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Published in J. Environ. Qual. 34:287-298 (2005).
© ASA, CSSA, SSSA
677 S. Segoe Rd., Madison, WI 53711 USA

TECHNICAL REPORTS

Landscape and Watershed Processes

Evaluating Colloidal Phosphorus Delivery to Surface Waters from Diffuse Agricultural Sources

Louise Heathwaitea,*, Phil Haygarthb, Rachel Matthewsb, Neil Preedyc and Patricia Butlerb

a The Lancaster Environment Centre, Lancaster University, Lancaster, LA1 4YQ, UK
b Institute of Grassland and Environmental Research, North Wyke, Okehampton, EX20 2SB, UK
c Environment Agency, Burghill Road, Westbury on Trym, Bristol, BS10 6BF, UK

* Corresponding author (louise.heathwaite{at}lancs.ac.uk)

Received for publication September 9, 2003.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Colloid-facilitated phosphorus (P) delivery from agricultural soils in different hydrological pathways was investigated using a series of laboratory and field experiments. A soil colloidal P test was developed that yields information on the propensity of different soils to release P attached to soil colloids. The relationship between turbidity of soil extracts and total phosphorus (TP) was significant (r2 = 0.996, p < 0.001) across a range of agricultural soils, and a strong positive relationship (r2 = 0.86, p < 0.001) was found between "colloidal P" (H2O–CaCl2 extracts) and turbidity. Linear regression of the proportion of fine clay (<2 µm) for each soil type evaluated against the (H2O–CaCl2) colloidal P fraction gave a weak but positive relationship (r2 = 0.38, p = 0.082). The relative contribution of different particle-size fractions in transporting P in agricultural runoff from grassland soils was evaluated using a randomized plot experiment. A significant difference (p = 0.05) in both TP and reactive phosphorus (RP) in subsurface flow was recorded for different particle-size fractions, with most TP transferred either in association with the 2-µm fraction or with the 0.001-µm or smaller fractions. Total P concentrations in runoff were higher from plots receiving P amendments compared with the zero-P plots; however, these differences were only significant for the >0.45-µm particle-size fractions (p = 0.05), and may be evidence of surface applications of organic and inorganic fertilizers being transferred through the soil either as intact organic colloids or attached to mineral particles. Our results highlight the potential for drainage water to mobilize colloids and associated P during rainfall events.

Abbreviations: RP, reactive phosphorus • TP, total phosphorus • TSP, triple superphosphate


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
PHOSPHORUS PLAYS a pivotal role in the ecology of freshwater environments. Although P accounts for only 2 to 4% of the dry weight of most cells, biomass production has been shown to be highly sensitive to small amounts of P (Redfield, 1958). Consequently, excess P in freshwaters derived from agricultural sources will enhance plant growth and accelerate eutrophication irrespective of the fact that the magnitude of P loss is small in agronomic terms (Environment Agency, 2000; Mainstone and Parr, 2002). In the UK, annual losses of P from agricultural land to water have increased from around 0.62 kg ha–1 in the 1930s to 1.24 kg ha–1 in the 1990s (Defra, 2002). Under intensive dairy farming in the UK, for example, approximately 26 kg P ha–1 accumulates in the soil each year from manure and inorganic fertilizer applications, forming a reservoir for future P loss to water (Defra, 2004). Haygarth et al. (1998b) have estimated that unless the import of P to agricultural land is modified, the soil P reservoir may double in the next 30 yr. This buildup of soil P increases the potential for off-site movement of P because the solubilization of P from soil surfaces and soil biota into soil water increases with increasing soil P (Heckrath et al., 1995), and detachment of sediment-associated P at the soil surface is encouraged where farming practices generate soil erosion (Kronvang, 1990; Chambers et al., 2000).

A major gap in current knowledge is our understanding of how nutrients are retained within complex landscapes and released to adjacent streams and water bodies via surface and subsurface flow paths (Kamprath et al., 2000). Phosphorus may be transported in dissolved, colloidal, and particulate forms (Sims et al., 1998). Soil water-extractable P has been widely used as a method for predicting P transfer (Sharpley, 1982), and provides quantitative data on the potentially mobilizable store of P within the soil. Studies investigating this relationship have often concentrated on "dissolved" forms of P that are determined on supernatants filtered through 0.45-µm membranes. While P in soil solutions passing through a 0.45-µm membrane can be operationally defined as "dissolved," no natural cut-off exists between "dissolved" fractions of <0.45 µm and "particulate" fractions of >0.45 µm. Furthermore, this arbitrary separation of P forms may not help us to understand fully the modes of P transfer from land to water. Critically, the colloidal size range from 1 nm to 1 µm spans the arbitrary 0.45-µm divide, and consequently tends to be ignored in studies of the mechanisms of P transfer on land and delivery to water. Colloids and fine particles may be especially important for P transfer because of their large specific surface area and thus enhanced adsorption characteristics. Haygarth et al. (1997), for example, were able to show that much of the P in filtrates of <0.45 µm was associated with colloids and particles of >1000 molecular weight. These colloids include inorganic solids, organic macromolecules, the debris of organisms, or aggregates of these forms (Buffle and Van Leeuwen, 1992). For colloid-facilitated transport to be a significant mechanism of P transfer, mobile particles must be present in sufficiently large quantities (Kretzschmar et al., 1999). Although quantitative theories predicting the release of soil colloids are lacking, we know that colloid-sized particles have a large specific surface area (Newman et al., 1994), and thus increased adsorption capacity (Beckett and Chittleborough, 1994). Consequently, both inorganic and organic colloids may be important in the transport of strongly sorbing contaminants such as P from land to water (Hens and Merckx, 2002; Jarvis et al., 1999).

