Published in J. Environ. Qual. 34:184-191 (2005).
© ASA, CSSA, SSSA
677 S. Segoe Rd., Madison, WI 53711 USA
Associated Release of Magnesium and Phosphorus from Active and Abandoned Dairy Soils
M. S. Josan,
V. D. Nair*,
W. G. Harris and
D. Herrera
Soil and Water Science Department, 106 Newell Hall, P.O. Box 110510, University of Florida, Institute of Food and Agricultural Sciences, Gainesville, FL 32611-0510
* Corresponding author (vdna{at}ifas.ufl.edu)
Received for publication March 31, 2004.
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ABSTRACT
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Dairy manure application to soils can result in phosphorus (P)-related degradation of water quality. The P in these manure-impacted soils can be labile even years after abandonment and under conditions normally associated with high P stability. Failure of P to stabilize with time compounds the environmental consequences of dairy manure disposal, especially on sandy soils. The objectives of this study were to compare chemical characteristics of active and abandoned dairy manure–impacted soils and minimally impacted soils and to assess the continuous release of P in relation to sparingly soluble salts using repeated water extractions, X-ray diffraction, and speciation modeling of column leachates. Soil samples from Ap horizons were collected from nine highly manure-impacted (total P > 1000 mg P kg–1 soil) areas on four active and five abandoned dairies and four minimally impacted soils (total P < 200 mg P kg–1 soil). Soil extracts were analyzed for electrical conductivity (EC), soluble reactive phosphorus (SRP), Ca, Mg, Na, and K. The EC of the soil solutions decreased as active dairy > abandoned dairy > minimally impacted soils. Release of Mg and SRP were significantly correlated (r2 = 0.68) and did not decline after abandonment; Ca release was not correlated with SRP (r2 = 0.01), and declined significantly (p < 0.05) after abandonment. Speciation data from column leachates suggested that Mg-P phases and/or the most soluble Ca-P phases could control P solution activities. An implication of this study is that P stabilization via crystallization of calcium phosphates (even at near-neutral pH) may be preempted by Mg-P association. Thus, mechanisms to minimize P release may require P-retaining soil amendments or management of animal rations to eliminate Mg-P formation.
Abbreviations: DOC, dissolved organic carbon • EC, electrical conductivity • SRP, soluble reactive phosphorus • XRD, X-ray diffraction
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INTRODUCTION
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ANIMAL MANURES can increase concentrations of P and other components in soils (Sharpley et al., 1995; Nair et al., 1998; Sims et al., 1998). Dairy manure accumulation in soils can increase the potential for P loss to surface waters via erosion (Sharpley and Smith, 1983) and/or subsurface drainage (Mansell et al., 1991), and degrade the quality of water. The P in these manure-impacted soils (total P > 1000 mg P kg–1 soil) can be labile after years of abandonment and under conditions normally associated with high P stability like high pH and Ca (Nair et al., 1995; Wang et al., 1995). The fate of added P depends on the soil's potential for P assimilation into stable forms. Many soils effectively retain P, but sandy soils can be exceptions due to paucity of P-retaining minerals (Neller et al., 1951; Ozanne et al., 1961; Gillman, 1973; Burgoa et al., 1991; Mansell et al., 1991, Graetz and Nair, 1995; Harris et al., 1996; Nair et al., 1998; Novak et al., 2003). Thus, the stability (resistance to dissolution) of manure-derived forms of P is an especially relevant environmental concern in sandy soils.
The release of P from manure-impacted soils with successive extractions has been studied by several researchers using different soil extractants. Sharpley (1996) observed that the release of P with 0.01 M CaCl2 using Fe strips as the adsorbent exponentially decreased with successive extractions. Nair et al. (1995) found that by repeatedly extracting a manure-impacted soil with 1 M NH4Cl solution, almost 80% of the P in the surface horizons of highly manure-impacted soils could be identified as labile P (readily soluble P) and was associated with Ca- and Mg-P forms. They speculated that Ca- and Mg-P associations were loosely bound, probably by some weak adsorption mechanism or in the form of poorly crystalline solids, and were available for sustained leaching under suitable conditions. Cooperband and Good (2002) presented evidence that sparingly soluble Ca- and Mg-P minerals (more soluble than apatite) controlled solution P concentrations in soils amended with poultry manure. However, they were unable to directly identify these forms.
