphorus. In addition to excellent recovery of total phosphorus, the Acid Persulfate method combined with analysis of Al and Fe by inductively coupled plasma atomic emission spectrometry provides a robust estimate of total SS. Due to the large quantity of samples that can result from water quality monitoring, an indirect measure of total SS could be very valuable when time and budget constraints limit the number of procedures that can be run on a single water sample. © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Editorial handling by Joyanto Routh. Keywords: Agricultural runoff Phosphorus Persulfate digestion

1. Introduction Phosphorus (P) in runoff from agricultural fields can lead to impairment of fresh surface water quality (Carpenter, 2005; Carpenter et al., 1998; Daniel et al., 1998; Gerdeaux, 2009; Schelske, 2009; Sharpley et al., 1994). As a result, substantial research effort has been put forth to quantify agricultural runoff P as related to on-field practices (Dayton et al., 2003; Hart et al., 2004; Jiao et al., 2011; Sharpley, 1995; Sharpley et al., 1994; Sharpley and Smith, 1989; Sharpley and Syers, 1979; Skwierawski

* Corresponding author. 410B Kottman Hall, 2021 Coffey Rd., Columbus, OH 43210, United States. E-mail address: [email protected] (E.A. Dayton).

et al., 2008; Lee and Jones-Lee, 2004). The gross forms of P typically reported are runoff dissolved P (DP), operationally defined as P in solution that passes a 0.45 mm filter, and runoff total P (TP), which is the sum of DP plus sediment bound P associated with runoff suspended sediments (SS). While the analysis of runoff DP is well prescribed and leaves little room for variability in methodology, there are several methods and variations of sample preparation reagents and procedures for runoff TP. Acid persulfate digestion (AcidPD) by USEPA method 365.3 (USEPA, 1978), as recommended by Southern Extension and Research Activity-17 (SERA-17, Kovar and Pierzynski, 2009), has been applied to small plot simulated rainfall runoff (Allen and Mallarino, 2008; Bundy et al., 2001), stream monitoring (Bishop et al., 2005; Gentry et al., 2007; Lowrance et al., 2007), and tile drainage (Gentry et al., 2007). A method developed by the

http://dx.doi.org/10.1016/j.apgeochem.2017.01.011 0883-2927/© 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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United States Geological Survey, in which an alkaline/acid persulfate digestion (USGS) starts out alkaline and ends at an acidic (<2.2) pH due to the thermal degradation of persulfate to sulfuric acid, was developed using surface and ground water samples (Patton and Kryskalla, 2003). Other similar alkaline/acid persulfate digest methods have been used to evaluate sea water (Valderrama, 1981), and lake water (Hosomi and Sudo, 1986; Lambert and Maher, 1995). In addition, an alkaline persulfate (AlkPD) method (Koroleff, 1983) has been used to evaluate agricultural edge-of-of-field runoff water (King et al., 2016; Williams et al., 2016). While all of these methods have been employed in a variety of applications, little work has been done to evaluate their performance as determined by percent recovery of TP in agricultural runoff samples that often contain substantial amounts of SS. Stuntebeck et al. (2011) reported SS up to 3.1 g L
1 and TP up to 42.6 mg L
1 using a sulfuric acid digestion USEPA method 365.4 (USEPA, 1974) in a 2003e2008 Wisconsin edge-of-field monitoring study. The alkaline/acid digestion procedures have been shown to deliver good TP recoveries (90e100%) from standard reference materials (chlorella and pond sediment) in lake water suspensions up to 0.1 mg L
1 TP (Lambert and Maher, 1995). However, the study recommended dilution of samples containing more than 0.1 mg L
1 TP to achieve optimum recovery. Hosomi and Sudo (1986) also reported good recoveries with the alkaline/acid digestion procedure when applied to a standard reference material prepared as a suspension at a dry mass SS concentration of 0.05 g L
1. However, these studies were approximately 400 times lower in TP and sixty times lower than the maximum sediment concentrations reported by Stuntebeck et al. (2011) in agricultural edge of field runoff samples. In addition to the sample preparation reagents and procedures for TP analysis, there is variation in the analytical instrumentation and methods used to analyze the digestion solution. Most of the TP results, regardless of the preparation procedure, report using manual or automated colorimetry (Allen and Mallarino, 2008; Bishop et al., 2005; Bundy et al., 2001; Gentry et al., 2007; Hosomi and Sudo, 1986; King et al., 2016; Koroleff, 1983; Lambert and Maher, 1995; Lowrance et al., 2007; Valderrama, 1981; Williams et al., 2016). Colorimetric analysis of TP relies not only on conversion of sediment bound P to dissolved P in the digestion but also on full conversion of all dissolved P forms to PO4
3. However, while instrumental analysis by inductively coupled plasma atomic emission spectrometry (ICP-AES) or mass spectrometry (ICP-MS) also relies on the digestion method to convert sediment bound P to dissolved P, ICP is insensitive to dissolved P forms and measures all P forms in solution. In addition to analytical differences between colorimetric and ICP analysis, there can be a substantial difference in equipment and operating cost. Manual colorimetry using a spectrophotometer is a very inexpensive method of providing accurate TP measures when the digestion method efficiently converts all forms of P to PO
3 4 . Automated colorimetry instrumentation is an expensive initial investment, but subsequent operating costs similar to those of manual colorimetry. As a result, the most costly analysis is by ICP, with similar instrumentation cost as automated colorimetry, but higher operating costs with the large amount of argon required to fuel the plasma. However, analysis of post-digest TP by ICP offers the advantage of evaluating additional elements in the digestion solution (Cantarero et al., 2002) with little additional effort. The ability to simultaneously measure multiple elements, thus providing an opportunity to estimate SS, may offset the higher cost of ICP analysis. The major metals that make up soil (Smith et al., 2005) are aluminum (Al) and iron (Fe) and therefore SS from edge of filed runoff. Previous research has used SS (Sadeghi et al., 2012) and turbidity (Yao et al., 2016) measures to predict elemental content

