SSSAJ Grow Your Career with SSSA
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (4)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Zhang, M. K.
Right arrow Articles by Li, Y. C.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Zhang, M. K.
Right arrow Articles by Li, Y. C.
GeoRef
Right arrow GeoRef Citation
Agricola
Right arrow Articles by Zhang, M. K.
Right arrow Articles by Li, Y. C.
Related Collections
Right arrow Heavy Metals
Right arrow Soil Pollution
Right arrow Phosphorus
Soil Science Society of America Journal 67:1158-1167 (2003)
© 2003 Soil Science Society of America

DIVISION S-4—SOIL FERTILITY & PLANT NUTRITION

Phosphorus and Heavy Metal Attachment and Release in Sandy Soil Aggregate Fractions

M. K. Zhanga, Z. L. He*,a,b, D. V. Calvertc, P. J. Stoffellac, X. E. Yanga and Y. C. Lid

a College of Resource and Environmental Sciences, Zhejiang University, Huajiachi Campus, Hangzhou 310029, P.R. China
b University of Florida, Institute of Food and Agricultural Sciences, Indian River Research and Education Center, 2199 South Rock Road, Fort Pierce, FL 34945
c University of Florida, Institute of Food and Agricultural Sciences, Indian River Research and Education Center, 2199 South Rock Road, Fort Pierce, FL 34945
d University of Florida, Institute of Food and Agricultural Sciences, Tropical Research and Education Center, Homestead, FL 33031

* Corresponding author (zhe{at}mail.ifas.ufl.edu)


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The presence of P and heavy metals in different forms or in association with different size fractions influences availability and discharge of these elements from watersheds. Understanding the association of P and heavy metals with size fractions can improve evaluation of leaching potential of P and heavy metals from soils. In this study, five aggregate-size fractions, ranged from 1.00 to 0.50 to <0.053 mm, were separated from seven Florida sandy soils by dry sieving. Each aggregate fraction was characterized by phosphate sorption, sequential fractionation of P, total, water- and Mehlich III-extractable concentrations of P and heavy metals. Size differences in sand, silt, and clay aggregates influence the amount and strength of element binding. Elemental attachment (particularly heavy metals) increased with decreasing aggregate sizes. Phosphorus and heavy metals in the sandy soils are readily transported to surface waters with suspended fine particles. Higher percentages of water-extractable, Mehlich III-extractable P, and heavy metals were found in both the 0.50- to 0.25- and 0.25- to 0.125-mm aggregate fractions, suggesting that P and heavy metals in these two fractions had higher release potential. The sequential fractionation of P suggested that the 1.00- to 0.50-mm fraction contained a larger percentage of Ca-bound P, whereas the 0.50- to 0.25-, 0.25- to 0.125-, and 0.125- to 0.053-mm fractions had higher ratios of labile P (H2O-P and NaHCO3–P). Phosphorus release from smaller aggregate fractions is faster with a higher P1/P168 ratio than from larger aggregate fractions because of larger amounts of water soluble P attached in the smaller aggregate fractions.

Abbreviations: ICP-AES, Inductively Coupled Plasma Atomic Emission Spectrometer • IP, orthophosphate P • OP, organic P