The typically high Ca concentrations and pH (approximately 8) for leachates of sandy soils amended with dairy manure should favor the formation of relatively stable calcium phosphates (e.g., apatite, tricalcium phosphate) (Wang et al., 1995). However, leaching and fractionation studies showed that P remains in relatively labile form in soils of "high-intensity areas" near dairy barns (Nair et al., 1995; Wang et al., 1995). Also, evidence of crystalline phosphate minerals in these soils is lacking (Harris et al., 1994). Possibly, crystallization of stable phosphate forms is inhibited by components such as Mg or dissolved organic carbon (DOC) (Harris et al., 1994). Alternatively (or perhaps additionally), the phosphate in the manure-amended soils could be in a sparingly soluble form that releases P slowly but at environmentally problematic concentrations.
The present study tests the hypothesis that abandoned and active dairy manure–impacted soils release comparable amounts of P because solution P is controlled by a sparingly soluble Mg-P phase that requires many years for depletion. We use the term "sparingly soluble" in this study in a qualitative way to refer to components that continue to be released even years after dairy site abandonment and cessation of heavy manure loading. The hypothesis was tested using both repeated extractions (wide soil to solution ratio and minimum soluble salt) and column leaching methodology (narrow soil to solution ratio, closer to equilibrium conditions, and common ion effects). Specific objectives were to (i) compare the chemical characteristics of active and abandoned dairy manure–impacted soils and minimally impacted soils and (ii) assess the continuous release of P in relation to Mg and Ca, using repeated extractions, X-ray diffraction (XRD), and speciation modeling from column leachate data.
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MATERIALS AND METHODS
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Soil Sampling
Soil samples were collected from manure-impacted soils at four active dairies (AD-1 to AD-4) and five abandoned dairies (AB-1 to AB-5) and four minimally impacted soils (MI-1 to MI-4) from peninsular Florida (Suwannee River and Lake Okeechobee basins). The active dairy manure–impacted soils are those soils that are currently receiving dairy manure, whereas the abandoned dairy manure–impacted soils are no longer receiving high levels of dairy manure daily (no longer operating as dairies). Abandoned dairy sites were formerly high-intensity areas similar to the active sites. Years of abandonment ranged from 12 to 32 yr. Soils from six of the eight sites included in this study were collected by tile spade to a depth of 25 cm or the bottom of the Ap horizon, whichever was shallower, from representative locations within the high-intensity areas, and were composited. Ap horizon soil samples were also taken at each site using a 4.0-cm-diameter soil auger for bulk density calculations. Representative soil profiles, to a 2-m depth, at these six sites were sampled to include Bt and Bh horizons, if present within that depth. Samples were taken to 2 m for classification and characterization purposes.
Parent materials for all soils were sandy marine sediments. Slopes were <2%. Drainage classes were estimated to be poorly to somewhat poorly drained. Active sites were bare or nearly bare, and abandoned sites were grassed. Two of the active sites were Spodosols and two were Ultisols. All abandoned sites were Spodosols. One active site (AD-1) and one abandoned (AB-1) site had fill material greater than 25 cm thick, which made their classification moot for the purposes of this study. Samples from one active (AD-4) and one abandoned dairy site (AB-2) were supplied by other researchers. These soil profiles were not described and sampled for these two sites, but both sites were located in areas dominated by Spodosols. The samples were collected to approximate depths of 25 and 15 cm for active and abandoned dairies, respectively. All samples were either dried shortly after collection or stored moist under refrigeration. Samples were air-dried and crushed through a 2-mm sieve before use.
Soil Characterization
The pH and electrical conductivity (EC, dS m–1) of soils were measured with a glass electrode using 1:2 soil to water ratio. Total P and metals (i.e., Ca, Mg, Fe, Al, Na, and K) in the soils were determined by using the ignition method (Anderson, 1974). Phosphorus was analyzed by using ascorbic acid colorimetry (Murphy and Riley, 1962) (Method 365.1; USEPA, 1993). All metals were analyzed by atomic absorption spectrophotometry.
