Journal of Environmental Quality 30:1059-1063 (2001)
© 2001 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
TECHNICAL REPORT
Waste Management
Biosolids Effects on Phosphorus Retention and Release in Some Sandy Florida Soils
Peng Lu and
George A. O'Connor
Soil and Water Science Dep., Univ. of Florida, Gainesville, FL 32611-0510
Corresponding author (gao{at}ufl.edu)
Received for publication March 3, 2000.
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ABSTRACT
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The soil solid phase components most responsible for P sorption in Florida soils are Fe and Al oxides. Thus, we hypothesized that land application of biosolids would significantly increase a soil's P retention by increasing its content of P-sorbing solids, especially when biosolids with high Fe and Al concentrations are applied to soils that sorb P poorly. Biosolids effects were quantified by a series of single-point isotherms on soils from two field studies sampled for up to 4 yr after initial biosolids application. Biosolids additions had little effect on P retention in a soil with abundant oxalate-extractable Fe and Al and a correspondingly large native P-sorbing capacity. However, biosolids significantly increased P retention in a soil with low oxalate-extractable Fe and Al content and low native P-sorbing capacity. Biosolids effects on P retention lasted 1 to 3 yr after application, depending on biosolids source and rate of application, and generally mimicked persistence of increased extractable Fe and Al concentrations in the poorly P-sorbing soil. Disappearance of added Fe and Al (and, hence, P retention capacity) from the surface horizons over time was relatively rapid, perhaps due to abundant organic acid production associated with biosolids degradation. Phosphorus in biosolids containing (or tailored to contain) abundant Fe and/or Al can be expected to behave as a slowly available P source, and to be less subject to leaching losses than completely soluble P sources.
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INTRODUCTION
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BIOSOLIDS typically contain 10 to 20 g total P kg-1, and are good sources of P for crops (USEPA, 1995; Peters and Basta, 1996). Land application of biosolids increases available soil P, plant uptake of P, or both (Kelling and Walsh, 1977; Kirkham, 1982). Most P in biosolids is inorganic (Chae and Tabatabai, 1981), and bioavailability varies from 10 to 100% of that in soluble fertilizers (de Haan, 1981). Differences in biosolids-P bioavailability are attributable to several factors. Notable is the addition of Fe, Al, or Ca in the treatment process necessary to meet effluent P limitations, which reduces P solubility in the biosolids residual (Soon and Bates, 1982). Such chemically treated biosolids can contain large (several percent by weight) amounts of Fe and Al oxides or CaCO3, all of which are capable of sorbing soluble P.
Specific sorption is the major reaction mechanism of P in most sandy Florida soils (Lu and O'Connor, 1999). The Fe and Al oxides are the major solid phase components responsible for P sorption (Reddy et al., 1998). Thus, applying materials containing large amounts of Fe and Al oxides could be expected to increase P sorption and reduce P lability. We hypothesized that land application of biosolids could significantly increase a soil's P retention by increasing the content of P-sorbing solids, especially when high Fe and Al biosolids are applied to soils that sorb P poorly.
Surprisingly, P retention by biosolids-amended soils has been little studied. Lee et al. (1981) compared P sorption on samples of a calcareous loam amended with biosolids treated with Ca(OH)2, FeCl3, or Al2(SO4)3, and on an unamended control. Amendment with any of the biosolids increased P retention, but the high Ca biosolids had the greatest effect in this (likely) Ca and P dominated system. The authors describe biosolids effects on high and low energy P sorption sites, but biosolids-induced changes in Ca-phosphate solids solubility (e.g., O'Connor et al., 1986) could have also been important.
In a previous study (Lu and O'Connor, 1999), we determined how P sorption in some sandy Florida soils varied with pH, P load, time, complementary ligands, and soil oxalate-extractable Fe and Al contents. That study included soil samples incubated in the lab with biosolids, and provided preliminary verification of our hypothesis. This study focused on biosolids-amended soils, field-equilibrated for up to 4 yr, and used single point isotherm measurements to quantify P retention. Our objectives were to: (i) determine the effects of biosolids on P retention and release, (ii) determine how long the effects (if any) persisted, and (iii) explain the basis for the effects and their persistence.
