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Published in Vadose Zone Journal 3:462-470 (2004)
© 2004 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA

SPECIAL SECTION: COLLOIDS AND COLLOID-FACILITATED TRANSPORT OF CONTAMINANTS IN SOILS

Particle Leaching and Particle-Facilitated Transport of Phosphorus at Field Scale

L. W. de Jonge*,a, P. Moldrupb, G. H. Rubæka, K. Scheldea and J. Djurhuusa

a Danish Institute of Agricultural Sciences, Dep. of Agroecology, Research Centre Foulum, P.O. Box 50, 8830 Tjele, Denmark
b Aalborg University, Dep. of Life Sciences, Environmental Engineering Section, Sohngaardsholmsvej 57, 9000 Aalborg, Denmark
* Corresponding author (Lis.W.de.Jonge{at}agrsci.dk).

Received 2 October 2003.



   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Strongly sorbing compounds such as P, pesticides, and heavy metals can be transported through soils while being adsorbed to mobile colloidal particles. While the rapid leaching of nonadsorbing chemicals is relatively well understood, the particle-facilitated transport of highly sorbing chemicals such as P requires further investigation. The aim of this work was to study spatial variations in particle-facilitated transport of P at the field scale, and investigate which soil-physical or chemical parameters relate to the observed variations. Leaching experiments were performed in the laboratory on 42 undisturbed soil columns sampled in a grid covering 25 by 30 m of an agricultural field. The columns were equilibrated in the laboratory to a pressure head of –20 cm and irrigated at a rate of 10 mm h–1 with an artificial rainwater solution. The experiments exhibited considerable variation among the columns in the accumulated mass of particles and P leached during the 3.5 h of irrigation. Columns taken from the lower part of the field showed the highest mass of leached particles. These columns had higher clay contents and contained more continuous macropores. The mass of particles was negatively correlated to the average electrical conductivity of the effluent, and positively correlated to the macropore flow velocity. The accumulated masses of particulate organic and inorganic P were linearly related to the accumulated mass of particles leached. About 75% of the leached P was transported in a particle-facilitated manner. Overall, soil structure controlled to a large extent the leaching of particles and particle bound P.

Abbreviations: BTC, breakthrough curves • DIP, dissolved inorganic P • DOC, dissolved organic C • DOP, dissolved organic P • DRP, dissolved reactive P • EC, electrical conductivity • NTU, nephelometric turbidity • PIP, particulate inorganic P • POP, particulate organic P • TDP, total dissolved P • TP, total P • TRP, total reactive P • WDC, water-dispersible colloids


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
PHOSPHORUS IS FREQUENTLY the limiting element in freshwater systems, and its control is of prime importance in reducing the accelerated eutrofication of freshwaters. Whereas point-source pollution has been reduced in many areas of the world, water-quality problems remain a concern. To further reduce the P loading of freshwaters in areas of intensive agricultural production it is necessary also to address the diffuse losses of P from agricultural land (Sharpley and Rekolainen, 1997).

During the 20th century about 1.4 t P ha–1 has accumulated on Danish agricultural lands. Due to regulations on N fertilization and reduced application of inorganic fertilizers, the national yearly surplus during the past several decades has been declining to 17 kg P ha–1 in 1998–1999 (Dalgaard et al., 2003). Point-source discharge of P to Danish fresh waters has been reduced by more than 80% from 1989 to 1999, in part because of large investments in waste water treatment. Diffuse losses of P from agricultural lands now constitute more than 50% of the yearly contribution of P to Danish freshwaters (Kronvang et al., 1995). In spite of these significant reductions in discharge, P is still problematic for the water quality of many Danish lakes and inlets. To fulfill the requirements of the EU water framework directive on water quality (EU, 2000) within the set timeframe, it is mandatory to address the diffuse losses of P from agricultural lands. Phosphorus leaching in artificially drained soils is now recognized as an important contributor of P to fresh waters in certain areas (Sims et al., 1998); however, the processes controlling this type of P loss and the spatial distribution of such losses are only poorly understood.

