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Published online 4 January 2008
Published in J Environ Qual 37:154-163 (2008)
DOI: 10.2134/jeq2007.0141
© 2008 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
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TECHNICAL REPORTS

Waste Management

Phosphorus and Nitrogen Sorption to Soils in the Presence of Poultry Litter-Derived Dissolved Organic Matter

Keith W. Goynea,*, Hee-Joong Junb, Stephen H. Andersona and Peter P. Motavallia

a Dep. of Soil, Environmental, and Atmospheric Sciences, 302 ABNR Building, Univ. of Missouri, Columbia, MO 65211-7250
b National Institute of Agricultural Science and Technology, Suwon, Korea

* Corresponding author (goynek{at}missouri.edu).

Received for publication March 21, 2007.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 
Two environmental aspects associated with land application of poultry litter that have not been comprehensively evaluated are (i) the competition of dissolved organic matter (DOM) and P for soil sorption sites, and (ii) the sorption of dissolved organic nitrogen (DON) relative to inorganic nitrogen species (e.g., NO3– and NH4+) and dissolved organic carbon (DOC). The competition between DOM and P for sorption sites has often been assumed to increase the amount of P available for plant growth; however, elevating DOM concentrations may also increase P available for transport to water resources. Batch sorption experiments were conducted to (i) evaluate soil properties governing P sorption to benchmark soils of Southwestern Missouri, (ii) elucidate the impact of poultry litter-derived DOM on P sorption, and (iii) investigate DON retention relative to inorganic N species and DOC. Soils were reacted for 24 h with inorganic P (0–60 mg L–1) in the presence and absence of DOM (145 mg C L–1) using a background electrolyte solution comparable to DOM extracts (I = 10.8 mmol L–1; pH 7.7). Soil P sorption was positively correlated with metal oxide (r2 = 0.70) and clay content (r2 = 0.79) and negatively correlated with Bray-1 extractable P (r2 = 0.79). Poultry litter-derived DOM had no significant negative impact on P sorption. Dissolved organic nitrogen was preferentially removed from solution relative to (NO3––N + NO2––N), NH4+–N, and DOC. This research indicates that poultry litter-derived DOM is not likely to enhance inorganic P transport which contradicts the assumption that DOM released from organic wastes increases plant-available P when organic amendments and fertilizer P are co-applied. Additionally, this work demonstrates the need to further evaluate the fate and transport of DON in agroecosystem soils receiving poultry litter applications.

Abbreviations: DOM, dissolved organic matter • DON, dissolved organic nitrogen • DOC • dissolved organic carbon • OM, organic matter • LMWOAs, low molecular weight organic acids • SOC, soil organic carbon


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 
POULTRY production in Missouri is a large and rapidly expanding industry. In 2002, 273 million broiler chickens and 18.4 million turkeys were produced within the state (USDA-NASS, 2004). Poultry production primarily occurs within confined animal feeding operations (CAFOs) located within the southwestern counties of Barry, Jasper, Lawrence, McDonald, and Newton (USEPA, 2006). Poultry production in these counties generates approximately 4.75 x 105 tons of litter per year (MDNR, 2000). Storage and transportation of poultry litter can be costly for producers; therefore, most of the litter produced is repeatedly land-applied on-site or on land within 80 to 160 km from a poultry producing facility (Sharpley et al., 1993; Pelletier et al., 2001). Repeat applications of litter can cause nutrient build-up in the rhizosphere and increased nutrient migration to surface and ground water resources (Kingery et al., 1994; Sharpley, 1997; Sauer et al., 2000; Sims and Pierzynski, 2005).

Numerous studies have investigated various agronomic benefits (e.g., fertilizer source and organic soil amendment) and environmental issues (e.g., P accumulation in soil, P loss via surface runoff, P induced eutrophication of surface waters) surrounding land application of poultry litter for use as a nutrient source or for waste disposal (Sims and Wolf, 1994; Sharpley and Rekolainen, 1997; Evers, 2002; Motavalli et al., 2003; Pote et al., 2003; Sharpley et al., 2003; Sistani et al., 2004). However, less is known about the impact of manure-derived DOM on P sorption to soil (Guppy et al., 2005b; Sims and Pierzynski, 2005). Two environmental and agronomic aspects associated with land application of poultry litter that have not been extensively explored are (i) competition between manure-derived DOM and P for sorption sites and (ii) the relative sorption of DON compared with inorganic N species (NO3– and NH4+) and DOC.

