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Soil Science Society of America Journal 64:2018-2024 (2000)
© 2000 Soil Science Society of America

DIVISION S-4-SOIL FERTILITY & PLANT NUTRITION

Phosphorus Fractionation in Biosolids-Amended Soils

Relationship to Soluble and Desorbable Phosphorus

R.O. Maguirea, J.T. Simsa and F.J. Coaleb

a Dep. Plant and Soil Sci., Univ. of Delaware, Newark, DE 19717-1303 USA
b Dep. Nat. Res. Sci., Univ. Maryland, College Park, MD 20742-5821 USA

rmaguire{at}udel.edu


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
Phosphorus has been identified as a major factor involved in decreasing water quality through its role in eutrophication, and there is now a focus on controlling nonpoint agricultural P sources. This work was conducted to identify how biosolids applications under current regulations have affected the forms and release potential of P in agricultural soils. We collected samples from eight farms with a history of biosolids amendments, selecting fields that had setback areas (where biosolids applications were not permitted) to allow comparison of amended and unamended soils. We analyzed these soils for P fractions (soluble P, Al-P, Fe-P, reductant soluble P, and Ca-P; their sum equals total P), sequentially desorbable P (Fe-strip), oxalate P, Al and Fe, Mehlich-1 P, and the degree of P saturation. Our results show that following a N-based biosolids nutrient management plan can significantly increase total P (from 403 to 738 mg kg-1) and initially desorbable P (from 32 to 61 mg kg-1). The main soil components associated with P retention (Alox and Feox) also tended to be increased by biosolids amendment and this may help mitigate P release. Biosolids amendment significantly increased Fe-P (from 137 to 311 mg kg-1), probably due to Fe added to biosolids during production, and there was also a strong trend for higher Al-P where biosolids had been applied. Desorbable P was initially greatest from biosolids sites, but with increasing extractions, the release converged towards that from the setback areas. Mehlich-1 P and Pox were good predictors of desorbable P release, as measured by one and five sequential extractions with Fe-strips. Desorbable P, by both one and five Fe-strip extractions, was more closely correlated with Al-P than Fe-P, especially in setback areas, indicating that Al-P is probably the most important source of desorbable P independent of biosolids amendment. This work indicates the importance of considering P availability at agricultural biosolids application sites and of maintaining setback areas near water bodies, where no biosolids may be applied, to reduce the risk of P losses.

Abbreviations: B, biosolids application site • Desorbable-P1, desorbable P from one extraction with Fe oxide impregnated filter paper strips • Desorbable-P5, cumulative desorbable P from five extractions with Fe oxide impregnated filter paper strips • DPS, degree of P saturation • Pox, Feox, Alox, oxalate extractable P, Fe, and Al • OM, organic matter • PAN, plant-available N • S, setback area with no history of biosolids application

Abbreviations: *, **, *** Significant at the 0.05, 0.01, and 0.001 levels of probability


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
PHOSPHORUS is well known for its role in eutrophication and has been suggested as one of the causative factors in outbreaks of Pfiesteria spp., a toxic dinoflagelate that has been linked to human health problems (Burkholder and Glasgow, 1997). Buildup of soil P due to the long-term land application of manures and fertilizers, and the increased risk this poses for P loss to surface waters, is well documented in the mid-Atlantic region of the USA (Sharpley et al., 1996; Sims et al., 2000). Changes in legislation and policy now mandate more intensive management of all forms of agricultural P, including biosolids P. For example, Maryland's Water Quality Improvement Act of 1998 mandated N- and P-based nutrient management plans for those using commercial fertilizers by 2001 and for those using animal manures or municipal biosolids by 2005 (Simpson, 1998). While under the Delaware Nutrient Management Act of 1999, the "application of P to `high' P soils cannot exceed a 3 yr crop removal rate" (Sims, 1999). In practice this means high P soils will only receive P applications once every 3 yr.