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
SOIL P AND HEAVY METALS are involved in the global environment and influence water quality. Understanding the impacts of soil P and heavy metals in an ecological system requires knowledge of P and heavy metal pools in soil aggregate (or particle) fractions. Recent studies have shown that fine soil fractions are often preferentially transported to surface water through runoff, and nutrients and toxic heavy metals or pesticide attached in the fine fractions are discharged along with the runoff (Ghadiri and Rose, 1991; Vansteenbergen et al., 1991; Farenhorst and Bryan, 1995; Uusitalo et al., 2001). To evaluate the leaching risk of P and heavy metals from soil to the environment, determination of metal concentrations in aggregate (particle)-size fractions of the soil is critical. Attention has been drawn to the role of these pools in the relationship of P dynamics to water quality (He et al., 1995; Wang et al., 2001). Aggregate (or particle)-size separation has been widely used to distinguish pools of different soil organic matter and nutrient quality (Christensen, 1992; Agbenin and Tiessen, 1995). Soil is not a homogeneous mass but a rather heterogeneous body of material. Soil chemical composition varies over distances of a few millimeters (Santos et al., 1997; Wilcke and Amelung, 1996; Wilcke and Kaupenjohann, 1997). The chemical composition and behavior of plant nutrients and heavy metals in soil are dependent on chemical properties and composition of the soil matrix, so the variation of composition in the soil matrix may lead to significant variation of composition and behavior of soil nutrients and heavy metals. The distribution of a specific element in the solid phase can be important for controlling its initial rate of leaching. Elements that are attached on particle surfaces will be more readily accessible to the soil solution. Consequently, the elements may be leached more rapidly, especially if they are present in water-soluble forms, than elements that are uniformly distributed throughout the whole matrix. Several microelements are not distributed uniformly throughout fossil fuel wastes but are attached in the smaller particle sizes and on the particle surfaces (Eary et al., 1990). Some studies (Anderson et al., 1981; Catroux and Schnitzer, 1987; Tiessen and Stewart, 1983) have reported that a major part of the organic matter in predominantly inorganic soils is usually found in the silt- and clay-size fractions. Nitrogen release (mineralization) increases with decreasing particle size (Catroux and Schnitzer, 1987; Cameron and Posner, 1979; Hinds and Lowe, 1980). Linquist et al. (1997) studied the role of aggregate size on the sorption and release of P in a Haiku clay (Typic Palehumult) and found that P sorption increased as mean aggregate diameter decreased, and P release from aggregates was linearly correlated with the reactive mass of aggregates. They concluded that aggregation affects short- and long-term plant P availability. The distributions of P in different forms or in association with different particle-size fractions are affected by pedogenic processes, which, in turn, affect P availability. The association of P with different particle sizes has been extensively investigated, but the relationships are far from clear (Agbenin and Tiessen, 1995; Day et al., 1987; Hanley and Murphy, 1970; Syers et al., 1969). Syers et al. (1969) reported that Ca-P was the dominant P form in sand and silt fractions. In contrast, Juo and Ellis (1968) measured Ca-P in clays of Michigan soils and concluded that physical breakdown of particles had probably been more rapid than chemical weathering. Agbenin and Tiessen (1995) reported the distribution of P forms in different size fractions changed with soil types and soil development.

The methods used for separation of size fractions included wet sieving, dry sieving, and physical dispersion (Christensen, 1985; Edwards and Bremner, 1967; Genrich and Bremner, 1974; Hinds and Lowe, 1980; Linquist et al., 1997). The wet sieving and dispersion methods may not be suitable for size fractionation to determine labile forms of elements. This is particularly evident for sandy soils, since sandy soils have low retention capacity for nutrients. Minimal information is available on the attachment and release potential of P and heavy metals in the various aggregate size fractions of sandy soils.