Organic compounds released into solution from surface-applied organic amendments during and after rainfall events may inhibit P sorption via competitive interactions for sorption sites. Additionally, the formation of DOC-metal complexes can enhance dissolution reactions that reduce the number of available sorption sites and alter surface charge chemistry (i.e., increase negative surface charge) to an extent that P sorption is diminished due to electrostatic repulsion (Guppy et al., 2005b). Cumulatively, competitive interactions between DOC and P are believed to provide agronomic benefits by increasing plant-available P when organic amendments are land-applied (Othieno, 1973; Singh and Jones, 1976; Hue, 1991; Ohno and Erich, 1997). The same chemical processes reducing P sorption may, however, increase P available for surface transport to surface waters via runoff or enhance downward migration of P in soil. Phosphorus leaching may be particularly problematic in sandy soils (Djodjic et al., 2004). In contrast, mineral-sorbed or particulate organic matter (OM) can also increase P sorption by increasing the number of sorbed cations available to form cation bridges with P (Appelt et al., 1975; Perrott, 1978; Guppy et al., 2005b).

Studies have shown that low molecular weight organic acids (LMWOAs), humic and fulvic acids, and sorbed organic matter can inhibit P sorption via competitive interactions for sorption sites (Sibanda and Young, 1986; Hue, 1991; Violante and Gianfreda, 1993; Bolan et al., 1994; Staunton and Leprince, 1996; Bhatti et al., 1998). In addition, Mikutta et al. (2006) observed that citrate can reduce P sorption up to 28% by clogging micropores in goethite.

Few studies have evaluated the influence of water-soluble dissolved organic compounds released from land-applied organic amendments on soil P sorption (Othieno, 1973; Ohno and Crannell, 1996; Ohno and Erich, 1997; Guppy et al., 2005a). Othieno (1973) investigated the effects of surface application of mulch and mulch leachate on the downward migration of fertilizer P through soil, plant utilization of applied P, and forage yield. Results from this study suggest that surface mulch increased P migration and vertical P distribution, and improved forage yield and applied P utilization. The author estimated that 30 and 47% of the increases in fertilizer P uptake and plant tissue P, respectively, were attributable to factors such as DOM and P competition for sorption sites. However, as noted by Guppy et al. (2005a), Othieno (1973) failed to account for P released from the organic amendments which could compete with fertilizer P for sorption sites, thus increasing fertilizer P available for plant uptake.

More direct investigations have also been performed to elucidate impacts of organic amendment-derived DOM on P sorption (Ohno and Crannell, 1996; Ohno and Erich, 1997; Guppy et al., 2005a). Ohno and Crannell (1996) studied the influence of green and animal manure-derived DOM on P sorption to acidic soil as a function of DOC concentration (0–240 mg C L–1) at a fixed P concentration (1.25 g P kg–1 soil). Green manure-derived DOM inhibited P sorption as a function of DOC concentration, whereas animal manure-derived DOM enhanced P sorption or had no significant effect. The ability of green manure-derived DOM to inhibit P sorption to a greater extent was attributed to the lower molecular weight of these compounds in comparison to animal manure-derived DOM. Subsequently, animal manure-derived DOM was less capable of forming complexes with Al and Fe oxides and competition with P for adsorption sites was reduced in comparison to green manure (Ohno and Crannell, 1996). Complexation of DOM from green manure with Al oxides was confirmed in subsequent kinetic experiments (Ohno and Erich, 1997).

In contrast, Guppy et al. (2005a) observed no net decrease in P sorption to highly weathered soils when inorganic P (0–800 mg kg–1 soil) and leachate DOM from soybean (50 and 500 mg C L–1) and Rhodes grass (9.6 and 96 mg C L–1) were reacted simultaneously. Transient decreases in P sorption were observed after the P and fulvic acid fraction of leachates were reacted with soil for 1.5 h, but no significant decreases were detected after 6 d. Due to lack of agreement in the literature and the few studies investigating effects of soluble DOM extracted from organic amendments on P sorption, additional studies are required to further understanding of competitive interactions between P and DOM in soil.

Little is known about the behavior of DON in agricultural soils and the potential of these organic compounds to interact with or migrate through soils relative to inorganic N species. However, evidence suggests that DON is an important component of the N cycle that has been overlooked in agroecosystems (Murphy et al., 2000; Siemens and Kaupenjohann, 2002; Jones et al., 2004; Christou et al., 2005). Studies investigating the flux of DON through agricultural soils have demonstrated that DON can account for 10 to 20% of total N movement through the soil profile at depths > 0.5 m (Murphy et al., 2000; Siemens and Kaupenjohann, 2002). It has also been shown that DON accounts for a greater proportion of total N flux in soil receiving manure application relative to non-manured soil (Murphy et al., 2000).

Additionally, decreases in DON concentrations in soil solution as a function of depth are often reported to be significantly lower than decreases in DOC (Siemens and Kaupenjohann, 2002), suggesting that DON is less readily sorbed to the soil matrix compared to DOC. These observations are consistent with studies investigating changes in DOC and DON in forested ecosystems (Qualls and Haines, 1991; Michalzik et al., 2001). Cumulatively, this information suggests that DON leached through manure-amended soils may be transported to surface or ground waters, mineralized to NH4+ or NO3– through microbial decomposition (Jones et al., 2004), and potentially contribute to N pollution of drinking water resources.