Land application of biosolids is a widespread and well-established process governed by the national 503 rule in the USA (USEPA, 1994). This rule alludes to the need for biosolids applications at "agronomic rates" that do not over-apply P, but in practice biosolids are usually applied according to crop N requirements as long as loading rates for some trace elements are not exceeded. Several researchers have shown that applying biosolids following a N-based management plan will lead to an oversupply of P relative to crop requirements (Kelling et al., 1977; Kick, 1981). For example, Pierzynski (1994) calculated that applying biosolids that contained 15 g kg-1 plant-available N (PAN) and 10 g kg-1 total P to meet corn (Zea mays L.) N requirements (150 kg PAN ha-1) added 110 kg P ha-1 relative to P removal in the harvested grain of {cong}25 kg P ha-1. This over-application of P in turn can lead to an accumulation of P in soils (Kelling et al., 1977; Peterson et al., 1994). However, the plant availability of biosolids P can be less than that of manure or fertilizer P. McCoy et al. (1986) attributed low availability of P in biosolids to the FeCl3 and Al2(SO4)3 often added at the wastewater treatment plant to bind P, while Corey (1992) found that as the (Al + Fe)/P ratio increased in biosolids, the availability of P to plants decreased.

Chemical fractionation of soil inorganic P provides a method for identifying the predominant individual forms of inorganic P in soils, most commonly soluble P, Al-P, Fe-P, occluded P, and Ca-P (Chang and Jackson, 1957). Fractionation of inorganic P is commonly carried out to characterize the effects of soil type and P source (fertilizer vs. manure vs. biosolids) on the fate and potential availability and mobility of P in soils (Sharma and Verma, 1980; McCoy et al., 1986; Sudhir et al., 1987; Tekalign and Haque, 1991). In acid soils, the potential availability of P has been reported to increase in the order: Ca-P and occluded P < Fe-P < Al-P < soluble P (Hanley, 1962; Debnath and Mandal, 1982; Hartikainen, 1989). Most P in biosolids is commonly in the form of Fe-P and Al-P, as frequently Fe and sometimes Al are added during the treatment process (Soon and Bates, 1982). This affects the fate of biosolids P in soils, Chang et al. (1983) found that 5 yr of biosolids applications to calcareous soils changed the predominant form of soil P from Ca-P to Al-P and Fe-P and this was still the case 3 yr after termination of biosolids applications. McCoy et al. (1986) found that triple superphosphate P was four to seven times more available to plants than biosolids P, and they attributed this in part to biosolids Fe and Al. Addition of lime to biosolids during production can lead to biosolids containing substantial amounts of Ca-P in biosolids, but Sui et al. (1999) found that biosolids HCl-P (Ca-P) can transform to inorganic NaHCO3–P (labile-P) and H2O-P after application to acid soils.

If we are to understand how biosolids P behaves with respect to plant availability and potential mobility in the landscape, it is important to know the fate of biosolids P in soils. Therefore we characterized the inorganic P fractions in agricultural soils of the mid-Atlantic region amended with biosolids during the past decade according to current N-based management plans. As farmers tend to apply biosolids only intermittently and normally apply fertilizers and manures in intervening years, we compared the application sites to nearby setback areas where biosolids applications were not permitted due to proximity to water courses and houses. We also estimated the potential availability and mobility of the inorganic P fractions by five repeat extractions with Fe oxide coated filter paper strips. This work should prove useful in the mid-Atlantic region of the USA and elsewhere where discussion is currently underway on P-based nutrient management plans, including the need for managing sources of P differently, based on their potential for P loss in runoff.


   Materials and methods
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results and discussion
 Conclusions
 REFERENCES
 
Site Selection
We selected eight sites in Delaware (2), Maryland (3), and Virginia (3), each with a history of biosolids applications according to current N-based regulations (Table 1) . These sites were representative of the local range of cropping systems, biosolids, and soil types, and each site had received biosolids from only one wastewater treatment process. Infrequent applications were most common, but annual additions were permitted at Sites 5 and 6 due to the installation of groundwater monitoring wells. The P rate applied as biosolids was not always available, but for the data available, the mean rate was 185 kg P ha-1 per biosolids application. Each site had a good setback area, where biosolids applications were not permitted under state regulations due to proximity to houses, wells, roads, or watercourses. These setback areas were identical in all ways to the biosolids application sites with respect to cropping, cultivation, and fertilization, except for the biosolids applications. The setback areas received no extra fertilization or lime to compensate for the lack of biosolids applications. Soil series information was obtained from USDA-NRCS soil survey manuals. Soils at four of the sites (Sites 1, 3, 4, and 8) were Ultisols (all Hapludults, except Site 3, which was an Endoaquult), while those at two sites (Sites 2 and 7) were Entisols (Quartzipsamments) and those at Sites 5 and 6 were Alfisols (Endoaqualfs).