The objectives of this study were to: (i) evaluate P and heavy metal distribution in various aggregate-size fractions; (ii) determine the release potential of P and heavy metals from different size fractions of sandy soils.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Soil Samples and Aggregate Fractionation
Seven composite samples (S1, S2, S3, S4, S5, S6, and S7) were collected from the 0- to 15-cm layers of cultivated sandy soils used for commercial citrus and vegetable crop production in St. Lucie and Martin counties, Florida. For comparison, a composite loamy soil sample (L1) was also taken from the 0- to 15-cm layer in a vegetable field in Homestead, FL. The soils had a 10- to 25-yr history of commercial citrus and vegetable crop production. About 60 P kg ha-1 was annually applied as basal fertilizer in September for vegetable fields, while annual rate of 40 P kg ha-1 was broadcasted in February and October for citrus groves. In addition, gypsum was often used as soil amendment as basal application in the vegetable crop and citrus production system, and applied at same time. The soils were sampled in July 2000, before P fertilizer application. Each sample was composited from five samples taken from five locations across the same field. Field-moist soil samples were air-dried and subsamples of the air-dried bulk soil samples were dry screened and fractionated into five (or six) different aggregate-size fractions. The remaining soil samples were ground, and passed through a 2-mm sieve before physical and chemical analyses. Size fractions were separated by manually moving the sieve in an up and down motion about 50 times. Approximately 1.5 kg of each soil was separated into different size fractions in similar way to sand-size fractions in USDA method (Day, 1965). The >1.00-mm aggregates were collected and sieving was repeated with the <1.00-mm fraction for next sample-sized sieve. This procedure was repeated until all the aggregate-size fractions (1, 0.5, 0.25, 0.125, and 0.053 mm) were obtained. All aggregate fractions were weighed and used for extractions of available elements, and studies of P adsorption and release dynamics. Subsamples of size fractions were ground to pass through a 0.125-mm sieve for determining total concentrations of elements. Names and selected physical and chemical properties of the soils are presented in Table 1. The mineralogy of the tested sandy soils is dominated by 1.4-nm intergrade minerals, montmorillonite, kaolinite, and quartz for clay fraction, and quartz for silt and sand fractions (USDA, 1980).


View this table:
[in this window]
[in a new window]
  		Table 1. Names and selected physical and chemical properties of soils sampled from vegetable and citrus production areas in Florida.

 
Characterization of Aggregate Fractions
Soil particle composition was determined using a micropipette method (Miller and Miller, 1987). Soil pH was measured both in water and in a 1 M KCl solution at a soil/solution ratio of 1:1 using a pH/ion/conductivity meter (Accumet Model 50, Fisher Scientific, Norcross, GA). Total C was determined using a CN-Analyzer (Vario MAX CN Macro Elemental Analyzer, Elemental Analysensystem GmbH, Hanau, Germany). Electrical conductivity (EC) was measured in water at a 1:2 soil/water ratio using the same pH/ion/conductivity meter. Soil available P was extracted using both 0.5 M NaHCO3 (Olsen-P, 1:20 soil/solution ratio) and Mehlich-I reagent (Mehlich I-P, 1:4 soil/solution ratio), and P concentration in the extract was determined by the molybdenum-blue method (Olsen and Sommers, 1982). Available P and heavy metals in different aggregate fractions were determined using the Mehlich-III method (Mehlich, 1984). A 2.5-g air-dried fraction sample was weighed out into 50-mL polystyrene centrifuge tubes and 25-mL Mehlich-III extractant was added, the suspension was shaken for 5 min, and filtered through Whatman 42 filter paper (Whatman International Ltd., Maidstone, England). Concentrations of metals in the extract were analyzed using an Inductively Coupled Plasma Atomic Emission Spectrometer (ICP-AES, Ultima, JY Horiba Inc. Edison, NJ). Water-soluble P and metals were determined by shaking a 4-g sample in 40 mL of deionized water for 8 d, centrifuging the suspension at 7500 x g for 30 min at 20°C, filtering the supernatant solution through a Whatman #42 filter paper, and then measuring the concentrations of Cd, Co, Cr, Cu, K, Mn, Ni, P, Pb, and Zn in the filtrate using the ICP-AES. For determination of total concentration of P and metals in the soils and aggregate fractions, 0.5 g of the finely ground soil sample was digested in a solution containing 9 mL of HNO3 and 3 mL of HF using a microwave digestion system (O·I·Analytical, College Station, TX). The concentrations of P and metals were determined using the ICP-AES. Analyses were performed on two replicate samples for each aggregate fraction.