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MATERIALS AND METHODS
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Field Equilibration and Soil Sampling
Surface soil samples (0–15 cm depth) came from two field studies of bahiagrass (Paspalum notatum Flugge) response to biosolids applications. The small scale field study was conducted at the University of Florida Ona Research and Education Center in south Florida. Details of the study are given in Nguyen (1998). Plots (2.5 by 5.5 m) were established on Immokalee fine sand (sandy, siliceous, hyperthermic Aeric Alaquods) that had been previously limed to pH 6.2. Native (not previously fertilized) land was sodded with bahiagrass, and allowed to establish for 30 d. The plots were then amended (surface-applied and raked) with two Class A biosolids from New York, NY, and Tampa, FL. Initial biosolids application rates were based on a high N rate for bahiagrass of 179 kg N ha-1 (=x), assuming 40% of biosolids total N to be plant available in the first season. Application rates were 1x, 2x, and 4x in Year 1. The 1x rate equaled 7.5 Mg ha-1 of Tampa material and 10 Mg ha-1 of New York material. Fertilizer (NH4NO3) controls (1x) and absolute controls (no biosolids or fertilizer) were also included. Each treatment was replicated (randomized complete block design) four times. In Year 2, the plots were split: amendments were reapplied to half of the plots, the other half served as residual treatments (no further amendments). In Years 3 and 4, no amendment or fertilizer was applied to any plot. Single soil samples were taken at random locations within each plot at 15-cm increments to just above the spodic horizon at about 75 cm. Holes were backfilled with soil from outside the plot boundaries. Soil samples were taken at the beginning of each growing season (immediately after amendment application), and at season's end (approximately 6 mo later). Soil samples were air dried, sieved (2 mm) to remove organic debris, and stored in plastic bottles for analysis.
A larger scale (0.8-ha plots) field study was conducted at the University of Florida Santa Fe Beef Research Center in northern Florida. Details of the study are given in O'Connor and McDowell (1999). Plots consisted of established bahiagrass pasture on Millhopper sand (loamy, siliceous, hyperthermic Grossarenic Paleudults). Treatments consisted of Tampa, FL, and Baltimore, MD, Class A biosolids surface applied (initially) at the same 1x and 2x rates used in the small scale study. The 1x treatment equaled 7.5 Mg ha-1 for Tampa material and 22.4 Mg ha-1 for the Baltimore material. Fertilized (1x) and nonfertilized control plots were included. In Year 1, each biosolids treatment was replicated six times in a randomized block design. In the second year, the replicates were reduced to three to allow reapplication of treatments to half the plots and residual studies in the remaining plots. In the third year, accumulative biosolids applications rates were adjusted to 3x or 6x by adding varying amounts of biosolids. Thus, for example, reapplied 1x plots from Year 1 received an additional 1x application rate (= total of 3x). Residual 1x plots of Year 2 received an additional 2x application to yield a cumulative load of 3x. Soil sampling was in 15-cm increments to 45 cm from four quadrants of each plot. These samples were composited for analysis. Sampling occurred at the start of the season (immediately following amendment application), and at season's end (about 6 mo later). Samples were air-dried, sieved to remove organic debris, and stored in plastic for later analysis.
Details of the sorption studies are given in Lu and O'Connor (1999). Briefly, single-point isotherm data were collected from 1:10 soil/solution suspensions at an initial P concentration of 40 mg P L-1. The resulting 400 mg P kg-1 soil load was chosen because most sandy Florida soils reach (or approach) maximum P sorption at this load (Harris et al., 1996). Sorption studies were conducted at room temperature (about 23°C), with 10-3 M CaCl2 as the background electrolyte. Suspensions were equilibrated for either 1 or 5 d, as determined in preliminary studies to be appropriate (Lu and O'Connor, 1999).
Soil-less and zero initial-P blanks accounted for nonsoil P removal and contributions of native soil P to the equilibrium solutions, respectively. Standard methods (Olsen and Sommers, 1982) were used for other measurements, including pH, EC, and soluble P. Oxalate-extractable Fe and Al contents were determined (Sheldrick, 1984) on all soil samples. All studies were conducted in triplicate, and included typical quality assurance–quality control protocols of spike recoveries, certified standard analyses, and new standard curves for each set of samples. Spike and certified standard analyses were within 10% of expected values.