Previous studies have documented an increase in nutrient loads and fecal coliform bacteria densities in Ozark Plateau streams that are draining watersheds which contain a significant number of animal livestock operations (Petersen et al., 1998; Schumacher, 2001; Haggard et al., 2003). Therefore, benchmark soils of this region of Missouri were used to investigate the following objectives: (i) to elucidate important soil physical and chemical characteristics governing P sorption to soils of Southwestern Missouri; (ii) to determine if DOM derived from poultry litter competes with inorganic P for sorption sites in soil; and (iii) to investigate if DON is retained more or less strongly to soil surfaces than inorganic forms of N (NO3– and NH4+) and DOC.


   Materials and Methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results and Discussion
 Conclusions
 REFERENCES
 
Soils
The A and B horizons of three Ultisols collected in southwestern Missouri were obtained from the Missouri Cooperative Soil Survey archives and used in sorption experiments. Soil samples were chosen based on their (i) predominance of the soil series in Southwestern Missouri, (ii) use in agriculture (samples were collected from pastures and hay production fields), and (iii) representation of various Bray-1 P and organic carbon contents (Table 1 ). Samples obtained from the archives had been previously passed through a 2-mm sieve and air-dried before storage.


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  		Table 1. Selected properties of Southwestern Missouri soils used in sorption experiments.

 
Soil pH was measured in water using a 1:5 (w/v) soil to solution ratio. Total organic carbon (TOC) and total nitrogen (TN) contents were determined using a LECO TruSpec CN analyzer (St. Joseph, MI). Bray-1 P was measured by the University of Missouri Soil and Plant Testing Laboratory (Columbia, MO) using standard Missouri soil test procedures (Nathan et al., 2006). Citrate-bicarbonate-dithionite extractable Fe and Al were determined following standard methods (Loeppert and Inskeep, 1996). Metal concentrations in extracts were quantified using a Varian Liberty inductively coupled plasma (ICP)–atomic emission spectrometer (AES; Palo Alto, CA). Soil particle size analysis data were obtained from the Missouri Cooperative Soil Survey Pedon Database (Missouri Cooperative Soil Survey, 2007) which contains both soil physical and chemical analyses of the samples used in this research (Table 1).

Poultry Litter and Water-Soluble Poultry Litter Extract
Broiler litter (manure and wood shaving bedding material) was sampled from a poultry house near Neosho, Missouri. Twenty subsamples were randomly collected within the house, bulked, and thoroughly mixed. The composite samples were placed in plastic bags, sealed, transported on ice to the laboratory, and stored under frozen conditions at –15°C. Poultry litter composition (Table 2 ) was determined by Penn State Agricultural Analytical Services Laboratory (University Park, PA) using recommended methods of manure analysis (Peters, 2003). Total carbon and TN were measured using a Fisons NA1500 Elemental Analyzer (Dearborn, MI). Ammonium and nitrate concentrations and pH (1:2 litter/water) were determined using ion-specific electrodes. Organic nitrogen was calculated by difference (TN minus NO3––N and NH4+–N). All other analytes (P, K, Ca, Na, Mg, S, Mn, Fe, Cu, and Zn) were quantified using ICP–AES after microwave-assisted acid digestion.


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  		Table 2. Chemical characteristics of sampled broiler poultry litter (dry mass basis).

 
Poultry litter extract was prepared by reacting 250 g (oven-dry mass) of litter in 1 L of 0.5 mmol L–1 HgCl2. Mercuric chloride was added to inhibit microbial growth (Wolf et al., 1989). The litter suspension was placed on a platform shaker at a low speed for 24 h and extraction was conducted in the absence of light at 20°C (± 1°C). Following centrifugation at 10,000 rpm for 30 min, the supernatant solution was decanted. The extract was centrifuged a second time at 12,000 rpm for 30 min and supernatant solution was filtered though glass fiber prefilters in tandem (Millipore Type AP20 and AP15) and a 0.45 µm nominal pore size Durapore membrane filter (Millipore Corp., Billerica, MA). After filtration, stock solutions were stored in acid-washed HDPE bottles at 4°C in the absence of light. Subsample aliquots used for cation or DOC and TN analyses were acidified to pH 2 using trace metal grade 6 mol L–1 HNO3 and 6 mol L–1 HCl, respectively.