Phosphate Sorption by Various Aggregate Fractions
Phosphate sorption was measured in all of the aggregate fractions separated from the different soils. Portions of aggregate samples (1.00 g for the >0.05-mm size fractions, 0.50 g for the <0.05-mm size fraction) were placed in polystyrene centrifuge tubes with 30 mL of 0.02 M KCl solution containing 0, 1.5, 3.0, 4.5, 6.0, 7.5, and 10.0 mg P L-1 for the >0.05-mm size fractions (or 0, 2.5, 5.0, 7.5, 10.0, 15.0, and 20 mg P L-1 for the <0.05-mm size fraction) were added to each tube, respectively. The tubes were shaken on an end-to-end shaker (180 cycles min-1) for 24 h at 25°C and then the suspension was centrifuged. Phosphorus concentrations in the supernatant solutions were determined using the molybdenum-blue method (Olsen and Sommers, 1982). A simple Langmuir equation [Q = KC Qm/(1 + KC), where Q is amount of P adsorbed (mg P kg-1 soil), C is equilibrium concentration (mg P L-1), Qm is adsorption maximum, and K is the constant related to the P binding strength] was employed to describe the P sorption isotherms. Adsorption maximum (Qm) was obtained from the Langmuir equation.

Sequential Fractionation of Phosphorus
The procedure suggested by Sui et al. (1999) was employed in this study to operationally define the soil P pools. A 0.5-g aggregate sample was placed into a 50-mL centrifuge tube and sequentially extracted with 30 mL each of deionized water, 0.5 M NaHCO3 (pH = 8.2), 0.1 M NaOH, and 1 M HCl. Each extraction was run for 16 h on an end-to-end shaker (180 cycles min-1). After each extraction, the suspensions were centrifuged at 6000 x g for 30 min at 20°C and filtered through a Whatman 42# filter paper. After final extraction, residual P was determined in the soil material left in the centrifuge tubes by digestion with H2SO4–H2O2 (Tiessen and Moir, 1993). Total P in the NaHCO3 and NaOH extracts was determined by digesting aliquots of the extracts with acidified ammonium persulfate (Greenberge, 1992). Orthophosphate P (IP) in the initial filtrates and in the digests of each extract was determined colorimetrically by the molybdenum-blue method. Differences between the total and the inorganic P represent organic P (OP).

Phosphorus Release Dynamics in Water from Aggregate Fractions
A batch technique was employed for this study. Ten to twelve sets of aggregate fraction samples, each at 2.00 g, were shaken continuously with 40 mL of deionized water on an end-to-end shaker (180 cycles min-1). The suspension samples for each size fraction were collected periodically during a 192-h period, centrifuged at 7500 x g rcf for 30 min and then filtered through a Whatman 42 filter paper, and analyzed for P concentration using the molybdenum-blue method. Kinetic equations of P released from the aggregate samples were obtained by correlating the amount of cumulative released P with time.

Statistical Evaluation
Comparison of chemical components for different aggregate-size fractions was conducted using a one-way analysis of variance (ANOVA); element pools in various fractions were characterized by "attachment factors" (A). The attachment factor A was defined as the concentration ratio of a specific element in each aggregate fraction over bulk soil. Attachment of a specific element in a aggregate fraction is indicated by A > 1, whereas A < 1 reflected depletion of the element.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Distribution of Aggregate-Size Fractions in Different Soils
In all the samples taken from the sandy soils, the aggregate-size fraction was dominated by the 0.50- to 0.25- and 0.25- to 0.125-mm size fractions. These two fractions, on the average, accounted for 80.6% of the dry soil weight (Table 2). Due to a lack of clay and organic matter (Table 1), >1-mm aggregate-size fraction was minimal in all the sandy soil samples. These results are different from those of Linquist et al. (1997), who found that about 50% of soil aggregates exceeded 1 mm in diameter. The proportion of <0.053-mm aggregate was below 1.5% in all sandy soil samples. In contrast, the loamy soil sample had an increase both in small and large aggregate fractions, while distribution differences in percentages of various aggregate-size fractions for the loamy soil sample was much less than those from sandy soils. Differences in sand, silt, and clay contents among 1.0- to 0.50-, 0.50- to 0.25-, and 0.25- to 0.125-mm aggregate fractions in the sandy soil samples was not significant (Table 3). However, significant differences in clay, silt, and sand contents were found between the <0.053-, the >0.125-, and the 0.125- to 0.053-mm aggregate fractions. Clay content was higher in the <0.053-mm fraction than in the >0.125- and 0.125- to 0.053-mm fractions, whereas sand and silt contents decreased in the order of <0.053-, 0.053- to 0.125-, and >0.125-mm fraction. Each aggregate from the sandy soils mainly consisted of same size of particle (Table 3). The 1.0- to 0.50-, 0.50- to 0.25-, 0.25- to 0.125-, 0.125- to 0.053-mm aggregate fractions contained 94.3% of the 1.00- to 0.50-mm, 95.8% of the 0.50- to 0.25-mm, 94.4% of the 0.25- to 0.125-mm, and 85.2% of the 0.125- to 0.053-mm particles, respectively. These results suggest that the sandy soils were very weakly aggregated; the aggregates separated from the sandy soils were mainly individual particles. Therefore, behavior and properties of the aggregates separated from the sandy soils are more comparable with those of individual particles than those of aggregates from the highly aggregated soils. Linquist et al. (1997) found that all aggregate-size fractions except the two smallest fractions (<0.027 and 0.072 mm) from an Ultisol had similar particle-size distribution and clay mineralogy.