The hydrological pathways along which P can be transferred include surface runoff, subsurface lateral flow (including preferential flow routes), artificial drainage channels, and vertical percolation. In many agricultural systems, surface runoff during high-intensity rainfall events is the main pathway of P delivery to water (Gburek and Sharpley, 1998; Catt et al., 1998). However, subsurface pathways are important where environmental and soil conditions favor rapid flow through macropores and drains (Chapman et al., 2001; McDowell and Sharpley, 2001), and this pathway may offer an important route for P delivery from land to water (Heathwaite and Dils, 2000). Critically, field tracer experiments by Becker et al. (1999) and Cumbie and McKay (1999) have shown that colloidal particles can be transported relatively rapidly in subsurface pathways compared with solutes. Furthermore, McGechan et al. (2002) suggest that P attached to fecal colloidal particles may be preferentially transported via soil macropores in subsurface pathways. The authors argue that unlike P transfer after sorption onto inorganic colloids, which is largely dependent on the erodibility of the soil (Fraser et al., 1999; Kronvang, 1990; Sharpley and Smith, 1990), P transfer in association with fecal colloids is independent of soil erosion because the colloids are already present in the excreta. The potential importance of colloid-facilitated P transfer from livestock sources is supported by research on grassland systems by Haygarth et al. (1998b), which has shown them to be a major source of diffuse P reaching surface waters. In the UK, grassland accounts for 70% of the land area and receives high P inputs, in particular that derived from dairy manure (usually including bedding material) and dairy slurry (from storage tanks), but also swine or poultry manure in some regions, both of which are a concentrated source of P that may be vulnerable to rapid transport to water (Sharpley and Moyer, 2000). For the UK, estimates of annual P transfer rates from grassland soils are of the order 2 to 3 kg P ha–1 (Haygarth and Jarvis, 1997; Haygarth et al., 1998a). However, Preedy et al. (2001) showed that P losses of this magnitude can occur in just a few hours where heavy rainfall followed spreading of manures or inorganic fertilizers on grassland; such events have been termed "incidental" P losses. Preedy et al. (2001) found that storm-driven P transfer mobilized both particulate and dissolved forms of P but the ratio appeared to vary with the magnitude of runoff. Sharpley et al. (1996) found that the high surface roughness characteristics of grassland soils tended to generate P loss through the solubilization of dissolved inorganic P. Clearly, the predominant process of P transfer will depend on the scale at which measurements are being made and the point in the landscape at which samples are collected. However, given the magnitude and potential environmental impact of incidental events such as those reported by Preedy et al. (2001), it is important to determine whether particulate and colloidal forms of bioavailable P are vulnerable to transport.

In this paper, we report the results from laboratory and field experiments that examined the potential for colloidal material to act as a vehicle for the transfer and delivery of P from agricultural soils to receiving waters. The first experiment took a range of agricultural soils and compared two methods, extraction and filtration, to determine the potential release of P attached to colloids of <2 µm. To undertake this comparison we developed a sequential filtration protocol and used the protocol to investigate the relationship between particulate and colloidal P desorption with water and the physical and chemical properties of soil. The second experiment builds on the first and led to the development of a soil colloidal P test that can be used to determine the propensity for different soils to release colloids. The third experiment measured the colloid-size fractions and associated P forms transferred in the field for grassland soils using a randomized plot experimental approach with a control (zero-treatment) and three different fertilizer applications (inorganic fertilizer, dairy manure slurry, and fertilizer plus manure).


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Development of a Sequential Filtration Protocol
A protocol for membrane and ultrafiltration was developed in two stages. First, we recorded the recovery rates for two model P compounds, dihydrogen orthophosphate and inositol hexaphosphoric acid, following sequential filtration using three test solutions to check the effect (if any) of the membrane or ultrafilters on P composition in the filtrate. Second, we compared the effect of sequential and nonsequential filtration on the P concentration (µg P L–1) in the filtrate using samples of leachate collected from field lysimeters. For sequential filtration, all samples were filtered through successively finer filters. For nonsequential filtration, we took a bulked <2-µm sample and filtered this through the finer membranes individually.