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  		Table 1 Soil series information and details of biosolids applications at each site

 
Soil Characterization
At each site, we composited multiple cores ({approx}20) from a depth of 0 to 20 cm, which were then air-dried and ground to pass a 2-mm sieve prior to analysis. The following analyses were carried out in triplicate. Soil pH was measured at a soil/water ratio of 1:1, organic matter (OM) by loss on ignition and Mehlich-1 P (1:4 soil/0.05 M HCl + 0.0125 M H2SO4, 5-min reaction time) were all determined following the standard methods of the University of Delaware Soil Testing Laboratory (Sims and Heckendorn, 1991). Sequential chemical extraction was carried out using 1 M NH4Cl, reaction time 30 min for soluble P; 0.5 M NH4F, reaction time 1 h for Al-P; 0.1 M NaOH, reaction time 17 h for Fe-P; 0.3 M Na3C6H5O7, 1 M NaHCO3, and 1 g Na2S2O4, reaction time 15 min in a water bath at 85°C for reductant soluble P; and 0.25 M H2SO4, reaction time 1 h for Ca-P (Olsen and Sommers, 1982). We determined sequentially desorbable P using a 1:40 soil/0.01 M CaCl2 + Fe-oxide coated filter paper strip, 16-h reaction time. After removing the Fe-strip from the sample a fresh strip was added and shaken for another 16 h until five extractions had been carried out on the same sample. Desorbable P was dissolved from the Fe oxide filter paper strips by shaking for 1 h in 1 M H2SO4 (Chardon et al., 1996). Acid ammonium oxalate P, Al, and Fe (Pox, Alox, and Feox) were determined using a 1:40 soil/0.2 M acid ammonium oxalate (pH 3) with a reaction time of 2 h (McKeague and Day, 1966). We calculated the degree of P saturation (DPS) as follows . Values reported for {alpha} have ranged from {approx}0.4 to 0.7. We used , as proposed by van der Zee and van Riemsdijk (1988) for long-term P reactions in acid, sandy soils in the Netherlands.

All solutions from the Fe-oxide strip and Mehlich-1 P extractions were analyzed for P colorimetrically by the molybdate blue method of Murphy and Riley (1962), all other solutions were analyzed for P by inductively coupled plasma atomic emission spectroscopy (ICP-AES). Statistics were conducted using the data analysis package in Microsoft Excel 2000 (Microsoft, Seattle, WA).


   Results and discussion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
Conclusions
 REFERENCES
 
Effect of Biosolids Applications on Soil Characteristics
At five of the eight sites studied, soil OM was higher for the biosolids application sites than the setback areas, but the difference was not significant when averaged for all sites (Table 2) . The soils tended to be slightly acidic, as is common in the mid-Atlantic region, and there was no consistent trend for higher pH in either application site or setback area. However at the three sites where lime-treated biosolids were applied (Sites 1, 3, and 4; Table 1), the pH was higher in the biosolids site than the setback area. At six of the eight sites Alox was higher where biosolids had been applied, while Feox and Pox were always higher in biosolids-amended soils and were significantly greater on average for the biosolids sites compared with setback areas (Table 2). It has been reported that [Feox + Alox] can be used as a measure of the P sorption capacity of acidic soils (van Riemsdijk et al., 1984; Lookman et al., 1996), while Pox is a measure of total sorbed P that has also been shown to be well correlated with total P (van der Zee and van Riemsdijk, 1988; Pautler and Sims, 2000). When the significantly higher [Feox + Alox] (P < 0.05) and Pox (P < 0.05) values in the biosolids sites compared with the setback areas are considered, these results suggest that biosolids applications have increased not only the P content, but also the P sorption capacity of the soils we studied. Therefore despite the significantly higher Pox values in the biosolids sites, only six of eight sites had higher DPS in the application sites and there was no significant difference between biosolids site and setback area DPS values, when averaged for all soils (Table 2).