View this table:
[in this window]
[in a new window]
  		Table 2. Compositions of dry-separated aggregate-size fractions of soils sampled from vegetable and citrus production areas in Florida.

 

View this table:
[in this window]
[in a new window]
  		Table 3. Average particle compositions of different aggregate fractions of soils sampled from vegetable and citrus production areas in Florida.

 
Total P and Heavy Metals in the Aggregate Fractions
Large proportions of P and heavy metals were attached to the fine fraction (<0.053 mm), although fine aggregate fractions from the sandy soils were very small (Fig. 1). The <0.053 mm fractions contained, on average, 13.0 to 19.1% of the total P, Cu, Zn, Pb, Cr, Co, Cd, and Ni. About 28.2 to 38.1% of the total P, Cu, Zn, Pb, Cr, Co, Cd, and Ni were present in the 0.125- to 0.053-mm fractions. The 1.0- to 0.5-mm fraction, on average, contained 4.7 to 15.7% of the total P, Cu, Zn, Pb, Cr, Co, Cd, and Ni. The P and heavy metal pools of both the 0.5- to 0.25- and 0.25- to 0.05-mm fractions were very similar (Table 4). However, compared with their high percentage (average 80.6%) in aggregate compositions (Table 2), proportions of P, Cd, Cr, Cu, Co, Ni, Pb, and Zn in the total contents for these two fractions, averaging from 38.9 to 45.5%, were relatively low (Fig. 1). The results suggest that attachment of the heavy metals and P increase with decreasing aggregate sizes. Since smaller aggregate in the sandy soils contained more silt and clay (Table 3), and specific surface area increases with decreasing particle size (Agbenin and Tiessen, 1995); concentration of these elements with decreasing aggregate sizes was an indication of surface attachment.



View larger version (58K):
[in this window]
[in a new window]
  		Fig. 1. Average distributions of P and heavy metals in different aggregate fractions of the sandy soils.

 

View this table:
[in this window]
[in a new window]
  		Table 4. Attachment factor (A) of total elemental concentrations in different aggregate fractions of soils sampled from vegetable and citrus production areas in Florida.{dagger}