It is well documented that particle-facilitated transport is an important mechanism for leaching of strongly sorbing pollutants, such as pesticides (e.g., Seta and Karathanasis, 1997; de Jonge et al., 1998, 2000; Sprague et al., 2000), heavy metals (e.g., Karathanasis, 1999; Sen et al., 2002; Barton and Karathanasis, 2003), and radionuclides (e.g., McCarthy and Zachara, 1989; Kersting et al., 1999). Particle-facilitated transport has also been found to play an important role in the leaching of P (Heckrath et al., 1995; Grant et al., 1996a; Stamm et al., 1998; Laubel et al., 1999; Djodjic et al., 2000), being a strongly sorbed nutrient. For a thorough review of the literature please refer to Kretzschmar et al. (1999).

Laboratory experiments have demonstrated that changes in ionic strength and pH play an important role in colloidal transport. Release rates generally increase with decreasing ionic strength and increasing pH (e.g., Grolimund et al., 1996; de Jonge et al., 1998). Most experimental work regarding particle transport in unsaturated soils has been done in laboratory experiments on either packed or undisturbed soil cores, while only a limited number of field studies have been performed. To our knowledge, no one has addressed variations in the potential leaching at the field scale of in situ particles and strongly sorbing compounds.

The objectives of our work were (i) to investigate the leaching of in situ particles and P across an agricultural field, (ii) to determine how important particle-facilitated transport is in the leaching of P, and (iii) to identify which factors determine the leaching of particles and P when soil type, soil water content, irrigation, and chemistry of the irrigation water are kept constant.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The top soil of the agricultural field near Røgen, Denmark is a sandy loam (Typic Hapludalf) with average clay, silt, and sand contents of 13.4, 15.5, and 68.8%, respectively. The clay mineralogy consists of even amounts of vermiculite, illite, and kaolinite. The organic C content is 1.5%. Oxalate-extractable Fe and Al representing poorly crystalline oxides of Fe and Al are 3.2 and 1.5 g kg–1 soil, respectively, according to McKeague and Day (1966). Bicarbonate-extractable P (Olsen et al., 1954) is 44 mg kg–1 soil, and total P (Kafkafi, 1972) is 734 mg kg–1 soil.

Forty-two stainless-steel columns (20-cm diameter, 20-cm length) with beveled edges were pushed into the soil to depths of 2 to 22 cm with a hydraulic press and excavated. The soil extending from both sides of the cylinders was carefully trimmed; no visual smearing or sealing occurred. The cores were sampled in a rectangular grid with spacing of 5 m. Each sampling point was displaced randomly either 1 m away from the grid or kept at the grid crossing (Fig. 1) . This was done to avoid repeated phenomena caused by cultivation of the field. The field was sloping slightly downhill (<1°) in the direction from Sampling Point 7 toward Sampling Point 1. The samples were taken in early spring when a winter wheat (Triticum aestivum L.) crop was growing at the field.



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  		Fig. 1. Sampling grid.

 
The soil columns were stored at 2°C until further experimental use. Soil specific characteristics along with measured variables are given in Table 1. Soil clay and silt contents and the fraction of water-dispersible colloids (WDC) were determined on disturbed soil samples taken from the same soil depths and adjacent to the soil columns. Clay and silt contents were determined using the hydrometer method (Gee and Bauder, 1986). The WDC was determined using 8-mm aggregates and a soil/solution ratio of 1:8 (modified after Kjaergaard et al., 2004a). The solution was artificial rainwater consisting of 0.012 mM CaCl2, 0.015 mM MgCl2, and 0.121 mM NaCl; pH = 7.82; EC = 2.24 x 10–3 S m–1. After applying the solution to the soil in Pyrex glass bottles, the bottles were slowly turned over 10 times and then allowed to rest to allow sedimentation of particles with diameter >2 µm, according to Stokes Law. From the supernatant 30 mL were removed by pipette and transferred to a vial for measurements of turbidity. Turbidity was measured with a Hach 2100AN (Hach, Loveland, CO) turbidimeter equipped with an EPA filter, measuring at wavelengths of 400 to 600 nm. The nephelometric turbidity (NTU) is proportional to particle concentration; the correlation used for calculating particle concentration (mg L–1) from turbidity (NTU) is given by Schelde et al. (2002). The mass of WDC was reported relative to the total amount of soil used.