including Al and Fe in natural waters by regressing elemental content (y) vs. turbidity or SS (x). In edge of field runoff monitoring, water samples that are digested for TP and measured by ICP could also acquire data for Al and Fe in digestion solution. As a result, major elements Al and Fe from agricultural runoff are variables that can be evaluated for prediction of SS, which would normally require an additional laboratory method to obtain. Due to the variation in methodology for determination of TP and an additional laboratory procedure required to measure SS, the objectives of the current study are to i. compare the performance of three persulfate digestion methods (AcidPD, USGS, and AlkPD) for TP percent recovery across a wide range of suspended sediments, and ii. evaluate the ability of using Al and/or Fe in digestion solution to predict SS as a surrogate to the traditional gravimetric method. 2. Materials and methods 2.1. Soil suspensions for testing In order to evaluate performance, percent recovery of TP was determined using suspensions prepared from soils collected from 21 agricultural fields in Ohio. In order to calculate the TP in suspension, soil samples were analyzed for dry mass TP by USEPA Method 3051a, a microwave assisted aqua regia digestion method (USEPA, 2007a). National Institute of Standards and Technology (NIST) San Joaquin soil standard reference material (SRM) was utilized to evaluate the ability of USEPA Method 3051a to recover TP from sediments. San Joaquin is an agricultural soil with a certified TP content of 620 mg P/kg. In addition, as measure of P lability, the plant available P (Mehlich, 1984) was also determined and is presented as M3-P. 2.2. Total P recovery Soil suspensions (2 from each of the 21 fields), with known TP and SS were prepared to target a range of 0.10e1.5 g L
1 SS. Suspensions were homogenized by overhead stirring (1500 rpm) while 10 mL subsamples were extracted and then subjected to AcidPD, USGS, and AlkPD (Table 1). 2.3. Suspended solids prediction Due to the results observed in the persulfate method comparison, the Fe and Al recovered in the AcidPD digestion solution was selected for evaluation of a predictive relationship with SS. A subset (297) edge of field runoff samples collected in calendar year 2013 were selected to provide an expected range in SS based on a range in Fe and Al in the AcidPD. The SS in the 297 subsamples were determined gravimetrically by filtration of 100 mL of homogenized sample through oven dried (103  C) and weighed 0.45 mm filter (American Public Health Association et al., 1995). The resulting collection on the filter paper was again oven dried (103  C) and mass of SS determined by difference. The reporting limit for SS was established at 10 times the standard deviation of the mass of 10 blank filters processed along with the samples. The 100 mL sample volume resulted in a detection limit of 0.02 g SS L
1. 2.4. ICP analysis Instrumental analysis, for all digestion and extractions for P, Al, and Fe, was conducted by ICP-AES according to USEPA method 6010C (USEPA, 2007b). Quality control operations included, method detection limit (MDL) determination, initial calibration and verification and blank, continuing calibration verification and blank every 10 samples and a low limit of quantitation verification every