Soil Extraction
Soil (20 g) was repeatedly extracted with 200-mL of deionized water eight times. The soils were shaken for 5 min and equilibrated for progressively longer intervals (0, 0.5, 3, 6, 12, 24, 36, and 48 h) with successive extractions. After each extraction, the samples were centrifuged at 738 x g for 5 min. The supernatants were collected and filtered through a 0.45-µm filter. All extractions were performed at room temperature (298 K). The collected filtrates were analyzed for pH, EC, soluble reactive phosphorus (SRP), Ca, Mg, Na, K, Fe, and Al.
All P determinations were performed on a UV-visible recording spectrophotometer at an 880-nm wavelength via a molybdate-blue colorimetric procedure (Murphy and Riley, 1962) (Method 365.1; USEPA, 1993). The filtrates were analyzed for metals by atomic absorption spectroscopy. Total carbon and total inorganic carbon was determined in all the filtrates using a carbon analyzer (TOC-5050A; Shimadzu, Kyoto, Japan) (Method 5310A; American Public Health Association, 1992). Dissolved organic carbon in these filtrates was then determined by difference between the total and inorganic carbon concentrations.
Particle Size Fractionation and Mineralogical Analysis
Air-dried soil samples (50 g each) were treated with bleach (10% sodium hypochlorite), adjusted to pH 9.5 at a 1:20 soil to bleach ratio (overnight) to oxidize organic matter (Lavkulich and Wiens, 1970). The supernatants were siphoned off and the soils were transferred to 250-mL centrifuge bottles and washed three times with 1 M NaCl solution to remove bleach. Samples were then washed with deionized water to remove salt. Water adjusted to pH of approximately 9.5 with Na2CO3 was added to promote dispersion. Sand was collected by sieving, and clay and silt by centrifugation (Whittig and Allardice, 1986). Oriented mounts for clay were prepared for X-ray diffraction (XRD) by depositing 250 mg of clay as a suspension onto a porous ceramic tile under suction. Clay was saturated on the tiles with Mg and solvated with glycerol following an initial XRD scan. Silt was mounted as an oriented dry powder on a low-background quartz crystal mount.
Soil Leaching Characterization and Chemical Equilibrium Modeling
Leachates from surface samples of four active and four abandoned dairy manure–impacted soils were collected from a column approach as reported by Wang et al. (1995). This approach entails adjusting soil to mean moisture content (25%) on wet weight basis and bulk density representative of the samples in the field (bulk density was 1.2 ± 0.1 Mg m–3). Leaching was conducted for 7 d at 0.6 mL min–1 deionized water and under 600 cm suction for a pore volume. Leachates were analyzed for metals (Ca, Mg, Na, K, Fe, and Al) by inductively coupled plasma–atomic emission spectroscopy (ICP–AES) using USEPA Method 200.7 (USEPA, 1993). Soluble reactive phosphorus concentrations were measured using ascorbic acid colorimetry (Method 365.1; USEPA, 1993). Anion concentrations were determined using USEPA Method 325.2 for chlorides (USEPA, 1993), automated colorimetry with the use of an Alpkem autoanalyzer for nitrates (Method 353.2; USEPA, 1993), the semiautomated colorimetry method for ammonium (Method 350.1; USEPA, 1993), and ion chromatography with separator AS-14 (Dionex, Sunnyvale, CA) for sulfate (Method 300.0; USEPA, 1993). The pH of leachates was determined and the ionic strength calculated from EC measurements (i.e., ionic strength = 0.013 x EC; Griffin and Jurinak, 1973). Dissolved organic carbon was also determined by TOC-5050A (Shimadzu) (Method 5310A; American Public Health Association, 1992). Interferences from the inorganic carbon in leachates were first removed by sparging with CO2–free gas after acidification of the sample (Sharp and Peltzer, 1993).
Visual-MINTEQ Version 2.3 (Department of Land and Water Resources Engineering, 2004) was used as a chemical equilibrium model for the calculation of metal species and solubility equilibrium indices for leachates.
Statistical Analysis
Analysis of variance (ANOVA) was performed to determine the significance of differences between the means. Nonparametric statistics (the Kruskal–Wallis test) was used as an alternative when the assumptions of the ANOVA were not confirmed. The Student–Newman–Keuls procedure was used to compare all possible pair wise differences of means at the 5% level of significance. Computations were performed in Minitab Version 14.0 (Minitab, 2004).