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  		Table 1. Summary statistics of the data.{dagger}

 
Before leaching, each column was saturated from the bottom with a mimicked soil water solution consisting of 0.652 mM NaCl, 0.026 mM KCl, 2.44 mM CaCl2, and 0.255 mM MgCl2. The columns were subsequently drained and equilibrated to a pressure of –20 cm (relative to the center of the column) for 3 d. At this time the columns were weighed to determine the actual soil water content and the air-filled porosity.

The irrigation system for the column experiments consisted of a solution reservoir, a peristaltic pump, a sample injector (Rheodyne, Rohnert Park, CA) equipped with a 50-mL stainless-steel sample loop, and a stainless-steel irrigation head equipped with 29 needles (0.5 by 16 mm) with 30-mm spacing (Fig. 2) . The column was automatically rotated throughout the experiments at three revolutions per hour to obtain a homogeneous irrigation pattern at the surface. The soil column was placed on an 0.8-mm stainless-steel screen to obtain an atmospheric pressure lower boundary condition. The time needed for water to exit the lower boundary was recorded. The effluent was collected through a glass funnel in beaker glasses before subsequent measurements on the effluent were performed. The columns were irrigated at an intensity of 10 mm h–1 for 3.5 h with the same artificial rainwater solution as mentioned above. A 1-h rainstorm with an intensity of 10 mm h–1 occurs at a frequency of about two times per year in Denmark. At time = 0 to 10 min a 50-mL pulse containing 0.3 mM KBr2 was applied to the column through an injection loop (Fig. 2). Bromide was used as a conservative tracer.



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  		Fig. 2. Experimental setup.

 
Effluent samples were taken throughout the experiments and analyzed for turbidity, pH, EC, Br–, dissolved organic C (DOC), total P (TP), total dissolved P (TDP), total reactive P (TRP), and dissolved reactive P (DRP). Dissolved P was defined as the P present in the supernatant after centrifuging for 10 min at 5420 g (lower cut-off particle diameter of 0.24 µm). Reactive P concentrations were determined by spectrophotometry using a colorimetric technique with ascorbic acid reduction as described by Murphy and Riley (1962). Total P concentrations were determined using acid persulphate digestion in an autoclave (120°C, 200 kPa) (Koroleff, 1983) followed by the colorimetric technique described by Murphy and Riley (1962). The fraction of particulate inorganic P (PIP) was estimated as the difference between TRP and DRP. The fraction of dissolved inorganic P (DIP) was set equal to the DRP, while the fraction of dissolved organic P (DOP) was determined as the difference between TDP and DRP. Finally, the fraction of particulate organic P (POP) was estimated as (TP-TDP) subtracted (TRP-DRP). A summary of the different P forms is present in Fig. 3 .



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  		Fig. 3. Operationally defined fractionation of P forms in the effluent. DRP, dissolved reactive P; TRP, total reactive P; TDP, total dissolved P; TP, total P; PIP, particulate inorganic P; DIP, dissolved inorganic P; POP, particulate organic P; and DOP, dissolved organic P. Adapted from Haygarth et al. (1998).

 
The outflow rate was quantified by measuring the elapsed time and the mass of the effluent that was sampled. Bromide was measured using a high pressure liquid chromatography system (Metrohm, Herisau, Switzerland). The duration of each flow experiment was 3.5 h. Afterwards the columns were irrigated for 1 h at a rate of 10 mm h–1 with a dye solution (Brilliant Blue R250, CI number 42660, 1 g L–1), and dissected into four layers to visualize the flow pathways. The dye-colored areas were drawn on a plastic sheet and reported as the dyed area relative to the total surface area of the column.

Field maps of the data (Fig. 5 and 8) were prepared with the Matlab software package (MathWorks Inc., Natick, MA) using triangle-based linear interpolation based on Delaunay triangulation of the spatial data.



