Published in J. Environ. Qual. 33:1521-1527 (2004).
© ASA, CSSA, SSSA
677 S. Segoe Rd., Madison, WI 53711 USA
TECHNICAL REPORTS
Waste Management
Phosphorus Speciation in Manure-Amended Alkaline Soils
Jeremy C. Hansena,
Barbara J. Cade-Menunb and
Daniel G. Strawna,*
a Department of Plant, Soil & Entomological Sciences, University of Idaho, Moscow, ID 83844-2339
b Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305-2115
* Corresponding author (dgstrawn{at}uidaho.edu).
Received for publication August 28, 2003.
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ABSTRACT
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Two common manure storage practices are stockpiles and lagoons. The manure from stockpiles is applied to soils in solid form, while lagoon manure is applied as a liquid. Soil amendment with manure in any form introduces a significant amount of phosphorus (P) that exists in both organic and inorganic forms. However, little is known about P speciation in manure stored under different conditions, or the subsequent forms when applied to soils. We used solution 31P nuclear magnetic resonance (NMR) spectroscopy and conventional P fractionation and speciation methods to investigate P forms in dairy manure and liquid lagoon manure, and to study how long-term amendment with these manures influenced surface and subsurface soil P speciation. Our results show that the P forms in solid and lagoon manure are similar. About 30% of the total P was organic, mostly as orthophosphate monoesters. On a dry weight basis, total P was much higher in the solid manure. In the manure-amended soils the total P concentrations of the surface soils were similar, regardless of manure type. Total P in the subsurface soil was greater in the lagoon-manure-amended soil than the solid-manure-amended subsurface soil. However, the fraction of organic P was greater in the subsurface of the solid-manure-amended soil. The NMR results indicate that the majority of organic P in the soils is phytic acid, which is enriched in the surface soils compared with the subsurface soils. These results provide insight into P speciation and dynamics in manure-amended soils that will further increase our understanding on how best to manage manure disposal on soils.
Abbreviations: ICP–AES, inductively coupled plasma–atomic emission spectroscopy • NMR, nuclear magnetic resonance
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INTRODUCTION
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ALTHOUGH ORGANIC FORMS of phosphorus (P) play an important role in the biological availability of soil P, the concentrations, forms, and dynamics of organic P in soil are poorly understood. Organic P in animal manure has also received less attention than inorganic P forms, although as much as 40% of the total P in manure may be organic (Gerritse and Vriesema, 1984), and long-term additions of animal manure increase soil organic P concentrations (Zhang and MacKenzie, 1997). In light of the contribution of soil organic P to the P nutrition of plants (Oehl et al., 2001), and the potential for eutrophication from the transfer of organic P forms to water (Haygarth and Jarvis, 1999), it is important to have a clear understanding of the organic P forms supplied to soil from manure, and the behavior of these P species within the soil environment.
The fate of organic P in manure-amended soils is dependent on the chemical forms of the organic P. Orthophosphate diesters appear to be mineralized more rapidly than orthophosphate monoesters (Condron et al., 2004; Turner et al., 2002). Within the orthophosphate monoesters, myo-inositol hexakisphosphate (phytic acid) has a higher charge density than other monoesters such as sugar phosphates or mononucleotides, allowing it to form relatively stable complexes in soil that are protected from microbial degradation (Celi et al., 1999; Greaves and Webley, 1965).
The forms of organic P applied to soil in manure will depend on the nature of the animal manure. Manure from different animal species is known to contain different P forms and concentrations (Sharpley and Moyer, 2000; Turner, 2004). The handling and storage of manure before land application can also affect P forms and concentrations due to differences in microbial species, oxygen, pH, and temperature. Although dramatic differences in water-soluble and total organic P concentrations were observed between composted and raw dairy manure when leached by simulated rainfall (Sharpley et al., 2000), there has been little research into differences in the chemical species of organic P that may result from manure storage.