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RESULTS AND DISCUSSION
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Soil Chemical and Mineralogical Characteristics
Active dairy soils had higher pH (7.1–7.9) than the pH of abandoned dairy manure–impacted soils (6.0–7.2) (Table 1). Higher pH values in manure-impacted soils are due to Ca and Mg additions in the manure (Nair et al., 1995). The minimally impacted soils had low pH (3.8–5.7). One of the abandoned dairy soils (AB-1) had extremely high total Ca concentrations (>35000 mg kg–1), probably as a result of fill material added to the soil. Total P concentrations were similar for active and abandoned dairy soils (>1000 mg kg–1), but were much lower for minimally impacted soils (<200 mg kg–1) (Table 1).
The native P retention capacity of sandy soils is low, and is almost certainly exceeded by the levels of P loading characteristic of high-intensity areas near dairy barns (Nair et al., 1995, 1998). However, dairy manure also contains high levels of Ca and Mg, and increases the soil pH. Calcium phosphate minerals are predicted to be stable under these conditions using chemical equilibrium modeling (Wang et al., 1995). These conditions are associated with P fixation and plant availability problems (Delgado and Torrent, 2000; Zhou and Li, 2001). Quartz was the dominant mineral in all size fractions of all soils except for one abandoned dairy sample (AB-1) influenced by fill material, in which calcite was dominant in the clay and silt. The silt fraction of an active dairy site, also influenced by fill, had high calcite as well. Calcite was detectable in all samples and was probably derived from either manure or amendments used to stabilize the soil for heavy animal traffic. Other minerals present in minor to moderate amounts included kaolinite (two abandoned dairies and one active dairy), smectite (one abandoned dairy), and hydroxyl-interlayered minerals (three active dairies). No phosphate minerals were directly identified in the samples analyzed via XRD. Pierzynski et al. (1990) and Harris et al. (1994) were also unable to identify distinct P minerals in excessively fertilized and dairy manure–impacted soils, respectively. Either the phosphate phases are noncrystalline or mineral concentrations are too low for detection (<1%) without further preconcentration (e.g., selective dissolution, density separation, etc.), or both. The presence of a broad "amorphous hump" on clay XRD plots suggested the presence of appreciable noncrystalline material, probably biogenic silica (Harris et al., 1994).
Repeated Extractions
The EC of repeated water extractions was greatest for active dairy soils, intermediate for abandoned dairy soils, and least for minimally impacted soils, with no overlap (Fig. 1)
. This trend reflects the salts in manure and their partial depletion via leaching on abandonment and cessation of manure loading. Some decline in EC with successive extractions was observed for all samples, but was sharpest for the active dairy samples. The initial high EC and its rapidly declining trend for the active dairy soils are consistent with the abundance of highly soluble Na and K salts in the extracts (data not shown).
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Fig. 1. Changes in electrical conductivity (EC) (dS m–1) with repeated water extractions. AD, active dairy manure–impacted soil; AB, abandoned dairy manure–impacted soil; MI, minimally impacted soil.
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Calcium removed by repeated extractions followed the same trend as for EC with respect to site groupings (active > abandoned > minimally impacted), but differed in that there was no decline with successive extractions for active dairies (Fig. 2)
. Calcium removal declined with successive extractions for most other samples. The data suggest that Ca is present in both soluble and sparingly soluble forms; though Ca is released less by abandoned than active dairy soils, significant amounts of Ca are nonetheless extracted. The sparingly soluble form could be calcite, which was detected via XRD analysis. Active dairy manure–impacted soils had more Ca in the eighth extractions (62.6–95.3 mg Ca kg–1 soil) than abandoned dairy soils (32.9–62.0 mg Ca kg–1 soil).
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Fig. 2. Changes in Ca concentrations (mg kg–1) with repeated water extractions. AD, active dairy manure–impacted soil; AB, abandoned dairy manure–impacted soil; MI, minimally impacted soil.