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  		Fig. 5. Field variations in (a) the accumulated mass of leached particles (mg), (b) macropore flow velocity (m h–1), (c) clay content (%), and (d) water-dispersible colloids (mg kg–1).

 


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  		Fig. 8. Field variations in the accumulated mass of leached (a) particulate inorganic P (mg), (b) particulate organic P (mg), (c) dissolved inorganic P (mg), and (d) dissolved organic P (mg).

 

   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
A statistical summary of measured data at the 42 grid points (soil columns) is provided in Table 1. Tests for normal distribution, where W is the Shapiro–Wilk statistic, give the probabilities for a normal distribution. Thus, if P < 0.05 the normal distribution can be rejected at the 0.05 level. The variables, accumulated mass of particles in the column effluent, accumulated mass of PIP in the effluent, and accumulated mass of POP in the effluent, were all lognormally distributed with rather large coefficients of variation of 102, 105, and 78%, respectively. The accumulated masses of the dissolved P fractions in the effluent did not show the same skewed distribution nor large variation.

Leached Particles
All columns leached initially a large flush of particles into the effluent (Fig. 4) . Before experimentation the columns were equilibrated to a pressure head of –20 cm, thereby promoting water flow through the macropores. A potential of –10 cm at the bottom of the columns was chosen to be as close to water saturation in the soil matrix as practically possible, but still having drained macropores, a situation often occurring under field conditions. There might have initially been a slight redistribution of water due to the implementation of the new lower boundary condition, but it is unlikely that this affected the first flush. The breakthrough of water generally happened very fast, indicating transport in macropores as intended. For the initial flush, the particles were immediately assessable to the irrigation water and were brought into suspension and transported with the water to the bottom of the soil column. We believe that the particles initially were associated with the macropore walls, but were detached by the flowing mobile water. The peak value of particle concentrations ranged between 188 and 1849 mg L–1 with an average of 448 mg L–1. The particle concentrations later decreased to lower, more constant values, albeit not necessarily close to zero. These lower concentrations ranged between 6 and 550 mg L–1, with an average of 155 mg L–1. These levels corroborate the findings of others. For example, El-Farhan et al. (2000) observed concentrations of fine particles in the range of tens to hundreds of milligrams per liter, whereas Ryan et al. (1998) reported particle concentrations of hundreds of milligrams per liter. Kjaergaard et al. (2004b) found particle concentrations from 6 to 167 mg L–1 in the effluent from soil columns having clay contents ranging from 12 to 43%, and irrigated at a much lower rate of 1 mm h–1.



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  		Fig. 4. Particle concentration in column effluent as a function of accumulated outflow. The specific numbers refer to grid numbers in Fig. 1.

 
The "first flush" phenomenon has been observed in several other studies, both during laboratory column experiments (e.g., Jacobsen et al., 1997; de Jonge et al., 1998; Schelde et al., 2002) and field plot experiments (Villholth et al., 2000; El-Farhan et al., 2000). The phenomenon has been shown to repeat itself if irrigation is stopped and resumed after some time, with the peak concentration depending on the length of the irrigation break (Schelde et al., 2002). The accumulated mass of leached particles during the 3.5-h experiments ranged between 8.8 and 598 mg, with an average of 157 mg. This amount corresponds to 0.013% of the average soil column clay content. Still we emphasize that the irrigation intensity was relatively high, while under field conditions filtering of particles would also occur below a depth of 20 cm. Therefore, these numbers should not be extrapolated in time or space.

The concentrations of DOC in the effluent were more or less constant for each column and showed no initial spike as for the particle concentrations. The average effluent DOC concentrations ranged between 8.4 and 68.7 mg C L–1. The column outflow rates in general reached a constant level shortly after the breakthrough of effluent. For some columns the outflow rate varied over short time intervals. This sometimes also caused the leached particle concentration to vary, such as was the case for Columns 25 and 35 (Fig. 4). This phenomenon was also found by Jacobsen et al. (1997) and de Jonge et al. (1998) and is presumably caused by one or more of the macropores being blocked due to sieving of particles, followed by a sudden flush caused by the water pressure building up in the blocked pores.