Two common manure storage practices are dry stockpiles, which may also be composted, and lagoons, in which the manure is mixed with large amounts of water (Palmer, 1993). The stockpiled manure is subsequently applied to the soil in solid form, while the lagoon manure is applied as a liquid. In addition to supplying the soil with different organic P forms from the different storage conditions, the application methods may affect the dynamics of these organic P forms. The goal of this project was to determine differences in soil organic P resulting from the application of dairy manure subjected to different storage treatments. Our objectives were to (i) compare the P forms in solid manure with those in liquid lagoon manure and (ii) investigate soil P speciation following the long-term application of these manures to an alkaline soil.
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MATERIALS AND METHODS
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Soils
Manure-amended soil samples used for this study were collected from a dairy farm in Gooding County in southern Idaho. The soils are classified as Kecko loamy fine sand (coarse-loamy, mixed, superactive, mesic Xeric Haplocalcids). Soil physicochemical properties are listed in Table 1. For more than 10 years, solid manure has been applied to one field in the spring before planting and in the fall after harvest, while an adjacent field has been irrigated throughout the growing season with water from a liquid manure holding pond. Both soils were irrigated throughout the growing season.
Because our sampling objective was to collect samples that had physicochemical characteristics representative of the soils in the field so that extensive laboratory P speciation experiments could be conducted, we used composite sampling as described by Peterson and Calvin (1996). Field observation was used to confirm that soils were morphologically similar. Three sampling sites for each field were chosen from an area of approximately 1 ha. Samples were taken from the surface (0–10 cm) and subsurface (45–65 cm) horizons, encompassing the top portions of the A and B horizons. The surface and subsurface samples were kept separate. Each subsample was air-dried (approximately 23°C), sieved to less than 2 mm, homogenized, and then composited in equal gravimetric portions to obtain a representative sample.
Manures
The liquid manure lagoons were sampled in August 2001 and October 2001. The NMR analysis was done on the August sample. The solid manure was sampled from the manure pile in which the manure from holding pens was collected and stored for 1 to 3 mo before field application. All manure samples were freeze-dried, sieved to less than 2 mm, and stored until further analysis.
Speciation
Solution 31P-NMR spectroscopy was used to determine the speciation of P in soils and manures. The soils and manures were extracted with 0.5 M NaOH + 0.1 M Na2EDTA, using a 1:10 solid to solution ratio, at room temperature overnight on an end-over-end shaker (Cade-Menun et al., 2002). After centrifuging the extracts, the supernatants were filtered through a 0.2-µm polysulfone membrane (Pall Life Sciences, Ann Arbor, MI) and lyophilized. Freeze-dried extracts were dissolved in 0.4 mL 10 M NaOH and 2.6 mL D2O and allowed to stand for 30 min with occasional vortexing. Samples were then centrifuged for 20 min at approximately 1500 x g, transferred to NMR tubes, and stored at 4°C before analysis within 24 h. Solution 31P NMR spectra were acquired at 202.45 MHz on a Varian (Palo Alto, CA) UNITYINOVA 500-MHz spectrometer equipped with a 10-mm broadband probe. We used a 90° pulse, 0.68-s acquisition, and 4.32-s pulse delay. Temperature was regulated at 25°C. An equal number of scans (8000) was collected for each sample and compounds were identified by their chemical shifts (ppm) relative to an external orthophosphoric acid standard. Peak assignments were based on Cade-Menun and Preston (1996) and Turner et al. (2003b), after standardizing the orthophosphate peak in all samples to 6 ppm. The spectra were processed with NUTS software (Acorn NMR, 2000), using visual inspection and automated peak analysis tools for peak-picking and spectral integration, and the percentages were calculated based on total peak area. For phytic acid determination we multiplied the percentage area under the peak at 5.3 ppm by 6. This calculation is based on the observation that the 5.3-ppm NMR peak, which originates from the C-2 position on the inositol ring, has minimal interference from other signals and is proportional to one-sixth of the total phytic acid concentration (Turner, 2004). The NaOH-EDTA extracts were diluted 1:100 with deionized water after NMR analysis, and the total P in each sample was determined by inductively coupled plasma–atomic emission spectroscopy (ICP–AES).