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Table 1 Summary of procedures for three persulfate total phosphorus (TP) digestion methods; acid persulfate (AcidPD), alkaline/acid persulfate USGS, and alkaline persulfate (AlkPD). Digestion Procedure

Persulfate Digestion Reagent

Sample:Digestion Solution

Temp (C)

Time (min)

AcidPD (USEPA, 1978) USGS (Patton and Kryskalla, 2003) AlkPD (Koroleff, 1983)

0.4 M Na2S2O8a 0.15 M K2S2O8 þ 0.15 M NaOH 0.185 M K2S2O8 þ 0.375 M NaOH þ 0.485 M H3BO3

10 mL:1 mL þ 0.2 mL H2SO4 10 mL:5 mL 10 mL:1 mL

121 121 121

60 60 45

a

Modified from K2S2O8.

20 samples. In addition, the percent recovery of total P from NIST San Joaquin by the combination of USEPA method 3051a and 6010C was >90%, indicating that the method is suitable for determination of TP in the sediments utilized for this study.

Table 2 Characterization of agricultural soils used to create soil suspensions. Soil series/ texture, field slope, USEPA 3051a total phosphorus (P), and Mehlich3 (M3) extractable P. Field

Soil Series/Texture

3. Results and discussion

M3 P

mg kg
1

2.5. Statistical analysis Digestion method performance was evaluated using percent recovery of TP in suspension. In addition to percent recovery, simple linear regression of known vs. measured suspension TP by each digest method was used to support the percent recovery evaluation. Further, simple linear and multiple regression of TP recovery vs. SS and M3-P were used to determine if decreased method performance was related to SS and labile P as estimated by M3-P in the soil used to create SS. A Bayesian statistical model was used to evaluate Al and Fe to predict SS. One advantage of this modeling strategy is that it can handle censored data (below reporting limit) and complex variance structures via a universal model fitting tool; Markov chain Monte Carlo. The model fitted to the data is essentially a regression model. However, the standard regression model assumes that the error variance is equal across all observations. The fitted model allowed the standard deviation of the observations to increase linearly with the prediction.

3051a P

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 Min Max Mean

Bennington silt loam Bennington silt loam Blount silt loam Blount silt loam Blount silt loam Blount silt loam Blount silt loam Blount silt loam Blount silt loam Blount silt loam Blount silt loam Blount silt loam Centerburg silt loam Centerburg silt-loam Centerburg silt-loam Centerburg silt-loam Centerburg silt-loam Hoytville clay loam Hoytville clay loam Latty silty clay Paulding clay

344 694 417 531 757 863 926 962 1068 1118 1204 1530 849 412 441 694 787 661 668 557 320 320 1530 752

18.1 21.3 18.9 35.9 69.6 106 22.4 268 123 241 173 292 75.4 13.5 48.8 34.3 40.8 78.5 11.7 28.8 8.75 8.75 292 82.3