The aqueous chemical composition of preliminary litter extractions is shown in Table 3 . Preliminary extract chemical data were used to prepare an appropriate background electrolyte solution for sorption experiments. Samples were analyzed for pH, DOC, and total dissolved nitrogen (TDN) concentrations (TOC-VCSH total organic carbon analyzer equipped with a TNM-1 total nitrogen measuring unit and an ASI-V autosampler, Shimadzu Corp., Japan), and elemental concentrations (Elan DRC II ICP–MS with attached dynamic reaction cell, PerkinElmer Inc., Waltham, MA). Anion analyses were performed on non-acidified extract samples using a Dionex DX-600 ion chromatography (IC) unit (Dionex Corp., Sunnyvale, CA) and/or samples acidified to pH 2 with trace metal grade H2SO4 using a Konelab AquaKem 200 photometric analyzer (Labmedics, Salford Quays, UK). Anion concentrations (NO2–, NO3–, and PO43–) in preliminary poultry litter extracts were determined using both instruments, whereas supernatant solutions from sorption experiments were analyzed using only photometric analyses. Ammonium concentrations were determined using the Konelab photometric analyzer.


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  		Table 3. Aqueous chemical composition of preliminary water-soluble poultry litter extract.

 
Values of NO3––N + NO2––N are reported in Table 3 because significant concentrations of NO2––N were detected in the preliminary poultry litter extract solution (mean value = 217 mg L–1). Photometric analysis is unable to distinguish between NO3––N and NO2––N in solution due to transformation of NO3– to NO2– after reaction with the Cd column within the instrument. Thus, we have chosen to express these values as NO3––N + NO2––N. Dissolved organic nitrogen was calculated by difference (TDN minus NO3––N + NO2––N and NH4+–N) (Goyne et al., 2000).

Phosphorus Sorption Experiments
Phosphorus sorption experiments were conducted over a range of initial aqueous phase P concentrations (0, 5, 10, 15, 30, 60 mg L–1) in the presence and absence of poultry litter extract. Air-dried soils (1.000 g oven-dried mass) were added to 50 mL polypropylene co-polymer (PPCO) centrifuge tubes and suspended in a background electrolyte solution (I = 10.8 mmol L–1) containing 6.9 mmol L–1 KCl, 3.9 mmol L–1 NaCl, and 0.05 mmol L–1 HgCl2. The electrolyte solution was adjusted to pH 7.7 using a small volume of KOH/NaOH solution (I = 10.8 mmol L–1). Concentrations of K and Na in background electrolyte solution represent a 100x dilution of the major cations present in preliminary poultry litter extract solutions and pH 7.7 is representative of the poultry litter extract solution (Table 3). Reaction vessels containing background electrolyte solution with and without soil (samples and controls, respectively) were spiked with a volume of P stock solution to achieve the desired initial concentration. Phosphorus stock solution (pH 7.7) was prepared by diluting concentrated phosphorus standard (NH4H2PO4, SPEX CertiPrep, Metuchen, NJ) in the background electrolyte solution previously described. The total solution volume in reaction vessels was 30.00 mL. All sorption experiments were conducted in the absence of pH buffers to prevent competitive sorption between buffer constituents (e.g., phosphate or organic compounds) and added P or DOM, and to allow for measurement of pH shifts often indicative of ligand exchange mechanisms (Stumm, 1992).

In experiments investigating competitive sorption between phosphorus and DOM, soils suspended in background electrolyte solution with varying P concentrations were also spiked with poultry litter extract to achieve an initial DOC concentration (145 mg C L–1). This DOC concentration is representative of DOC in runoff and soil solution in fields having recently received manure application (Bhogal and Shepherd, 1997; Gallet et al., 2003; Pote et al., 2003). The DOC concentration is also representative of a 100x dilution of concentrated poultry litter extract. The PO43––P concentration of control samples containing no soil and no added inorganic P but spiked with DOM was 0.32 ± 0.01 mg L–1 (mean ± 95% CI).

After solution addition, triplicate samples and duplicate controls (no soil) were placed on end-over-end shakers (8 rpm) and allowed to react in the dark at 25°C (± 1°C) for 24 h (Kaiser, 2001; Lilienfein et al., 2004a, Lilienfein et al., 2004b). After reaction, samples were centrifuged at 18,500 rpm for 20 min, and supernatant solutions were filtered through a pre-rinsed Whatman Puradisc 0.45 µm nominal pore size membrane syringe filter (Whatman Inc., Florham Park, NJ). Solution pH was measured immediately following filtration. Samples were partitioned into acid-washed 15 mL amber HDPE bottles, acidified, and stored at 4°C until analysis of PO43––P, NO3––N + NO2––N, NH4+–N, DOC, and TDN as described previously.