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  		Table 2 Selected properties of soils at biosolids sites (B) and setback areas (S)

 
Sequential Chemical Fractionation of Soil Phosphorus
Soluble P consistently represented the smallest fraction of soil P, except for Site 7 that had low Ca-P (Table 3) . The average percentage of total P in a soluble form was 1.8% (0.4–3.3%) for soils from biosolids application sites, relative to 3.0% (<0.1–6.5%) for soils in setback areas. There was no significant difference in mean values (over all eight sites) for soluble P between biosolids sites and setback areas. For the biosolids sites, soluble P measured by the sequential fractionation method was significantly correlated with DPS ; however, this relationship was not significant for the setback areas . Soluble P and Mehlich-1 P were significantly correlated for both biosolids sites and setback areas .


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  		Table 3 Sequential chemical extraction of inorganic P from biosolids sites and setback areas

 
In all soils from both biosolids sites and setback areas, most (>75%) soil P was found as either Al-P or Fe-P (Table 3). This agrees with the work of several other authors who found Al and Fe to be the dominant soil components involved in P retention in acidic soils (Williams et al., 1971; Syers and Curtin, 1988; Mozaffari and Sims, 1996). The average concentration of Fe-P was significantly greater in soils from biosolids application sites (311 mg Fe-P kg-1) than in those from the setback areas (137 mg Fe-P kg-1). In seven of the eight sites, Al-P was greater where biosolids had been applied, and when averaged across all soils, the application sites contained twice as much Al-P as soils in the setback areas. Indeed there was a good correlation between Al-P and OM in the application sites , but not in the setback areas . This may be due in part to the interaction between OM and Al, with OM increasing the amorphous nature and hence reactivity of Al, as reported by Maguire et al. (2000) in biosolids-amended soils.

For the biosolids application sites, there was a good correlation between Alox and Al-P and Feox was also correlated to Fe-P . However, the setback areas exhibited different behavior, with Feox well correlated with Fe-P , but the relationship between Alox and Al-P was not significant . Hartikainen (1982; 1989) suggested that initial P sorption in soils occurred by reaction with Fe hydroxides, as the Fe-P bond is more stable than Al-P bonds. As the P content of a soil increases and the Fe hydroxides become more saturated with P, then Al hydroxides sorb a greater proportion of the added P than the Fe hydroxides. Other researchers have also shown that Fe hydroxides have a higher binding energy for P, while Al hydroxides have a larger P adsorption capacity (Bolan and Barrow, 1984; Adams et al., 1987; Parfitt, 1989). If P is predominantly sorbed by Fe in low P soils, then as the setback areas have a lower content of Pox than the biosolids sites, the setback areas should have a greater proportion of P associated with Fe. This is supported by the higher correlation between Feox and Fe-P in the setback areas compared with the application sites. Then as the biosolids sites have a greater Pox content, they would be expected to have a higher proportion of P sorbed by Al compared with Fe hydroxides than the setback areas. However, the ratio of mean Al-P/mean Fe-P is about the same for application sites and setback areas (Table 3). This is because biosolids applications add not only P, but also reactive Fe, which can sorb P.

Correlations of Al-P with Mehlich-1 P were significant for both biosolids sites and setback areas , but the relationship between DPS and Al-P was not significant. The relationships to Mehlich-1 P and DPS were similar for Fe-P, but the correlations with Mehlich-1 P ( in biosolids sites and 0.67* in setback areas) were not as good as for Al-P.

Reductant soluble or occluded P represents a fairly stable form of soil P encapsulated in the interior of mainly Fe oxides, such as hematite and goethite (Chang and Jackson, 1957; Smeck, 1985). We found no significant difference or trend for differences in occluded P between application sites and setback areas.

Calcium-bound P was only a minor constituent in all soils ({approx}10% of total P). This would be expected as Ca-P usually predominates only in calcareous soils. No consistent trend or overall significant difference was seen for the effect of biosolids application on Ca-P. There were significant correlations of Ca-P to Mehlich-1 P in the biosolids sites and setback areas , but the relationship of Ca-P to DPS was not significant.