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RESULTS AND DISCUSSION
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Soil and Biosolids Properties
Selected properties of the soils and biosolids are given in Table 1. The soils differ dramatically in their native P status, and in characteristics expected to affect P retention and release. The native Immokalee sand is extremely low in P and in oxalate-extractable Fe and Al, whereas the native Millhopper contains abundant P and oxalate-extractable Fe and Al (Table 1). Phosphorus sorption maxima for the native soils were proportional to oxalate-extractable Fe and Al contents, and were about 20 mg kg-1 for Immokalee and about 200 to 350 mg kg-1 for Millhopper (Lu and O'Connor, 1999). Biosolids also differ in P status and in the expected ability to alter soil P retention. Each material could be regarded as a low-analysis P source (15–24 g kg-1 P), but the Mehlich-I extractability data suggest that total P may not completely assess P solubility. Of more interest here is the range of Fe and Al contents in the biosolids. The Tampa and New York materials contain about 20 to 30 g kg-1 total Fe and Al, which is typical of solids from waste water treatment processes without chemical addition. The Baltimore material, however, reflects Fe or Al (primarily Fe, in this case) addition in the treatment stream to aid in P removal from waste effluent.
Phosphorus Sorption: Santa Fe Site (Millhopper Soil)
Phosphorus sorption was unaffected (p < 0.05) by additions of either Tampa or Baltimore biosolids at the Santa Fe site (Table 2). The Millhopper soil's large native Fe and Al content, and associated large P sorption capacity, apparently masked biosolids effects. The maximum application rate of Tampa material (6x, 100 Mg ha-1) contributed total Fe and Al that represents only about 1% of the native soil's total Fe and Al, and thus, was not expected to significantly affect P retention. Even the high Fe and Al Baltimore material, applied at the maximum rate (6x, 134 Mg ha-1), was calculated to add only about 4% additional Fe and Al to this soil. Therefore, no significant effect of Baltimore material was expected either. We conclude that the high P retention capacity of soils, like the Millhopper, will not be markedly augmented by biosolids additions. Such soils would, however, be expected to strongly retain large P inputs associated with biosolids applied at agronomic rates based on crop N needs. Indeed, despite cumulative biosolids-P additions of >1000 kg P ha-1, there was no evidence of P movement below the 0- to 15-cm depth over 3 yr (O'Connor and Sarkar, 2000).
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Table 2. Single-point P adsorption changes with time and application rate in Millhopper soil (0–15 cm) amended with Tampa and Baltimore biosolids.
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Phosphorus Sorption: Ona Site (Immokalee Soil)
In distinct contrast to the high sorption capacity of the Millhopper soil, the Immokalee soil sorbs much less P, and contains much less Fe and Al natively (Lu and O'Connor, 1999). Thus, even modest contributions of additional Fe and Al in biosolids were expected to affect P retention.
Phosphorus sorption over time for the Tampa residual plots (amended only in 1995) are presented in Fig. 1. Similar data for the New York residual plots are omitted for brevity. Increased P sorption was obvious, even by the end of Year 1 (0.5 yr since initial biosolids application), but the full effect was delayed to Year 2. The increase in P sorption was roughly proportional to biosolids rate (and added Fe and Al contents). Phosphorus retention remained significantly greater than in the control samples well into Year 3, but biosolids rate effects disappeared by the end of Year 3 (2.5 yr since initial application). In Year 4, there were relatively minor effects of biosolids application (or rates) on P sorption.
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Fig. 1. Single-point P sorption changes with time for Immokalee 0 to 15 cm soil amended in 1995 with Tampa biosolids: Residual plots (1x, 2x, 4x represent biosolids loading rates).
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Oxalate-extractable Fe and Al concentrations of the same Tampa residual soil samples generally mimicked effects on P sorption (Fig. 2). Extractable Fe and Al values generally increased with biosolids rate, and the effects were essentially constant through the end of Year 3 (2.5 yr since initial application). Thereafter, extractable Fe and Al values tended to decrease and, by the end of Year 4 (3.5 yr since initial application), were similar to those of the control treatments.