3.1. Soil suspensions Characterization of the soils used to create the soil suspensions as well as the soil series/texture of the source field is presented in Table 2. Total soil P ranged from 320 to 1530 mg kg
1 with a mean of 752 mg kg
1; M3-P ranged from 8.75 to 292 mg kg
1 with a mean of 82.3 mg kg
1. 3.2. Total P recovery Performance of the three persulfate digest methods varied widely. The AcidPD method was most effective, with percent recoveries ranging from 84.5 to 112% and a mean of 96.6% (Fig. 1). In addition, known suspension TP is highly correlated (r2 ¼ 0.98) with AcidPD P (Fig. 2A) and has a slope not significantly different from one, indicating that the AcidPD effectively recovers TP across a range of SS potentially transported with runoff water. Due to the good recovery of TP by the AcidPD method, there was no significant relationship (r2 ¼ 0.026, P < 0.01) between AcidPD recovery and SS (Fig. 3A). This result is expected since AcidPD is the prescribed method to determine TP, regardless of form (USEPA, 1978; Kovar and Pierzynski, 2009). The USGS method was the second most effective method, with percent recoveries ranging from 47.5 to 124% with a mean of 76.1% (Fig. 1). The regression of TP vs. USGS P (Fig. 2B) was highly significant (r2 ¼ 0.90) and slope of 1.31. In addition, there was a significant (r2 ¼ 0.35, P < 0.01) negative relationship between USGS P recovery and SS (Fig. 3B), indicating that slightly lower recovery of

Fig. 1. Total P digestion recovery with the Acid Persulfate (AcidPD), USGS, and Alkaline Persulfate (AlkPD) digestion procedures. Box boundaries indicate the 25th and 75the percentiles, whisker boundaries represent the 10th and 90th percentiles, and the line in the box represents the median.

TP can be expected as SS increases. This indicates that the USGS method may extract slightly less TP than the AcidPD, especially with higher SS, but the average recovery (76.1%) of the USGS method approaches a common QC limit (±20%) for digestion accuracy using laboratory control samples (USEPA, 2007c, 2014a,b).

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Fig. 2. Known suspension total P (TP) concentration vs. digestion solution P for acid persulfate (A), USGS (B), and alkaline persulfate (C) digestion procedures.

Fig. 3. Digest recovery of total P vs. suspended sediment for Acid Persulfate (A), USGS (B), and Alkaline Persulfate (C) digestion procedures vs. suspended sediment in the suspensions. Significant (P < 0.01) relationships for USGS and Alkaline persulfate digestion methods indicated (*).

As a result, the USGS method may be sufficient to meet the data quality objectives for many water quality projects. The USGS method also has the additional benefit of analyzing for total nitrogen (TN) without the need for an additional digestion procedure. The performance of the AlkPD was poor, with percent TP recoveries ranging from 9.79 to 56.5% and a mean of 24.6% (Fig. 1). This is also apparent in the regression of TP vs AlkPD P (Fig. 2C), with a slope of 3.64. In addition, AlkPD had the least significant correlation with TP (r2 ¼ 0.81), indicating that AlkPD does not extract a consistent fraction of TP and therefore a predicted TP using the regression (Fig. 1C) would have potential for error. In addition, there was a significant (r2 ¼ 0.21, P < 0.01) negative relationship between AlkPD recovery and SS (Fig. 3C). The poor performance is likely due to a final solution pH in the alkaline persulfate digestion of approximately 5.5 (Koroleff, 1983; Valderrama, 1981) which results in limited dissolution of sediment bound P. As a result, AlkPD underperforms at increasing levels of SS where P may be strongly sorbed to mineral surfaces (Lindsay, 2001). Unlike the AcidPD and USGS methods, considerably more variability in TP extracted was explained with P lability as demonstrated in the significant (P < 0.01) multiple regression (eq. (1)) relationship between TP and the combination of SS and M3-P, which indicates that TP recovery by AlkPD is positively related to M3-P and negatively related to SS.

%TP recovery ¼ 0:092ðM3
PÞ
10:4ðSSÞ þ 23:8; R2 ¼ 0:70 (1) Similar to the USGS method, there is a decrease in extraction efficiency with increased SS. However, the increase in efficiency associated with an increase in M3-P is a result of increased P solubility with increased M3-P. This result is in agreement with previous P solubility studies that demonstrated relationships between P solubility and M3-P both without (Pote et al., 1996; Sibbesen and Sharpley, 1997) and with inflections points (Dayton et al., 2014; McDowell et al., 2002; McDowell and Sharpley, 2001; Sharpley et al., 2001) after which the slope of the relationship P solubility and M3-P increases. For example (Dayton et al., 2014), found that below 181 mg P kg
1, a doubling in M3-P resulted in a 1.68 fold increase in P solubility, but above the inflection point a doubling in M3-P resulted in a 3.44 fold increase in P solubility. Due to these limitations of AlkPD to recover TP from samples, the AlkPD is the least applicable for agricultural runoff, especially when the source field M3-P is low and/or SS in runoff is high. 3.3. Suspended solids prediction The measured SS in 297 edge of field runoff samples ranged from 0.0012 to 3.12 g L
1 with a mean of 0.230 g L
1. The Al in the digestion solutions ranged from 0.830 to 150 mg L
1 with a mean of 13.4 mg L
1. Similarly, Fe in the digestion solution ranged from