Sorption Isotherms
Phosphorus sorption to soil was calculated as:

Formula 1[1]
where qp is the surface excess of phosphorus (mg kg–1), V is the volume of solution (L) added to reaction vessels, CS and CC are the equilibrium phosphorus concentrations (mg L–1) in supernatant solutions of soil suspensions (S) and for the corresponding controls (C) after the reaction period, and m is soil mass (kg) (Essington, 2004). Adsorption data were fit to a modified Langmuir equation (Eq. [2]) that includes a parameter (a) allowing for a nonzero y-intercept (Lilienfein et al., 2004a, 2004b).

Formula 2[2]
In the modified Langmuir equation, the parameters K, b, and a are adjustable. The adsorption constant (K; L kg–1) is a measure of isotherm intensity, C is the P concentration in solution after reaction, b (mg kg–1) is adsorption maxima, and a is the y-intercept which describes P release from reacted samples (mg kg–1). Inclusion of this term was necessary due to desorption of P observed in samples where no P was added.

Preferential Nitrogen Species Sorption
Solution phase carbon and nitrogen data acquired during phosphorus sorption studies conducted in the presence of added DOM, but no added inorganic P stock solution, were used to determine if DON sorption is different relative to DOC and inorganic nitrogen species sorption. This was accomplished by comparing the ratios of DOC/DON, DON/(NO3––N + NO2––N), and DON/NH4+–N in solution of samples (with soil) and controls (no soil and no added inorganic P) and after 24 h of reaction time. Significant differences between analyte ratios in controls and samples were used as an indicator of preferential analyte sorption. Ratio differences between soils were also evaluated.

Statistical Analyses
Nonlinear regression analysis was used to fit the Langmuir equation to sorbed P versus equilibrium P concentration data for the selected soils and to identify the modified Langmuir fitting parameters (Origin v.7.5 SR6; OriginLab Corp., 2003). Linear and nonlinear regression analysis was used to correlate phosphorus sorption to various soil properties. Analysis of variance was used to compare means for DON/(NO3––N + NO2––N), DON/NH4+–N, and DOC/DON ratio data, as well as determining differences among soils and horizons using the PROC GLM procedure with LSD tests at the 0.05 significance level (SAS 9.1; SAS Institute, 2003).


   Results and Discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
Conclusions
 REFERENCES
 
Phosphorus Sorption in the Absence of Dissolved Organic Matter
Phosphorus sorption to the A and B horizons of the three different soil series investigated is shown in Fig. 1a -f, and the sorption isotherms are well described by the modified Langmuir equation (Table 4 ). Comparison of isotherms for A horizon soils indicates that the Tonti soil sorbs P to the greatest extent and the Goss soil sorbs the least amount of P from solution (Fig. 1a, c, e). Data collected for B horizon soils indicates that P sorption follows the order Tonti > Goss > Clarksville (Fig. 1b, d, f).


Figure 1
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  		Fig. 1. Phosphorus sorption to Clarksville (a) A and (b) B soil horizons, Goss (c) A and (d) B soil horizons, and Tonti (e) A and (f) B soil horizons in absence and presence of dissolved organic matter (DOM) (145 mg C L–1). (Means of triplicate samples are shown and error bars that are larger than the symbol represent 95% C.I.). Lines represent fits to modified Langmuir equation.

 

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  		Table 4. Modified Langmuir parameters for phosphorus sorption in presence and absence of dissolved organic matter (DOM).

 
Linear and nonlinear regression analyses were performed to correlate properties of all the soils studied (n = 6) with the b parameter (P sorption capacity) of the modified Langmuir equation. The results demonstrate that P sorption is positively correlated with the sum of citrate–bicarbonate–dithionite extractable Fe and Al (AlCBD+FeCBD) (y = –122 + 78x; r2 = 0.70; p = 0.039) and clay content (y = 0.019 + 15.5x; r2 = 0.71; p = 0.036). Sorption capacity was negatively correlated with Bray-1 extractable P [y = 279 – 38 ln(x-2.1); r2 = 0.79], and a very weak negative correlation was observed with percent soil organic carbon (SOC) content (y = 337 – 68x; r2 = 0.21; p = 0.37). Due to the narrow ranges of Bray-1 P concentration (2.1–5.8 mg kg–1) and SOC content (0.37–0.60%), along with the probability that other soil factors (e.g., mineralogy and clay content) are expected to significantly affect P adsorption capacity in B horizons, linear regression analyses were performed using A horizon data only (n = 3). The analyses also show a negative correlation for b versus Bray-1 P and SOC content with coefficients of determination of 0.87 and 0.98, respectively, thus indicating that both extractable soil P and SOC content are important factors that may limit P sorption in soil A horizons.