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Fig. 2. Oxalate-extractable Al + Fe changes with time in Immokalee 0 to 15 cm soil amended in 1995 with Tampa biosolids: Residual plots (1x, 2x, 4x represent biosolids loading rates).
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Phosphorus sorption in the Tampa reapplied plots (Fig. 3) reflected the additional biosolids (and Fe and Al) applied in 1996 with even greater P retention than in the residual plots (Fig. 1, 2 yr since initial application). The effect of biosolids on P sorption, however, began to decrease by the end of Year 3, and essentially disappeared by the end of Year 4.
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Fig. 3. Single-point P sorption changes with time for Immokalee 0 to 15 cm soil amended in both 1995 and 1996 with Tampa biosolids: Reapplied plots (1x, 2x, 4x represent biosolids loading rates).
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Oxalate-extractable Fe and Al values in the reapplied plots were variable, but were roughly proportional to biosolids application rates (Fig. 4). Even by the end of Year 4, biosolids applied twice at the 2x and 4x rates maintained elevated extractable Fe and Al contents compared with the control.
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Fig. 4. Oxalate-extractable Al + Fe changes with time in Immokalee 0 to 15 cm soil amended in both 1995 and 1996 with Tampa biosolids: Reapplied plots (1x, 2x, 4x represent biosolids loading rates).
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Obviously, changes in oxalate-extractable Fe and Al concentrations of the samples do not totally explain changes in P sorption, but the correlations between the variables (Fig. 5) for individual biosolids sources are good. Other factors (e.g., changes in soil pH or ionic strength, presence of competing ions, organic ligands that complex Fe and Al, etc.) associated with biosolids applications could also influence P sorption. [Over the pH range experienced here, P adsorption is independent of pH (Lu and O'Connor, 1999)]. Nevertheless, variations in extractable Fe and Al explained about 70% of the variation in P sorption for both Tampa-amended and New York-amended Immokalee samples (Fig. 5). When data for both Tampa- and New York–amended samples are combined, the r2 value decreases, but the regression still accounts for about 66% of the variability in P retention. Linear correlation coefficients were similar (0.61 for both New York and Tampa sources individually, and 0.43 for the combined data), but smaller than the coefficients for the curvilinear models used in Fig. 5.
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Fig. 5. Single point P sorption vs. oxalate-extractable Al and Fe content in Immokalee soil amended with Tampa and New York biosolids (data represent all rates and sampling times).
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Apparently, the reactivity of oxalate-extractable Fe and Al toward P retention varies with the biosolids source. This could be associated with differences in biosolids Fe and Al oxide specific surface and crystallinity, the extent to which other oxyanions were previously bound to the oxides, etc. Pursuit of these factors was beyond the scope of this study, but may deserve further study as changes in P sorption are directly related to P phytoavailability and mobility in biosolids-amended soils (O'Connor and Sarkar, 2000).
The effects of biosolids Fe and Al varied with time following applications (Fig. 1 and 3), and were generally consistent with changes in oxalate-extractable Fe and Al with time (Fig. 2 and 4). Oxides of Fe and Al are usually considered stable and recalcitrant in most soils, but Spodosols characteristically form as a result of Fe and Al movement (eluviation) down the profile. Organic acids, resulting from organic matter degradation, complex Fe and Al and transport the metals through the profile. In Spodosols, a high organic matter (spodic horizon) develops where the solubilized organic matter is stabilized by bonding with Fe and Al and eluviated clay. Such pedogenic processes often require decades to centuries, but our data suggest much faster (2–3 yr) removal of Fe and Al added at the surface in biosolids. The high water tables (reducing conditions possible throughout the profile), excessive leaching (abundant rainfall), possibly abundant organic acid production (from degrading biosolids), etc. could all be responsible for the rapid decreases in oxalate-extractable Fe and Al and reduced P sorption we observed over time. Additionally, the solubility of Fe and Al forms in biosolids could be much greater than that of the normally recalcitrant soil forms and, thus, be more quickly lost from soils.