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0.0796 to 156 mg L
1 with a mean of 10.6 mg L
1. Of the 297 observations in the dataset, 46 were censored at the gravimetric sediment reporting limit (0.02 g L
1) for model fitting. The linear regression representation of the model fittings for Al and Fe are presented in Fig. 4. These results demonstrate that both Al and Fe are predictive of SS, but Fe has a slightly stronger correlation than Al. As a result, the laborious and expensive task of measuring SS gravimetrically on filters may be supplanted by the measurement of Fe in the AcidPD solution. In order to achieve lower detection limits by the gravimetric method, larger sample volumes are required which increases time and expense. In edge of field monitoring collection, large sample volumes are not always possible due to the way in which samples are collected by the instrumentation. Using the concentration of Fe in the digestion solution however, is independent of sample volume and therefore a more sensitive measure of SS. This was demonstrated in the 46 samples that were below the gravimetric detection limit using 100 mL of sample but not in the Fe measured by ICP using only a 10 mL subsample. The relationship between SS and Fe presumes SS from soil particulates and therefore may not be valid for SS from other sources such as manure or other organic amendments. Therefore the relationship between SS vs. Fe (Fig. 4) demonstrates an approach to predict SS and should be tested/validated with SS originating from other soil types or management systems before being applied to predict SS. 4. Conclusions Significant efforts are being put forth to address the surface water quality issues resulting from agricultural P runoff. Data presented here illustrates the importance of a demonstrable percent TP recovery in digestion method selection. Digestion performance is vital to drawing conclusions that may impact farmer management decision making and have policy implications. However, there is no standard method in the literature for quantifying runoff TP in agricultural research. The average TP recovery of 96.6% with the AcidPD confirms robust performance, especially for samples with SS present. The second most appropriate method was the USGS with an average TP recovery of 76.1%. As a result, if determination of runoff TP and TN is necessary, the USGS method may be sufficient depending on the data quality objectives of the project. The AlkPD performed poorly with an average 24.5% TP recovery. As a result application of AlkPD to edge of field monitoring may

Fig. 4. Gravimetric suspended solids in edge of field runoff samples vs. Acid Persulfate (AcidPD) Al and Fe solution concentrations.