Although the regression equations were developed using a limited number of data points, the analyses assist with interpretation of particular soil factors governing maximum P adsorption capacity. For example, the Tonti A horizon sorbed a significantly greater amount of P (mean ± 95% C.I. = 243 ± 11 mg kg–1) than did the Clarksville and Goss soils (132 ± 37 and 85 ± 37 mg kg–1) when reacted with the highest initial P concentration. This can be attributed to the Tonti soil having the lowest SOC content (1.26%), the lowest Bray-1 P concentration (9.7 mg kg–1), the highest clay content (14.6%), and the highest AlCBD+FeCBD concentration (5.44 g kg–1) among the A horizons investigated. These are all factors that positively enhance P uptake from solution. In contrast, the Goss A horizon soil has characteristics less favorable for P adsorption (i.e., highest SOC and Bray-1 P, and the lowest clay and AlCBD+FeCBD content) and therefore exhibits the lowest maximum P adsorption capacity among A horizons of the three observed soils. Similar observations can be made regarding correlations between soil properties and P sorption to B horizon soils.

Previous studies have documented analogous correlations between P sorption and soil properties as those documented in this study. Several studies have shown that higher soil clay content results in increased surface area and a greater number of sorption sites; thus, soils with higher clay content typically sorb P more readily than coarse-textured soils (McDowell et al., 2001; Penn et al., 2005; Sims and Pierzynski, 2005; Zhang et al., 2005). Soils containing greater concentrations of extractable soil test P have been shown to diminish P sorption capability, due to occupation of sorption sites by historically applied P (Siddique and Robinson, 2003; Lehmann et al., 2005). Particulate as well as adsorbed organic matter can increase the net negative soil charge to inhibit phosphate anion sorption as well as occupy sites where P could be sorbed (Iyamuremye and Dick, 1996; Daly et al., 2001; Guppy et al., 2005b; Sims and Pierzynski, 2005). Extractable Fe and Al oxides are well known to influence P adsorption (Lewis et al., 1981; McDowell et al., 2001; Giesler et al., 2005; Zhang et al., 2005) and this effect is attributed to the high reactivity of these solids with phosphate anions which results in formation of inner-sphere complexes on the mineral surface (Tejedor-Tejedor and Anderson, 1990; Bhatti et al., 1998; Sims and Pierzynski, 2005). Therefore, the results of this study corroborate the importance of certain soil properties on P sorption and demonstrate that these soils exhibit typical behavior with regard to factors influencing P sorption behavior. The latter is particularly important to consider when evaluating the influence of poultry litter-derived DOM on P sorption.

Potential Mechanism of Phosphorus Sorption
Figure 2 illustrates the change in suspension pH after reaction as a function of P sorption in absence of DOM. The data indicate that the pH of all samples is lower than the mean control pH and, in absence of added P, suspension pH is buffered to approximately soil pH (± 0.5 pH units; Table 1). However, suspension pH increases concurrently with increased P sorption in all soils despite the fact that pH in control samples remains relatively constant at pH 7.53 ± 0.06. Therefore, the data suggest that on adsorption, phosphate species displace hydroxyl functional groups into solution from the surface of soil minerals, such as Fe and Al oxides, via a ligand exchange mechanism as shown in Eq. [3] (Bhatti et al., 1998).

Formula 3[3]
This hypothesized mechanism of adsorption is supported by previous studies investigating P sorption to goethite and ferrihydrite using spectroscopic techniques (Tejedor-Tejedor and Anderson, 1990; Khare et al., 2005) and the positive correlation between P sorption and soil AlCBD+FeCBD content observed in this study. Differences in the slopes of suspension pH versus sorbed P can be attributed, presumably, to differences in soil buffering capacity.


Figure 2
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  		Fig. 2. Change in suspension pH (mean values shown) as a function of phosphorus sorption (qp) after reaction for (a) A horizons and (b) B horizons reacted with P in the absence and presence of dissolved organic matter (DOM). Line represents mean pH of control samples (no soil) after reaction.

 
Phosphorus Sorption in the Presence of Dissolved Organic Matter
Sorption isotherms for A and B horizon soils reacted with P in the presence of poultry litter-extracted DOM are presented in Fig. 1a-f. Modified Langmuir fitted parameters are shown in Table 4. Based on the relatively high coefficients of determination (0.90–0.99) the isotherm equation reasonably fits the sorption data in presence of DOM. However, the most important finding from this aspect of the study was that poultry litter extracted DOM did not reduce P sorption and may actually enhance soil uptake of this nutrient.

Comparison of the modified Langmuir fitted parameters (Table 4) indicated no significant differences in P sorption maxima (b) for the Clarksville A, Goss B, and Tonti B horizons when DOM was present in solution at a concentration of 145 mg C L–1 as compared to samples not spiked with DOM. Similarly, no significant differences were noted in isotherm intensity (K) and P desorption (a) from these soils when DOM was added to reaction vessels, relative to experiments conducted in absence of DOM. However, predicted maximum P sorption to the Goss and Tonti A horizon and the Clarksville B horizon soils in presence of DOM was significantly higher than corresponding soils reacted with only P. Differences in b between these particular samples in the presence and absence of DOM should be viewed with caution due to overlap of 95% confidence intervals for individual isotherm data points, particularly for data points at the upper end of the isotherms. Dissolved organic matter had little impact on the other two modified Langmuir parameters for these three soils (Table 4). Additionally, the presence of DOM in solution did not appear to affect the mechanism of P sorption based on similar patterns of pH change in the presence and absence of DOM (Fig. 2).