The final dye experiments showed that the average active macropore area was about 0.115 m2 m–2 with a standard deviation of 0.08. The dyed areas decreased with soil depth. The nonequilibrium flow conditions were confirmed by the shapes of the Br– breakthrough curves (BTCs) (data not shown), which were all very asymmetrical and showed extensive tailing. The Br– pulses were applied at the time when the irrigation was started. The Br– peak concentration generally occurred in the first few effluent samples. This suggests that water predominantly flowed through interaggregate pores or the larger pores stemming from earthworm burrows or old root channels. The macropore flow velocity of each column, calculated as soil column height divided by the breakthrough time, ranged between 0.11 and 0.82 m h–1. The breakthrough time in this paper is defined as the time between the start of irrigation and when effluent first exited the column.

We found considerable variations in the leaching of particles at the field scale (Fig. 5a) . The columns with the highest amount of leached particles originated mainly from the lower parts of the agricultural field. These areas also showed the highest macropore flow velocities (Fig. 5b), thus reflecting more extensive and continuous macropore systems. These results also indicate that soil structure in the lower part of the field is different from other parts of the field. The clay content ranged between 10.7 and 16.1% and the silt content between 13.9 and 17.2%. The higher clay contents were located in the bottom part of the field (Fig. 5c). The average total amount of leached particles was significantly higher at clay contents above 13% as compared with the lower clay contents.

The amount of WDC represents a potential amount of leachable colloids (Fig. 5d). Comparing Fig. 5a through 5d it is interesting to note that in the lower right corner of the field, where the amount of WDC and the macropore flow velocity were highest, we also found the highest mass of leached particles. This again suggests that soil structure plays an important role in the leaching of particles in the unsaturated zone. By comparison, the accumulated amount of particles was relatively low, at points where only the concentration of WDC was high, but where few if any continuous macropores were present. No relation was found between the amount of WDC and clay content. A complete correlation analysis was performed to identify the factors influencing particle and P leaching (Table 2). The two factors most strongly related to the amount of leached particles, PIP, and POP were the average effluent electrical conductivity and the macropore velocity.


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  		Table 2. Correlation matrix{dagger}.

 
The accumulated mass of leached particles was negatively correlated to the average electrical conductivity (EC) of the effluent (r = –0.672) and positively correlated to the macropore flow velocity (r = 0.782) (Fig. 6a) . A low electrical conductivity of the effluent must have been the result of fast transport of irrigation water, having a low EC, from the top to the bottom of the column. A low EC of the effluent is in this case hence an indirect structural measure or a measure of the continuity of macropores from the top to the bottom of the soil columns. This agrees with the fact that the columns having low average ECs generally were also those showing higher macropore flow velocities (Fig. 6a).



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  		Fig. 6. (a) Accumulated mass of leached particles as a function of average electrical conductivity of the effluent and the macropore flow velocity. (b) Accumulated mass of leached particles and dyed areas of active macropores as a function of bulk density.

 
Column bulk densities were found to increase with increasing clay content. At the same time the accumulated mass of leached particles increased with increasing bulk density (Fig. 6b). Columns with the higher bulk densities showed generally smaller dyed areas, representing the active flow volume. All this indicates that in the columns where a high amount of particles was transported, the transport took place in only a few macropores, whereas when a larger part of the soil matrix was involved in flow process, fewer particles were leached. Plotting the accumulated mass of leached particles as a function of the air-filled porosity at –20 cm soil water potential (Fig. 7) clearly reveals that only at clay contents >13% (high bulk density) large amounts of particles were transported. For these highly conducting columns it is evident that the lower air-filled porosity indicates macropore dominated transport and increased particle transport.



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  		Fig. 7. Accumulated mass of leached particles as a function of the air-filled porosity at –20 cm soil water potential. The encircled data points are from columns that ponded during the experiments.