Total inorganic P (estimated by sum of chemical fractions) was consistently higher (P < 0.05) for soils from biosolids sites than for setback area soils (average values of 738 and 403 mg kg-1, respectively; Table 3). Higher values for total P, and other forms of soil P, would be expected where biosolids had been applied simply because the setback areas received no P in years of biosolids application. Total P was similar in magnitude and highly correlated with Pox , for both biosolid sites and setback areas. This agrees with the work of Pautler and Sims (2000) who found Pox to be a good estimate of total soil P in soils of the mid-Atlantic region. Mehlich-1 P was also significantly correlated with total P in both biosolids application sites and setback areas , but the relationship between total P and DPS was not significant.

Sequentially Desorbable Phosphorus
A single Fe-strip extraction for desorbable P is useful for measuring the initial potential for soils to release P to runoff or leaching waters and to characterize the bioavailability of soil P (Sharpley et al., 1995; Menon et al., 1997). Indiati and Sharpley (1998) found that desorbable P came almost entirely from active P fractions (soluble P, Al-P, Fe-P, and Ca-P) in weakly acidic and calcareous soils high in Ca-P. However, there is little information available on the source of desorbable P in acidic soils where Al-P and Fe-P predominate. It is well-known that soils have the ability to continue to release P into solution over long periods of time and that the size of the easily desorbable pool of soil P will vary with soil properties (Sharpley and Menzel, 1989; Freese et al., 1995; Lookman et al., 1995). We conducted five consecutive extractions of the soils from these eight sites with Fe-strips to quantify the effects of biosolids application on the nature of P release into solution and the overall magnitude of desorbable soil P.

We observed a consistent P desorption pattern for all soils, with the amount of P released decreasing with each extraction step (Table 4) . On average, for all soils (biosolids sites and setback areas) the percentages of P desorbed in each of the five steps were 36, 20, 17, 14, and 13%. We also found that the amounts (61 mg kg-1) and percentages (38%) of the total desorbable P pool released in the first extraction step were significantly higher where biosolids had been applied than in the setback areas (32 mg kg-1 and 31%). In all successive extractions there was a continuing trend of greater P release by soils from the biosolids sites (average values of 32, 25, 20, and 20 mg P kg-1, respectively) compared with the setback areas (21, 18, 16, and 15 mg P kg-1, respectively). However, these differences were not significant, and the amount of P released by biosolids sites and setback areas converged with increasing number of extractions. This is best illustrated by examining the average ratio of P removed in the biosolid application site to the setback area. This ratio was {approx}2.0 for the first extraction, but decreased to 1.3 by the fifth extraction. From the first to second extract the average amount of P released decreased from 61 to 32 mg P kg-1 for soils from the biosolids sites and from 32 to 21 mg P kg-1 for setback area soils, while for all soils almost no decrease in average P release was observed from the fourth to the fifth extraction. Similar results of decreasing P desorption rate with P extracted were found by Lookman (1995), who attributed this to the existence of two pools of desorbable P, one with fast release kinetics and the other a slowly desorbing pool. If his findings are applied to our results, it suggests that biosolids applications have increased the size of the fast desorbing pool, as indicated by the magnitude of the average P released in the first extraction. Biosolids application also appears to have increased the size of the slowly desorbable P pool, as in seven of eight cases more P was released in the fifth Fe-strip extraction from the biosolids sites compared with setback areas.


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  		Table 4 Desorbable P removed by five repeat extractions (Extr.) with Fe strips for both biosolids sites (B) and setback areas (S)

 
Relationship between Sequentially Desorbable P and Pox, Mehlich-1 P, Degree of P Saturation, and Soil P Chemical Fractions
We evaluated Pox, Mehlich-1 P, and DPS for predicting desorbable P release from either one or cumulatively from five repeat extracts (Table 5) . For short-term P release (desorbable-P1), the correlation coefficients decreased in the order: Pox > Mehlich-1 P > DPS for the biosolids sites (0.93***, 0.89**, and 0.80*, respectively), but the opposite was true for the setback areas: Pox < Mehlich-1 P < DPS (0.74*, 0.82*, and 0.86**, respectively). However, for cumulative desorbable P from five repeat extractions (desorbable-P5) there was a consistent trend for both biosolids sites and setback areas, with the correlation coefficients decreasing in the order: Pox > Mehlich-1 P > DPS (0.95***, 0.92** and 0.74*, respectively, for the biosolids sites and 0.98***, 0.96***, and 0.62, respectively, for the setback areas; Table 5). Therefore, the best soil test for short-term P release, as measured by Fe oxide impregnated paper strips, will vary between soils according to their history. However, if we consider longer-term P desorption, Pox appears to be the best test for all soils, closely followed by Mehlich-1 P, while DPS was not as good.