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The release of Mg and P, in contrast to Ca, did not differ between active and abandoned dairy soils; however, like Ca, Mg and P release was less for the minimally impacted soils (Fig. 3 and 4)
. In effect, there appears to be no significant highly soluble phase of Mg and P that becomes depleted with abandonment. Data are consistent with Mg and P being mainly in sparingly soluble form(s), which would require a longer time for depletion. Phosphorus release for AD-1 was unexpectedly low given its high total P content (Fig. 4). Possibly the CaCO3 present in fill material noted in this soil provides sites for more stable P sorption than for other soils. Another fill-influenced soil, an abandoned dairy (AB-1), also had relatively low release of P.
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Fig. 3. Changes in Mg concentrations (mg kg–1) with repeated water extractions. AD, active dairy manure–impacted soil; AB, abandoned dairy manure–impacted soil; MI, minimally impacted soil.
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Fig. 4. Changes in soluble reactive phosphorus (SRP) concentrations (mg kg–1) with repeated water extractions. AD, active dairy manure–impacted soil; AB, abandoned dairy manure–impacted soil; MI, minimally impacted soil.
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The correlation between Mg and P release was much stronger than that between Ca and P release for both active (Fig. 5)
and abandoned dairy (Fig. 6)
soils, suggesting there may be a Mg-P phase in these manure-impacted soils. The r2 value of Mg with P changed from 0.68 to 0.71 for the active dairies and from 0.62 to 0.75 for the abandoned dairies when the first two extracts (most heavily influenced by soluble salts) were removed from the regression equation. Harris et al. (1994) attributed nonstabilization of Ca-P forms to the likely inhibitory properties of manure-derived components such as DOC, Mg, and Si. Such manure-derived components may also inhibit Ca-P formation at the high pH of the soils used herein, but Mg-P associations may also favor continued release of P with time. No MgCO3 was detected in these soils, whereas calcite was identified as nonphosphate Ca form. A previous study (Nair et al., 2003) using stepwise regression analysis of 0.01 M KCl-P in soil leachates with Ca, Mg, Fe, and Al as the independent variables also showed that the soluble P was related to Mg concentrations for dairy manure–impacted soils supporting the Mg-P association noted here. The regression analysis showed no positive relationship for a Ca-P association, also in agreement with the current studies. However, the poor correlation of Ca with P does not necessarily mean that there is no Ca-P phase in the soil, because calcite in the samples could confound the Ca-P relationship.
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Fig. 5. Relationship between soluble reactive phosphorus (SRP) and Mg and Ca for active dairy manure–impacted soils released during repeated water extractions.
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Fig. 6. Relationship between soluble reactive phosphorus (SRP) and Mg and Ca for abandoned dairy manure–impacted soils released during repeated water extractions.
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Nair et al. (1995) found that the percent distribution of P fractions within the surface horizon of a young active dairy versus an old active dairy indicated reduction in Ca- and Mg-P forms and a concomitant increase in recalcitrant P (p < 0.001). They observed a reduction in Ca- and Mg-P forms, with subsequent increase in recalcitrant P, in abandoned versus active dairy soils (p < 0.001). This could have been due to Ca leaching in the abandoned dairy soils resulting in a reduction in the Ca- and Mg-P fraction with abandonment. The fractionation scheme used by the authors, however, grouped Ca- and Mg-P forms together, so individual associations between Ca with P, and Mg with P, were not distinguished.
Release of DOC in repeated extracts leveled off after the third extraction for active and abandoned manure-impacted soils (data not shown). Active dairy manure–impacted soils released more DOC after eight extractions (48.7–85.4 mg DOC kg–1 soil) than abandoned dairy soils (25.2–32.7 mg DOC kg–1 soil). It is likely that DOC could have leached with time, or could have been complexed into more stable forms, resulting in decreases in DOC with abandonment.
Soil Leaching and Chemical Equilibrium Modeling
Concentrations of ions in the column leachates were much greater than in the repeated extractions (e.g., mean SRP was 33 mg L–1, Ca was 87 mg L–1, and Mg was 54 mg L–1 in the leachates vs. SRP of 4 mg L–1, Ca of 7 mg L–1, and Mg of 3 mg L–1 for the water extractions), because of the much narrower soil to solution ratio in the column study. The leachates, in contrast to repeated extractions, showed a negative relationship between SRP and Ca and Mg. We attributed this trend to a common ion effect in the leachates arising from the high concentrations of salts (consistent with high EC in the initial leachates).