Net sorption of poultry litter-derived DOM to A horizon soils (Fig. 3 ) was not detected; instead a net release of DOM was observed in the form of negative qDOC values. This result may be attributed to the dispersion of soil particles in suspension caused by addition of monovalent ions (K+ and Na+) in the background electrolyte solution (Haynes and Naidu, 1998), and the disruption of cation bridges between OM and clay minerals on exchange of polyvalent cations with monovalent cations (Reemtsma et al., 1999). Although the net release of DOM from A horizon soils may provide an explanation for the lack of competition between DOM and P for sorption sites, DOM was sorbed to B horizon soils and no significant impact on P loss from solution was observed. Apparent trends in Fig. 3 indicate that DOM sorption to B horizon soils decreased as suspension pH increased. Increased suspension pH results in higher net negative soil charge and higher net negative charge on DOM molecules as surface or organic functional groups are dissociated, ultimately resulting in greater repulsion between solutes and particle surfaces (Jardine et al., 1989).


Figure 3
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  		Fig. 3. Sorption of dissolved organic carbon (DOC) (qDOC) as a function of suspension pH. Data points represent mean values obtained from phosphorus sorption experiments conducted in presence of dissolved organic matter (DOM) (145 mg C L–1).

 
The possibility that poultry litter-derived DOM does not significantly impact P sorption is important and demonstrates that DOM leached from land-applied manure is not likely to enhance P transport in the environment by reducing inorganic P sorption. The data also indicate that the coupled application of P fertilizer and organic amendments may not increase plant-available P for all soils types, which is in general agreement with the work of the others (Ohno and Crannell, 1996; Guppy et al., 2005a). However, the results are in direct contrast to studies that have demonstrated improved P plant availability after surface mulching and direct application of mulch leachate to soil (Othieno, 1973) and those that have investigated competitive sorption between dissolved organic compounds and P (Sibanda and Young, 1986; Hue, 1991; Violante and Gianfreda, 1993; Bolan et al., 1994; Staunton and Leprince, 1996; Bhatti et al., 1998).

There are several plausible explanations as to why poultry litter-derived DOM does not inhibit P sorption to soil. Poultry litter-derived DOM may have a lower net negative charge density (mmolc g–1 C) than the LMWOAs investigated in previous P inhibition studies; thus, fewer functional groups may be available for binding to soil minerals. Ohno and Crannell (1996) investigated this explanation but no clear correlation was observed to substantiate this hypothesis. However, it should be noted that only four DOM samples were included in their regression analysis. Evaluation of larger data sets would be necessary to validate this hypothesis.

Ohno and Crannell (1996) did observe that greater molecular weight and reduced structural lability can affect the ability of DOM to undergo exchange or complexation reactions with soil surfaces due to increased steric restrictions. Higher molecular weight compounds with larger physical conformation may be excluded from sorption sites available to the smaller phosphate anion. For example, Zimmerman et al. (2004) demonstrated that amino acid polymers were excluded from mesopores (8 nm diameter) and Mikutta et al. (2006) documented entry of phosphate into micropores (<2 nm). Based on these studies, it is easy to envision the exclusion of DOM from pores accessible to phosphate anions. Fluorescence spectra collected by Ohno and Crannell (1996) also indicate that poultry litter-derived DOM has higher molecular rigidity, relative to plant-derived DOM, which may reduce the ability of DOM extracted from poultry litter to form multi-carboxylate bonds with mineral surfaces. Subsequently, DOM with greater molecular rigidity would compete less with P for sorption sites.

Sorption of Dissolved Organic Nitrogen Relative to Inorganic Nitrogen Species and Dissolved Organic Carbon
Compound concentrations ratios in control solutions (no soil and no added inorganic P) and samples spiked with DOM then reacted for 24 h were evaluated for preferential sorption of poultry litter-derived DON relative to other dissolved constituents (Fig. 4 –6). Figure 4 shows that the DON/(NO3––N + NO2––N) ratio for all three soil series investigated were significantly less than the solution ratio of reacted controls, with exception of the Clarksville soils. Although nitrate and DON may be released from A horizons, B horizon ratios were not compromised by the release of these compounds. These data illustrate that DON is preferentially sorbed to soil surfaces relative to the anionic forms of nitrogen, as might be expected due to repulsion of nitrate and nitrite from negatively charged soil surfaces.