Total P in the soil samples was determined by digestion in HF and aqua regia, followed by ICP–AES. Organic P in the soils was measured using the Saunders and Williams (1955) ignition method (Cade-Menun and Lavkulich, 1997) and ICP–AES. All samples were run in triplicate, and the percent relative standard deviation (RSD) was less than 4%.
To gain additional insight into the partitioning of P, a selective-sequential extraction experiment was conducted. In this experiment P was partitioned into fractions, including exchangeable P, P associated with calcium (Ca), organic matter (OM), Fe oxides, and residual phases of the soil. Although this method is operationally defined, results do provide insights into the speciation and relative availability of P for desorption into soil solution because the extractants are increasingly aggressive with respect to their extraction potential. Exchangeable P was extracted with 1 M Mg(NO3)2, adjusted to pH 7.0. The Ca-bound P was extracted with 1 M NaCH3COO buffer adjusted to pH 4.8. The Fe-oxide-bound P was extracted using a citrate, dithionate, and bicarbonate (CDB) extract at 85°C. Organic-matter-bound P was extracted with 6% NaClO adjusted to pH 9.5 at 95°C. The residual P was digested with aqua regia and HF in a microwave digestion vessel. This method was adapted from a sequential extraction method outlined by Tessier et al. (1979), and is similar to the sequential extraction method used to fractionate P in marine sediments (Ruttenberg, 1992). Total P in the extract was determined on ICP–AES. All selective sequential extractions were done in triplicate. Relative standard deviations for all extracts were less than 5%, except for the exchangeable fraction measurements that had a RSD of 20 to 30%.
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RESULTS
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Soil and Manure Characterization
Total P and organic P concentrations in the surface soils after the two manure treatments were similar (Table 2). However, differences between the manure treatments were observed in subsurface soils. Although the concentration of total P in the lagoon-manure-amended soil was nearly double that of the soil amended with solid manure, organic P concentration (mg kg–1 and as a percent of total P) was much higher in the solid-manure-amended soils.
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Table 2. Phosphorus concentrations in surface and subsurface soils from manure-amended, lagoon-liquid-amended, and native soils.
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Total and organic P concentrations in the solid and liquid lagoon manures are given in Table 3. Of the total P, 26% was present as organic P for the solid manure and 18 to 73% was present as organic P for the liquid lagoon manure. The results of the liquid lagoon manure represent samples taken from the same pond on different dates. It is expected that the total P and distribution of species are highly variable through different ponds, seasons, and effluent source, thus explaining the high variability in the two lagoon samples (DeRoughey et al., 2002). The second sampling of the lagoon water had more suspended organic material than the first sample, which could explain the difference in organic P concentrations.
Nuclear Magnetic Resonance
The percent recovery by the NaOH-EDTA extraction was 78% of the total P in the solid manure, and 59% of the total P in the liquid lagoon manure (Table 4). The percent recovery for the soils was variable, ranging from 13 to 42% of the total soil P (Table 4). This is comparable with recovery rates of 12 to 45% for other semiarid soils (Turner et al., 2003a). For all samples, the primary form of extracted P was orthophosphate. Orthophosphate monoesters, orthophosphate diesters, and pyrophosphate were also present (Table 5 and Fig. 1 and 2). Total P in the NaOH-EDTA solution (measured by ICP–AES), organic P (determined by NMR spectroscopy), and the percent organic P in the original soil and manure samples are reported in Table 4. The organic P in the NaOH-EDTA extract determined by NMR spectroscopy is close to the total organic P measured by the ignition method, suggesting that P species extraction ratios are proportional to the original composition. Orthophosphate was highest in the subsurface of the soil amended with liquid manure. The total percentages of orthophosphate monoesters in the solid and liquid manure samples were twice as high as those of the amended soils, mostly due to a higher percentage of P in orthophosphate monoesters other than phytic acid (Table 5). These unidentified monoester compounds could include glucose phosphates, mononucleotides, phospholipid breakdown products, and lower inositol phosphates (Turner et al., 2003b). The percentages of P in phytic acid in the surface and subsurface of the solid-manure-amended soil, and the surface of the lagoon-manure-amended soil, were similar to the percentages of phytic acid in the manure samples. The other orthophosphate monoesters were higher in the subsurface soils than the surface soils for both soils. In contrast to the two manure samples, very little P in any of the soil samples was present as orthophosphate diesters. Polyphosphate was not detected in any sample, while pyrophosphate was present in all samples, with higher percentages in both types of manure than the soils. A very small phosphonate peak was observed only in the solid manure sample.