We conducted oxalate-extractable Fe and Al analyses on a limited number of soil samples from the 15- to 30-cm depth at the Ona site. Data for the samples from the end of Year 3 (F'97) and both the beginning and end of Year 4 (B'98 and F'98, respectively) are given in Table 3. There is a general trend for extractable Fe and Al concentrations to be greater in the biosolids treatments than in the control, and for the effect to be roughly proportional to biosolids application rates (data for other rates not shown). The absolute values are small, however, and the standard errors are sufficiently large that no differences were significant (p < 0.05). It is clear, however, that the large losses of oxalate-extractable Fe and Al from the 0- to 15-cm depth (Fig. 2 and 4) are not accounted for by corresponding increases in the 15- to 30-cm depth. Possibly, the lost Fe and Al is now in the spodic horizon (not sampled).
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Table 3. Oxalate-extractable Fe and Al content changes with time in selected samples of Immokalee soil (15 to 30 cm) amended with Tampa and New York biosolids.
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Data for the Immokalee soil amended (once or twice) with both materials clearly demonstrate a positive effect of biosolids on P sorption. The effect is not permanent, but temporarily reduces the amount of soluble P available for possible leaching. Data from related greenhouse studies (D. Sarkar and G.A. O'Connor, unpublished data, 2000) suggest the increased sorption causes biosolids P to act as a more slowly available source of P than fertilizer P. Greenhouse and field data also suggest that biosolids naturally containing (or tailored to contain) abundant Fe and Al can slow P movement in some soils and alter P availability to plants (O'Connor and Sarkar, 2000). Literature-reported field data also support this contention (e.g., Soon et al., 1978). Several recent research articles (e.g., Gallimore et al., 1999; Ippolito et al., 1999) demonstrated the value of water treatment residuals, containing large amounts of Fe or Al, in reducing P losses from soil when applied alone, or mixed, with biosolids.
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SUMMARY AND CONCLUSIONS
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Biosolids applied to land at rates based on crop N needs frequently supply P far in excess of crop removals, and can cause P accumulations that endanger nearby water supplies. Biosolids, however, also contain other elements (notably Fe and Al) that can affect P retention in the amended soil through the addition of sorbing solid phases (Fe and Al oxides). We hypothesized that land application of biosolids could significantly increase soil P sorption, particularly some sandy Florida soils. We tested the hypothesis on soil samples collected from two field studies involving biosolids and sampled for up to 4 yr after the initial application.
Biosolids additions had little effect on P retention (single-point isotherms) in the Millhopper soil, which natively contains abundant oxalate-extractable Fe and Al and possesses a correspondingly large P-sorption capacity. Even high cumulative loads of a biosolids containing about 100 g total Fe and Al kg-1 insignificantly increased P retention capacities. Such soils could accept the large P inputs associated with biosolids application rates based on crop N needs with little concern for P leaching.
Biosolids significantly increased P retention in the Immokalee sand, which has low native extractable Fe and Al concentrations and low P sorption capacity. Increases in P sorption were well correlated with increases in oxalate-extractable Fe and Al contents of amended soils. Data suggest differences in biosolids Fe and Al reactivities toward P sorption, but a curvilinear regression for all data combined explained nearly 70% of the P-sorption variability in terms of extractable Fe and Al concentrations.
Biosolids effects on P sorption lasted 1 to 3 yr after application, depending on biosolids source and rate of application, and generally mimicked persistence of increased Fe and Al concentrations in the soil. Disappearance of Fe and Al (and, hence, P retention capacity) from the surface horizons over time was relatively rapid in the Immokalee Spodosol, perhaps due to increased organic acid production following biosolids degradation.
Although temporary, the increased retention of P effected by biosolids applications can have important implications. Phosphorus in biosolids containing (or tailored to contain) abundant Fe and/or Al can be expected to behave as a slowly available P source, and to be less subject to excessive leaching losses than completely soluble sources. This can be especially significant in many areas of Florida dominated by soils that sorb P poorly and allow extensive P leaching.
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NOTES
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Contribution of the Florida Agric. Exp. Stn. Journal Series no. R-07430.
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REFERENCES
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TOP
ABSTRACT
INTRODUCTION