Figure 4
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  		Fig. 4. Dissolved organic nitrogen (DON)/(NO3––N + NO2––N) ratio in equilibrium solution of soils reacted with dissolved organic matter (DOM) (145 mg C L–1) in absence of phosphorus. The line represents mean DON/(NO3––N + NO2––N) ratio in control solutions (no soil). Asterisk (*) indicates significant difference in the DON/(NO3––N + NO2––N) ratio in equilibrium sample solutions relative to control solutions (p ≤ 0.05). Bar indicates LSD(0.05) value; NS: not significant.

 
The DON/NH4+–N ratios in equilibrium solution of controls and samples reacted with DOM are shown in Fig. 5 . Ratios for all three reacted soils were below the control solution ratio except for the Goss A horizon, which exhibited net ammonium release into solution. Ratio shifts relative to the control ratio were significant for only the Goss and Tonti B horizons. Greater sorption of DON relative to cationic ammonium can be attributed to the presence of high K+ and Na+ concentrations in the background electrolyte solution and the competition of these ions, particularly K+, with NH4+ for soil surface exchange sites (Chappell and Evangelou, 2000).


Figure 5
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  		Fig. 5. Dissolved organic nitrogen (DON)/NH4+–N ratio in equilibrium solution of soils reacted with dissolved organic matter (DOM) (145 mg C L–1) in absence of phosphorus. The line represents mean DON/NH4+–N ratio in control solutions (no soil). Asterisk (*) indicates significant difference in the DON/NH4–N ratio in equilibrium sample solutions relative to control solutions (p ≤ 0.05, t-tests). Bar indicates LSD(0.05) value; NS: not significant.

 
With respect to DOC loss from solution relative to DON (Fig. 6 ), the ratios for all three soils were greater than the control solution ratio, although differences were significant for only one half of the soils studied. Excluding A horizon data from consideration due to net release of DOM into solution, the data indicate that DON in poultry litter-derived DOM was preferentially sorbed relative to DOC. This observation is in contrast to the work of Kaiser and Zech (2000) where it was observed that the ratio of DOC/DON remaining in solution after reaction of forest floor DOM with soil minerals tended to decrease. Field studies have also documented greater retention of DOC relative to DON as soil solution percolates through forested soils (Qualls and Haines, 1991; Michalzik et al., 2001) as well as agricultural soils amended with swine and cattle manure (Siemens and Kaupenjohann, 2002). The converse results observed in our study may be explained by differences in DOM chemical properties derived from varying OM sources. However, additional research must be conducted to further evaluate this finding of our study to determine if DON leached from poultry litter is a significant constituent of total N flux through poultry litter-amended soils.


Figure 6
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  		Fig. 6. Dissolved organic carbon (DOC)/dissolved organic nitrogen (DON) ratio in equilibrium solution of soils reacted with dissolved organic matter (DOM) (145 mg C L–1) in absence of phosphorus. The line represents mean DOC/DON ratio in control solutions (no soil). Asterisk (*) indicates significant difference in the DOC/DON ratio in equilibrium sample solutions relative to control solutions (p ≤ 0.05, t-tests). Bar indicates LSD(0.05) value; NS: not significant.

 

   Conclusions
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
REFERENCES
 
Phosphorus sorption to benchmark soils of southwestern Missouri was found to be positively correlated with AlCBD+FeCBD and clay content and negatively correlated with Bray-1 P concentration. More importantly, however, was the finding that poultry litter-derived DOM did not inhibit P sorption over a wide range of P solution concentrations. This indicates that DOM released from land-applied poultry litter is unlikely to compete with PO43– for sorption sites in soil and enhance inorganic P transport deeper in the soil profile or to water resources. The data also suggest that DOM competition with P for sorption sites in soil may not be responsible for increases in plant-available P when organic amendments and fertilizer P are land-applied simultaneously. Additionally, DON was found to sorb more readily to soil than inorganic forms of N and DOC under the conditions studied. Additional studies are needed to investigate the significance of DON leaching from poultry litter application to verify this occurrence in the field.


   ACKNOWLEDGMENTS
 
We thank Dr. Will McClain, Dr. Russell Dresbach, and Si Hyun Wu (Univ. of Missouri), Jennifer Nelson and Bettina Coggeshall (USDA-ARS) for their assistance with analytical equipment and analyses. Dr. Randy Miles, Univ. of Missouri, and the Missouri Cooperative Soil Survey are thanked for providing soil samples. Gratitude is also expressed to NRCS employees Tom DeWitt and Nathan Witt for assistance with locating and sampling poultry litter houses. We also thank the anonymous reviewers for their constructive comments. Financial support for this work was provided by the Univ. of Missouri's Food and Agriculture Policy Research Inst. (FAPRI) through grant X99739601-6, USEPA Region VII, under section 104(b)(3).


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 
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   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 





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