 
The attachment factors (A), defined as the ratio of concentrations of macro and microelements in each aggregate fraction relative to the concentration in the bulk soil, varied with aggregate size (Table 4). The smallest size fraction had the highest A value for all the measured elements. The next highest A values were found with the 1.00- to 0.50-mm fraction or 0.125- to 0.053-mm fraction. Aluminum, Co, Cu, K, Mn, Na, Pb, and Ba were more attached in the 0.125- to 0.053-mm fraction than in the 1.00- to 0.50-mm fractions, whereas other elements were more attached in the 1.00- to 0.50-mm fraction than in the 0.125- to 0.053-mm fraction. Higher Ca, Mg, P, and heavy metals in the 1.00- to 0.50-mm fraction may attribute to the residual gypsum applied. Large variations in P and heavy metal concentrations among the different aggregate-size fractions along with the similarity in the trends for attachment of P and heavy metals for each fraction suggest that aggregate-size fractionation is useful tool for identifying different pools of P and heavy metals in sandy soils. However, it may be not true for highly aggregated soils. Linquist et al. (1997) did not find similar results because there was little difference in clay and mineralogy among the fractions from a highly aggregated soil that they examined. The degree of attachment was significantly different for each of these elements (Table 4). The average attachment factors of the measured elements in the <0.053-mm fraction were >10. For example, the P attachment factor in the <0.053-mm fraction averaged 13.4, attachment factors of heavy metals including Cd, Co, Cr, Cu, Ni, Pb, and Zn in the <0.053-mm fraction, ranged from 16.4 to 31.7. The 0.50- to 0.25-mm fraction had the lowest A values for all measured elements. Compared with the sandy soil samples, differences in element concentrations in the loamy soil sample were much smaller (Table 4). The attachment factors of the elements in various fractions were generally below 3 except for Al, Fe, and Na, and decreased in the order of <0.053-, 0.125- to 0.053-, and >0.25-mm fraction.

Extractability of Phosphorus and Heavy Metals
Both deionized water and Mehlich-III solution are common extractants for testing available P and heavy metals (Kuo, 1996; Mehlich, 1984). Soils with higher extractable P and heavy metals are considered more likely to lose P and metals to surface waters by erosion and surface or subsurface runoff (Pote et al., 1999). Estimation of elemental extractability has been proposed as a means to predict potential release capacity of these elements in soils (Kuo, 1996; Mehlich, 1984; Pote et al., 1999). Extractability, defined as the percentages of the total amounts of elements in each aggregate size extracted by water and Mehlich-III extraction, are presented in Tables 5 and 6. Water extractability of P and heavy metals ranged from 0.11 to 29.8%. By comparing results in Table 5 with those in Table 4, it was found that the water extractability (%) increased with decreasing concentration of the element in the fractions. Water extractability for all the elements, except for Pb and Mn, was highest in the 0.50- to 0.25-mm fraction, followed by the 0.25- to 0.125-, 1.00- to 0.50-, and the 0.125- to 0.053-mm fraction; and the <0.053-mm fraction had the lowest (Table 5). However, for the loamy soil sample, the water extractability for all the elements except for K were generally below 2.8%, the highest extractability was found in the 1.00- to 0.50-mm fraction, followed by the 0.25- to 0.125- or the >1.00-mm fractions.


View this table:
[in this window]
[in a new window]
  		Table 5. Water extractability of P and metals for each aggregate-size fraction of soils sampled from vegetable and citrus production areas in Florida.

 

View this table:
[in this window]
[in a new window]
  		Table 6. Mehlich-III extractability of P and heavy metals for each aggregate fraction of soils sampled from vegetable and citrus production areas in Florida.

 
Mehlich-III extractability of the samples from the sandy soils also varied greatly for all elements. The highest extractability was in both the 0.50- to 0.25- and the 0.25- to 0.125-mm fractions. The Mehlich-III extractability in the two fractions was over 40% for Al, Ca, Cd, Co, Cr, K, Mg, Mn, Ni, P, and Ba, and 15 to 40% for Cu, Fe, and Zn. In contrast, the extractability in the <0.053-mm fraction was relatively low (below 4%) for heavy metals except for Cu and Zn. The Mehlich-III extractability of the loamy soil sample was generally lower than the sandy soils for all the elements. The extractability for P and heavy metals was generally below 10%. The higher extractabilities of P and heavy metals with water and Mehlich-III extractant in both the 0.50- to 0.25- and 0.25- to 0.125-mm aggregate fractions for the sandy soil samples suggest that P and heavy metals have higher release potential in these two fractions than any other fraction.