 
We found it remarkable how even small soil textural differences directly or indirectly can cause relatively large differences in soil bulk density and soil structure, thus creating conditions which seemed to have a dramatic effect on the leaching of particles. Soils with the higher bulk density and clay contents apparently had fewer medium-sized pores, thereby producing a more distinct two-domain flow process in which much of the water flows relatively quick through continuous macropores, with concomitant rapid transport of particles. Water in the low bulk-density soils flowed relatively more through medium-sized pores, evidently resulting in less favorable conditions for particle and particle-facilitated transport.

Leached Phosphorus
The leaching of the different P fractions, quantified as a function of accumulated outflow, followed different patterns. Leaching of PIP and POP followed the leaching of particles closely by also showing a characteristic first flush. Phosphorus sorption mainly occurs on Fe and Al groups of solid phase surfaces, especially those of iron and aluminum oxides (Schwertmann and Schiek, 1980; Parfitt, 1978). Phosphorus sorption in Danish soils is expected to occur predominantly on poorly crystalline forms of such oxides (Borggaard et al., 1990). Particulate inorganic P concentrations of the first flush ranged between 0.41 and 5.27 mg P L–1, with an average concentration of 1.48 mg P L–1. For POP the range was between 0.17 and 0.99 mg P L–1, with an average concentration of 0.38 mg L–1. The concentrations of leached dissolved P fractions were more constant throughout the experiment, with DIP effluent concentrations sometimes increasing slightly with time. The average effluent concentrations for DIP and DOP were 0.16 and 0.03 mg P L–1, respectively. For comparison, average concentrations of dissolved reactive P in the Danish monitoring program (weekly sampling of tile drains) were between 0.009 and 0.156 mg P L–1, while total P ranged between 0.01 and 0.199 mg P L–1 (Grant et al., 1996a, 1996b). The accumulated mass of leached DOP was not correlated to the accumulated mass of leached DOC (Table 2).

Variation in leached particulate P at the field scale (Fig. 8a and 8b) resembled the variations in accumulated mass of leached particles (Fig. 5a). The spatial distribution of leached dissolved P (Fig. 8c and 8d) differed markedly from the spatial distribution of particles and particulate P. The accumulated mass of leached PIP for each column and the accumulated mass of leached POP were linearly correlated to the accumulated mass of leached particles, producing r values of 0.995 and 0.779, respectively (Fig. 9 , Table 2). This means that the P concentration associated with the particles was constant. Our experiments always showed the same linear relationship between particulate inorganic P and the particle concentration in the effluent. This suggests that the sources of leachable particles were the same throughout the experiments, irrespective of the type of particles being transported during the initial first flush or later at the more constant levels.



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  		Fig. 9. Accumulated mass of leached P fractions as a function of the accumulated mass of leached particles.

 
For all columns, the total amount of leached P on average consisted of 54% PIP and 19% POP. As for the dissolved fractions, 23% was leached as dissolved inorganic P and 4% as dissolved organic P. Most of the leached P was hence due to particle-facilitated transport. Since we worked only with columns from the plough layer in the field, this study considered only the potential loss of P from this soil layer to deeper layers. However, several studies have demonstrated that P being transported to deeper soil layers or appearing in artificial drains originates from either the top soil, or from fertilizer P placed on the soil surface (e.g., Heckrath et al., 1995; Magid et al., 1999; Djodjic et al., 2002).

As expected the amount of leached PIP increased with increasing macropore flow velocity (r = 0.771) (Table 2) (Fig. 10) . The highest amounts of leached PIP were clearly from the columns with the higher clay contents, which, as discussed above, also showed the largest and most continuous macropores. We conclude from this that particle-facilitated transport of P to a large extent is controlled by soil structure.



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  		Fig. 10. Accumulated mass of leached particulate inorganic P as a function of macropore flow velocity.

 

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


   ACKNOWLEDGMENTS
 
This work was financed by the Danish FREJA program (Female Researchers in Joint Action) under the Danish Research Council. We wish to thank Palle Jørgensen, Stig Rasmussen, and Kaspar Rüegg for technical assistance.


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




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The SCI Journals Agronomy Journal Crop Science
Journal of Plant Registrations Soil Science Society of America Journal
Journal of Natural Resources
and Life Sciences Education		 Journal of
Environmental Quality