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Table 4. Recovery rate in NaOH-EDTA extracts (nuclear magnetic resonance [NMR] solutions), determined by inductively coupled plasma–atomic emission spectroscopy (ICP–AES). Also shown is organic P in solution determined by calculation from the organic P peaks in the NMR spectra, and total organic P in the original sample before extraction, determined after ignition.
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Table 5. Distribution of P forms in NaOH-EDTA extracts of each sample type, calculated from nuclear magnetic resonance (NMR) spectra by integration.
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Fig. 1. Phosphorus-31 nuclear magnetic resonance (NMR) spectra of manure and lagoon samples extracted with 0.5 M NaOH + 0.1 M Na2EDTA. The NMR parameters were 202.45 MHz, 90° pulse, 0.68-s acquisition, 4.32-s delay, 25°C, 8000 scans, and 20-Hz line broadening. The * marks the diagnostic peak for phytic acid, and the insets show expanded orthophosphate monoester and diester regions (off-set for the manure sample), and an expanded phosphonate region for the manure sample.
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Fig. 2. Phosphorus-31 nuclear magnetic resonance (NMR) spectra of solid-manure-amended (A) and lagoon-manure-amended (B) surface and subsurface soils extracted with 0.5 M NaOH + 0.1 M Na2EDTA. The NMR parameters were 202.45 MHz, 90° pulse, 0.68-s acquisition, 4.32-s delay, 25°C, 8000 scans, and 20-Hz line broadening. The insets show expanded orthophosphate monoester and diester regions.
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Selective Sequential Extraction
The results from the fractionation procedure are presented in Fig. 3. In the surface horizon of both amended soils, 14 to 17% of the total P exists in the exchangeable phase, while in the subsurface horizon of both amended soils less than 5% of the total P is found in the exchangeable phase. The Ca-bound fraction comprises 19 to 24% of the total P for the surface soils, and 10 to 17% for the subsurface soils. Ruttenberg (1992) showed that acetate buffer was effective at dissolving diagenic Ca-PO4 minerals as well as carbonate minerals. The Fe-oxide fraction for the surface and subsurface soils ranged from 9 to 12% of the total P. Only the surface soils had detectable concentrations of P associated with organic matter, with less than 5% for both. The lack of recovery of organic P in the organic extraction step is probably a result of readsorption of mineralized phosphate and/or lack of complete organic P extraction.
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Fig. 3. Results of sequential P extraction of manure-amended and lagoon-manure-amended surface and subsurface soils. The fractions shown are exchangeable, Ca-bound, Fe and Al oxide, organic matter, and residual P (%).