Phosphorus Adsorption and Fractionation
Phosphorus sorption capacity of soil is considered to affect the partitioning of P between soil solution and the solid phase and release of P from soil (Syers et al., 1973). Maximum sorption of P (Qm), obtained from the Langmuir equation, varied greatly in various aggregate fractions, and decreased in the order of <0.053, 0.125 to 0.053, 0.25 to 0.125, 1.0 to 0.25, and 0.5 to 0.25 mm. The Qm value in the <0.053-mm fractions ranged from 90 to 497 mg kg-1, with an average of 241 mg kg-1, whereas the Qm values in other fractions varied from 2 to 78 mg kg-1, the mean Qm values in the 0.125- to 0.053-, 0.25- to 0.125-, 0.5- to 0.25-, and 1- to 0.5-mm fractions were 28, 17, 6, and 16 mg kg-1, respectively. Phosphorus adsorption maximum ratios of aggregate fractions to the bulk soils reflected the difference in P adsorption capacity for various size fractions in the sandy soils (Fig. 2). The P adsorption capacities for the 0.125- to 0.053-, 0.25- to 0.125-, 0.5- to 0.25-, and 1.0- to 0.25-mm fractions were close to the bulk soils, whereas P adsorption capacity for the <0.053-mm fraction was higher than the bulk soil. However, P adsorption capacities of various fractions in the loamy soil sample were almost identical to the bulk soil sample. Variations in P adsorption capacity among various fractions in the sandy soil samples might be due to differences in Al, Ca, and Fe contents in different aggregate-size fractions (Fig. 3). Positive correlations occurred between P adsorption capacity and total Al, Ca, and Fe in all the different size fractions (Fig. 3).



View larger version (17K):
[in this window]
[in a new window]
  		Fig. 2. Phosphorus adsorption maximum ratios of each size fraction (Qmf) to the bulk soil (Qms) sampled from vegetable and citrus production areas in Florida.

 


View larger version (21K):
[in this window]
[in a new window]
  		Fig. 3. Phosphorus adsorption maximum as functions of total Al (Alt), Ca (Cat), and Fe (Fet) in all the size fractions of soils sampled from vegetable and citrus production areas in Florida.

 
The P fractions varied with size fractions in the sandy soil samples (Table 7). The H2O-P and NaHCO3–P (including NaHCO3–IP and NaHCO3–OP) were higher in the 0.50- to 0.25-, 0.25- to 0.125- and 0.125- to 0.053-mm fractions than in the 1.00- to 0.50- and <0.053-mm fractions. NaOH-P (including NaOH-IP and NaOH-OP) was lower in the 1.00- to 0.50-mm fraction than in any other fractions, whereas HCl-P was higher in the 1.00- to 0.50-mm fraction than in any other fraction. The results suggest that the 1.00- to 0.50-mm fraction contains a larger percentage of Ca-bound P, whereas P in the 0.50- to 0.25-, 0.25- to 0.125- and 0.125- to 0.053-mm fractions is more available and readily released into water. Compared with the sandy soil samples, the loam soil sample had relatively small variations in P fractions among different size fractions (Table 7).


View this table:
[in this window]
[in a new window]
  		Table 7. Average compositions of different P forms in various aggregate size fractions of soils sampled from vegetable and citrus production areas in Florida.

 
Release Kinetics of Phosphorus in Water
Phosphorus release in water varied with aggregate-size fractions and was time-dependent. In nearly all the cases, P release was more pronounced during the first hour than later on. Phosphorus release was the slowest from the 1.00- to 0.50- and 0.50- to 0.25-mm aggregate fractions. The release process could be described by the first-order reaction model: Q = a + b ln t, where Q is the quantity of P released at a specific time (t); a and b are constants; the a value is the estimated quantity of P released in the first hour, and b represents the average rate of P release after 1 h. The regression equations of P release as a function of time in various fractions are presented in Table 8. The a and b values tended to increase with decreasing size of fractions with only a few exceptions. In Soil S5, most of the available P was released in the first hour. The rate of P release became slower after 1 h; and therefore, the b value in the <0.053-mm fraction of this soil was smaller than any of the other fractions. In Soil S6, the b value in the <0.053-mm fraction was close to other fractions because of the relatively low available P in this soil. However, the ratio of P released during the first hour to that of the total 168-h period (P1/P168) could explain the differences in P release characteristics for the various fractions. The highest ratios were found in the smallest fraction and the ratio decreased with increasing aggregate size. The ratios might be related to specific surface area of the aggregate fractions. The smaller size fraction that had larger specific surface area had higher values of P1/P168. These results suggest that available P release is faster in the smaller size fraction than in the larger aggregate-size fraction. Agbenin and Tiessen (1995) also reported that labile and available P increased with decreasing particle size, the increases in surface area of finer fractions resulted in higher levels of labile and available P.