The leachates, presumably closer to equilibrium conditions as compared with the water extractions due to wider soil to solution ratios of the latter, were subjected to chemical equilibrium modeling. Modeling results for only the first, third, and seventh leaching events are presented (Table 2) because all other data show similar trends. We recognize that there are assumptions and uncertainties (e.g., approximation of organic speciation, nonequilibrium, kinetic effects, etc.) inherent in modeling complex solutions. We therefore restrict our interpretations to major trends indicated by the modeling results.
By considering negative saturation index (SI) values as undersaturated and positive values as supersaturated, solutions were undersaturated for all Mg-P minerals considered (Table 2), including struvite (NH4MgPO4·6H2O), suggesting that they would be dissolving during leaching events if they were present. However, undersaturation neither precludes nor confirms the presence of struvite or other sparingly soluble Mg-phosphate phases under non-equilibrium conditions. Leachates were supersaturated with respect to all Ca-P phases modeled, except brushite and monetite; however, some leachates were supersaturated even for these soluble Ca-P minerals considered.
Bohn and Bohn (1987) suggested that for soil solutions, a range of –1 < SI < 0 would be appropriate for designating "saturation" of a mineral. Based on this criterion of saturation, with data for all leaching events considered, Mg-P (i.e., struvite, farringtonite, and newberryite) and Ca-P (i.e., brushite and monetite) minerals were within the saturation range for some leachates. Hence, these minerals would be prospects for controlling P activity in active and abandoned dairy manure–impacted soils (Table 3). There were no clear distinctions between active and abandoned dairies based on the modeling results. The most thermodynamically stable mineral modeled, hydroxyapatite, would be stable (if present) but would not likely be a factor in P release from the soils. Over time, the formation of apatite would be thermodynamically predicted, but no apatite was detected in any soil by XRD. We hypothesized that release of P is controlled by a metastable phase (perhaps amorphous) of Mg- or Ca-P. These modeling results are consistent with other leachate studies involving similar dairy manure–impacted soils (Wang et al., 1995).
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Table 3. Percent of observations that are undersaturated, saturated, and supersaturated for selected minerals based on chemical equilibrium modeling of active and abandoned dairy column leachates.
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CONCLUSIONS
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The P released from high intensity areas in dairy soils during repeated water extractions was more closely associated with Mg than Ca release. Release of Mg and P was similar in both active and abandoned dairy sites. This is consistent with observations that old abandoned sites remain a source of P. Release of Ca was less for abandoned dairies than active dairies. Chemical modeling of column leachates indicated that all but the most soluble Ca-P minerals were supersaturated, whereas all Mg-P minerals were either undersaturated or near saturation. The observed data are consistent with the idea of a sparingly soluble Mg- or Ca-P phase controlling P release from these manure-impacted soils and maintaining elevated P concentrations in soils solutions even years after abandonment of the dairies. Further study is needed to determine if the P in the manure and soil is initially associated primarily with Mg or Ca. A finding that Mg-P is prevalent would diminish the prospect of stabilization via slow crystallization and transformation of P to more stable Ca-P forms. Also, loss of more soluble forms of Ca after abandonment would diminish the prospect of P released from a Mg-P phase being precipitated as Ca-P.
Knowledge of manure-derived components and their associations with P is pertinent to nutrient management for sandy soils with minimum P sorbing components. If a sparingly soluble Mg-P phase is responsible for the continued release of P from the manure-impacted soils, amendment additions (e.g., water treatment residuals) would be the only possible solution to stabilize the P because no stable Mg-P phase is expected to form. Alternatively, reduction of Mg-P via dietary management could be a viable strategy in reducing P solubility in the manure.
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ACKNOWLEDGMENTS
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We thank Dr. Don Graetz for providing us with some of the soil samples used in this study and Dr. George O'Connor for his assistance at various stages of this work.
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NOTES
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This research was supported by the Florida Agricultural Experiment Station and a grant from the USDA National Research Initiative, and approved for publication as Journal Series no. R-10100.
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ABSTRACT
INTRODUCTION