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  		Table 5 Correlation coefficients for desorbable P vs. Pox, Mehlich-1 P, degree of P saturation, Al-P, Fe-P, Ca-P, and soluble P fractions, for both biosolids sites (B) and setback areas (S)

 
Correlations of desorbable P to soil P fractions (Al-P, Fe-P, Ca-P, and soluble P) tended to be similar for desorbable-P1 than desorbable-P5 (Table 5). Therefore, as more P is desorbed, P release did not become harder to predict from soil P fractions. As Al-P and Fe-P are the dominant P fractions in these soils, it is not surprising that they are best correlated with desorbable P. The correlations with desorbable P tended to be closer for Al-P than Fe-P; also Fe-P in the setback areas was not significantly correlated with desorbable or desorbable P5 , while good equivalent correlations were seen with Al-P ( , respectively). Therefore in these soils, Al-P appeared to be a greater source of desorbable P than Fe-P, especially in the setback areas.


   Conclusions
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
REFERENCES
 
Land application of biosolids based on crop N requirements and preventing the over-application of nonessential trace elements has been shown to be an effective means to beneficially reuse these byproducts. Our results and past research show that following a N-based approach to biosolids management can significantly increase total soil P (measured by Pox) and bioavailable P (Mehlich-1 P, Fe-oxide strip P, total desorbable P). We found that soil components associated with P retention (Alox + Feox) were also increased and this may mitigate P release relative to where soil P is increased without a corresponding increase in (Alox + Feox). Indeed the sequential chemical fractionation data showed a significant increase in the content and percentages of Fe-P and a strong trend for higher Al-P concentrations in biosolids-amended soils relative to soils from setback areas. It was expected that P sorption would switch from being associated predominately with Fe hydroxides to Al hydroxides with increasing soil P saturation, but this was not observed, probably due to the Fe added to the biosolids during wastewater treatment. We found that release of desorbable P was greater from the biosolids sites than the setback areas, measured by both single and five repeat extracts with Fe-strips. This suggests biosolids increased the risk of P loss in runoff; however, it is important to note that the setback areas received no P in years of biosolids applications and thus were expected to contain less P. When we evaluated Pox, Mehlich-1 P, and DPS for estimating short-term desorbable P, we found that the correlation was dependent on whether or not biosolids had been applied. However Pox, closely followed by Mehlich-1 P, was a good predictor of desorbable P release in five repeat extractions from both biosolids sites and setback areas. Desorbable P was also more closely correlated with Al-P than Fe-P, especially in the setback areas, suggesting Al-P was the main source of desorbable P.Sharma Verma 1980

Received for publication March 27, 2000.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 




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Phosphorus Restrictions for Land Application of Biosolids: Current Status and Future Trends
J. Environ. Qual., November 1, 2003; 32(6): 1955 - 1964.
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C. J. Penn and J. T. Sims
Phosphorus Forms in Biosolids-Amended Soils and Losses in Runoff: Effects of Wastewater Treatment Process
J. Environ. Qual., July 1, 2002; 31(4): 1349 - 1361.
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P. A. Vadas and J. T. Sims
Predicting Phosphorus Desorption from Mid-Atlantic Coastal Plain Soils
Soil Sci. Soc. Am. J., March 1, 2002; 66(2): 623 - 631.
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R.O. Maguire, J.T. Sims, S.K. Dentel, F.J. Coale, and J.T. Mah
Relationships between Biosolids Treatment Process and Soil Phosphorus Availability
J. Environ. Qual., May 1, 2001; 30(3): 1023 - 1033.
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