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DISCUSSION
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The results of this study suggest that the form of applied manure will affect soil P forms and dynamics. The manures themselves were not very different with respect to their P forms. Nuclear magnetic resonance spectroscopy and chemical analyses showed that about 30% of the total P was organic, which corresponds to the values of Gerritse and Vriesema (1984) and Turner (2004). Orthophosphate monoesters were the predominant form of organic P in both types of manure. Phytic acid represented more than half of the orthophosphate monoesters of solid manure, and one-third of the monoesters of liquid manure. Phytic acid is the dominant P compound in most cereal grains fed to animals. However, because most animals cannot digest phytic acid (Taylor, 1965), high concentrations are common in animal manures (Peperzak et al., 1959; Turner, 2004). The other orthophosphate monoesters could not be identified as specific compounds in our spectra due to the overlapping of peaks. However, these may include glucose phosphates, mononucleotides, phospholipid breakdown products, and lower inositol phosphate compounds or isomers (Turner et al., 2003b). The higher percentage of P as other monoesters, DNA, and pyrophosphate in the lagoon manure suggests that there is more microbial activity because these P forms are commonly associated with microbes (Ghonsikar and Miller, 1973; Magid et al., 1996; Turner et al., 2003c).
One interesting difference between the two types of manure is their total P content. On a per gram dry weight basis, total P in the solid manure was substantially greater than that of lagoon manure, regardless of sampling date. However, the total P concentrations of the surface soils receiving the two types of manure are comparable (the lagoon-manure-amended soil is 3% higher). This suggests that either the net P application is similar, or the P retention capacity of surface soils of the fields is similar.
In the subsurface of the lagoon-manure-amended soil the concentration of inorganic P was greater than the concentration in the subsurface of the solid-manure-amended soil (Table 2). This difference could occur if the P in the lagoon manure exists in different forms than the P in the solid manure. However, our speciation results show that the P speciation fractionation is similar in the liquid and solid manures. Jensen et al. (2000) observed an increase in inorganic P mobility in manure-amended clay soils under saturated flow conditions but not under unsaturated flow conditions. Their results suggest that the addition of water with the manure may be an important factor for P mobility. In particular, if long-term manure amendment has nearly saturated the P sorption sites in the surface soils, P added in lagoon manure may be flushed to lower depths without reacting with surface minerals (Menzies et al., 1999), while the P applied in the solid manure may have more time to react with minerals near the surface. This suggests that long-term application of lagoon-manure P could be a larger environmental risk than P applied in the form of solid manure.
The surface soils are the only soils to show organic-matter-associated P during fractionation (Fig. 3). Phytic acid and pyrophosphate were also enriched in the surface soils (Table 5). Phytic acid, with six phosphate groups, has a high charge density, and can bind strongly to soil mineral surfaces (Celi et al., 1999, 2000). It can also precipitate as insoluble calcium salts in alkaline soils (Celi et al., 1999; Jackman and Black, 1951; Tunesi et al., 1999). Thus, we would expect any phytic acid applied from manure to be retained in these soils and have low mobility. The decrease in the surface soils of other monoester forms, which have fewer P groups and thus a lower charge density than phytic acid, may be due to the replacement or exclusion of these P forms on P exchange sites by phytic acid (Celi et al., 2000), allowing the less stable monoester P compounds to be mineralized by soil microbes or mobilized into the subsurface.
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CONCLUSIONS
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Our results show that P forms in solid manure and lagoon manure are similar. Both had about 30% organic P, found mostly as orthophosphate monoesters. Phytic acid was present in both manure types. The two manures differed in total P concentrations on a dry weight basis; solid manure had much higher P concentrations than lagoon manure.
Despite different amendment management strategies, the total amounts of P in the surface soils were similar. The biggest difference could be seen in subsurface soils, where total P concentration in the lagoon-manure-amended soil was greater than total P concentration in the solid-manure-amended soil. However, the P in the lagoon-manure-amended soil was only enriched in the inorganic fraction, suggesting that inorganic P applied in liquid manure is more mobile than organic P in these soils.
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ACKNOWLEDGMENTS
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This project was supported by the USGS State Water Resources Research Institute Program. We are grateful for the thorough reviews provided that greatly improved this paper, as well as assistance from dairymen and scientists in southern Idaho. The NMR analyses were performed at the Stanford Magnetic Resonance Laboratory with support funding from the Stanford University School of Medicine and the assistance of Dr. Corey Liu.
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TOP
ABSTRACT
INTRODUCTION