View this table:
[in this window]
[in a new window]
  		Table 8. Kinetic equations of the amount of P released as a function of time, and the amounts of P released during the first and 168 h from each size-aggregate fraction of soils sampled from vegetable and citrus production areas in Florida.

 

   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Sandy soils are often clay and organic matter deficiency, and weakly aggregated. Compositions and properties of the aggregates separated from the sandy soils are different from those of aggregates from the highly aggregated soils. Aggregate-size fractions in the sandy soils were dominated by the 0.5- to 0.25- and 0.25- to 0.125-mm fractions, both, on the average, accounting for 80.6% of the whole soil, which were related to their texture compositions. Size differences in sand, silt, and clay aggregates influence the amount and strength of element binding. Attachment of P and heavy metals in various fractions for the sandy soils tended to increase with decreasing aggregate size, suggesting that surface attachment mechanisms control the distribution of these elements among the different aggregate-size fractions, P, and heavy metals are readily transported to surface waters through suspended fine particles. The percentages of water-extractable and Mehlich III-extractable P and heavy metals were higher in both the 0.50- to 0.25- and 0.25- to 0.125-mm aggregate fractions, suggesting that P and heavy metals in these two fractions would be more readily released to surface runoff or leached to ground water. Phosphorus adsorption capacity in various fractions was mainly determined by Al, Ca, and Fe contents. Phosphorus adsorption maximum values of the <0.053-mm fractions in all the sandy soils were much higher than those of any other fractions. The results from sequential fractionation of P indicate that the 1.0- to 0.50-mm aggregate fraction contained a larger percentage of the Ca-bound P, whereas the 0.50- to 0.25-, 0.25- to 0.125-, and 0.125- to 0.053-mm fractions had higher ratios of available P forms (H2O-P and NaHCO3–P) and thus had greater P release potential. In addition, the available P in the smaller aggregate fractions appears to be more readily released than in the larger aggregate fractions.


   ACKNOWLEDGMENTS
 
This study was, in part, supported by a section 319 Nonpoint Source Management Program grant (DEP contract # WM746) from the U.S. Environmental protection Agency (US EPA) through a contract with the Nonpoint Source Management/Water Quality Standard Section of the Florida Department of Environmental Protection (FDEP) and by a grant (DEP contract # SP566) from the FDEP.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Florida Agric. Exp. Stn. Journal Ser. No. R-08572.

Received for publication January 15, 2002.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




This article has been cited by other articles:


Home page
 Soil Sci.Home page
D. V. Sarkhot, N. B. Comerford, E. J. Jokela, J. B. Reeves III, and W. G. Harris
Aggregation and Aggregate Carbon in a Forested Southeastern Coastal Plain Spodosol
Soil Sci. Soc. Am. J., September 28, 2007; 71(6): 1779 - 1787.
[Abstract] [Full Text] [PDF]

This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (4)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Zhang, M. K.
Right arrow Articles by Li, Y. C.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Zhang, M. K.
Right arrow Articles by Li, Y. C.
GeoRef
Right arrow GeoRef Citation
Agricola
Right arrow Articles by Zhang, M. K.
Right arrow Articles by Li, Y. C.
Related Collections
Right arrow Heavy Metals
Right arrow Soil Pollution
Right arrow Phosphorus

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
The SCI Journals Agronomy Journal Crop Science
Journal of Natural Resources
and Life Sciences Education		 Vadose Zone Journal
Journal of Plant Registrations Journal of
Environmental Quality		 The Plant Genome