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drastically underestimated runoff TP, especially as SS increases. In addition to excellent recovery of TP, the AcidPD method combined with analysis of Al and Fe by ICP provides a robust estimate of total SS. Due to the large quantity of samples that can result from water quality monitoring, an indirect measure of total SS could be very valuable when time and budget constraints limit the number of procedures that can be run on a single water sample. Acknowledgements This work was funded by USDA-NRCS Conservation Innovation Grant (69-3A75-12-231), The Ohio Soybean Council and Ohio Corn & Wheat. Thanks to Peter McDonough, Research Associate, and Annie Smith, Research Assistant, at The Ohio State University, for their assistance in this work. References Allen, B.L., Mallarino, A.P., 2008. Effect of liquid swine manure rate, incorporation, and timing of rainfall on phosphorus loss with surface runoff. J. Environ. Qual. 37 (1), 125e137. American Public Health Association, American Water Works Association, Water Pollution Control Federation, 1995. Standard Methods for the Examination of Water and Wastewater, nineteenth ed. American Public Health Association, Washington, D.C. (variously paged). Bishop, P.L., Hively, W.D., Stedinger, J.R., Rafferty, M.R., Lojpersberger, J.L., Bloomfield, J.A., 2005. Multivariate analysis of paired watershed data to evaluate agricultural best management practice effects on stream water phosphorus. J. Environ. Qual. 34 (3), 1087e1101. Bundy, L.G., Andraski, T.W., Powell, J.M., 2001. Management practice effects on phosphorus losses in runoff in corn production systems. J. Environ. Qual. 30 (5), 1822e1828. Cantarero, A., Lopez, M.B., Mahia, J., Maestro, M.A., Paz, A., 2002. Determination of total and dissolved phosphorus in agricultural runoff samples by inductively coupled plasma mass spectrometry. Commun. Soil Sci. Plant Anal. 33 (15e18), 3431e3436. Carpenter, S.R., 2005. Eutrophication of aquatic ecosystems: bistability and soil phosphorus. Proc. Natl. Acad. Sci. U. S. A. 102 (29), 10002e10005. Carpenter, S.R., Caraco, N.F., Correll, D.L., Howarth, R.W., Sharpley, A.N., Smith, V.H., 1998. Nonpoint pollution of surface waters with phosphorus and nitrogen. Ecol. Appl. 8 (3), 559e568. Daniel, T.C., Sharpley, A.N., Lemunyon, J.L., 1998. Agricultural phosphorus and eutrophication: a symposium overview. J. Environ. Qual. 27 (2), 251e257. Dayton, E.A., Basta, N.T., Jakober, C.A., Hattey, J.A., 2003. Using treatment residuals to reduce phosphorus in agricultural runoff. J. Am. Water Works Assoc. 95 (4), 151e158. Dayton, E.A., Whitacre, S.D., Holloman, C.H., 2014. Demonstrating the relationship between soil phosphorus measures and phosphorus solubility: implications for Ohio phosphorus risk assessment tools. J. Gt. Lakes Res. 40 (3), 473e478. Gentry, L.E., David, M.B., Royer, T.V., Mitchell, C.A., Starks, K.M., 2007. Phosphorus transport pathways to streams in tile-drained agricultural watersheds. J. Environ. Qual. 36 (2), 408e415. Gerdeaux, D., 2009. Phosphorus and eutrophication of fresh waters. Mechanisms and consequences in large lakes. Oceanis 33 (1e2), 75e86. Hart, M.R., Quin, B.F., Nguyen, M.L., 2004. Phosphorus runoff from agricultural land and direct fertilizer effects: a review. J. Environ. Qual. 33 (6), 1954e1972. Hosomi, M., Sudo, R., 1986. Simultaneous determination of total nitrogen and total phosphorus in freshwater samples using persulfate digestion. Int. J. Environ. Stud. 27 (3e4), 267e275. Jiao, P.J., Xu, D., Wang, S.L., Zhang, T.Q., 2011. Phosphorus loss by surface runoff from agricultural field plots with different cropping systems. Nutr. Cycl. Agroecosyst. 90 (1), 23e32. King, K.W., Williams, M.R., Fausey, N.R., 2016. Effect of crop type and season on nutrient leaching to tile drainage under a corn-soybean rotation. J. Soil Water Conserv. 71 (1), 56e68. Koroleff, J., 1983. Determination of total phosphorus by alkaline persulphate oxidation. In: Grasshoff, K., Ehrhardt, M., Kremling, K. (Eds.), Methods of Seawater Analysis. Verlag Chemie, Wienheim, pp. 136e138. Kovar, J.L., Pierzynski, G.M., 2009. Methods of phosphorus analysis for soils, sediments, residuals, and waters. In: Southern Cooperative Series Bulletin 408: Southern Extension and Research Activity (SERA), p. 17. Lambert, D., Maher, W., 1995. An evaluation of the efficiency of the alkaline persulfate digestion method for the determination of total phosphorus in turbid waters. Water Res. 29 (1), 7e9. Lee, G.F., Jones-Lee, A., 2004. Assessing the water quality impacts of phosphorus in runoff from agricultural lands. Environ. Impact Fertil. Soil Water 872, 207e219. Lindsay, W.L., 2001. Chemical Equilibria in Soils. John Wiley and Sons Ltd. Lowrance, R., Sheridan, J.M., Williams, R.G., Bosch, D.D., Sullivan, D.G., Blanchett, D.R., Hargett, L.M., Clegg, C.M., 2007. Water quality and hydrology in

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