For Stage I, five replicates of three standard test solutions (water and 0.01 and 0.1 M sodium chloride) were used. The test solutions contained 0.2 mg P L–1 model P compounds of dihydrogen orthophosphate (KH2PO4) and inositol hexaphosphoric acid (C6H18O24P6·KH2PO4 and C6H18O24P6) having molecular weights of 139 and 924, respectively. Consequently, retention through the smallest filter (0.0003 µm, or 3K nominal molecular weight [NMWL]) should not occur. The following membrane and ultrafilters were used: (i) Whatman (Maidstone, UK) GMF150 2-µm glass microfiber filter; (ii) Millipore (Billerica, MA) mixed cellulose nitrate and acetate CNA membrane filters for the size range of 1.2 µm (Millipore code RAWP), 0.8 µm (Millipore code AAW), 0.45 µm (Millipore code HAW), and 0.1 µm (Millipore code VCWP); and (iii) Amicon (Millipore) regenerated cellulose ultrafilters for the size range of 0.001 µm (10K NMWL) (Amicon code YM10) and 0.0003 µm (Amicon code YM3). The membrane filters were rinsed with Milli-Q ultrapure water (Millipore) and tested using 50-mL aliquots of each standard solution. The filter was conditioned with each solution before the aliquots were placed into a Sartorius (Goettingen, Germany) 250-mL polycarbonate filter holder and vacuum-filtered (<500 kPa of vacuum). The filtrate was retained in a polyethylene bottle and stored at 4°C as recommended by Haygarth et al. (1995). The ultrafilters were prepared by soaking them in Milli-Q water for 1 h; we changed the water three times to remove the glycerin and sodium azide pretreatments added during manufacture. After soaking, the ultrafilters were placed in an Amicon 150-mL stirred ultrafiltration cell and rinsed for 5 min with Milli-Q water. Seventy-milliliter aliquots of each standard solution were placed in the cell in turn, and ultrafiltered at 500 kPa. The first 20 mL of filtrate was discarded, with the remainder being retained in a polyethylene bottle and stored at 4°C. After use, the ultrafilters were restored by rinsing in 0.1 M NaOH for 30 min.

For Stage II, five replicate samples of leachate were collected in the field from two soil types with contrasting textures (sand and clay), and subjected to sequential and nonsequential filtration using the same sequence of membranes described above. The objective was to use soil waters containing particles and colloids that covered the spectrum of the filter sizes used to develop the protocol described in Stage I. The first set of samples were collected in gravity drainage from an undisturbed 80-cm-diameter, 135-cm-deep soil monolith (UK national grid reference [NGR]: SX 657 983) subject to natural rainfall (approximately 1150 mm yr–1). This Typic Haplaqualf belongs to the UK Newport soil association. For the second set of samples, surface runoff was collected from a 1-ha in situ lysimeter (NGR: SX 650 995) containing a Typic Haplaquept (UK Soil Series: Hallsworth). All soil water samples were fractionated within 1 h of collection to reduce the potential problems of coagulation and microbial growth. Twenty-milliliter aliquots from each of the filtrates as well as from each unfiltered sample were taken for TP determination.

The Soil Colloidal Phosphorus Test
Studies of the release of soil colloids using model systems have shown that release rates increase with decreasing ionic strength (Khilar and Fogler, 1984). To evaluate the potential for a soil to release colloids and associated P, we developed an extraction technique that determines the colloidal P content of a soil by varying the ionic strength of the extraction solution and exploiting the ability of cations to prevent particle release. We compared this extraction technique with a filtration procedure to check for consistency in terms of colloid release. As filtration is a time-consuming process, one of our objectives was to develop a colloidal P test that could be conducted more rapidly using an extraction approach. A rapid test is important given the potential for exchange between dissolved and colloidal or particulate P fractions following sample collection. The method described below was developed using three replicate soil samples of a Typic Haplaquept. The samples were taken to a 7-cm depth and passed through a 2-mm sieve; a subsample was dried overnight at 110°C for dry matter determination. The key physical and chemical properties of the soil are shown in Table 1. Fresh samples of equivalent dry weights were used in each of the methods described below. This decreased the effect of variability in aggregate stability on rewetting, and allowed conditions that were more representative of those in the field to be attained. The method was tested on five fresh replicates of seven agricultural soils (Table 1) using the same collection and preparation procedure outlined for the Typic Haplaquept above. To develop the extraction method, three separate extractions using solutions of different ionic strength (I) were undertaken: 0.01 M CaCl2 (I = 0.03), 0.03 M NaCl (I = 0.03), and deionized water (I = 0). In each case, a subsample was used of soil from each replicate. The method was then tested using the deionized water and 0.01 M CaC12 extracts alone. Extractable colloids and associated P were determined by end-over-end shaking of the sample with the extractant using a ratio of 1:30 (10 g dry wt. equivalent to 300 mL of solution) for 120 min at ambient temperature. The 1:30 ratio was selected on the basis of a review of the effect of batch extraction ratios in relation to P solubilization (Turner and Haygarth, 1999). After extraction, samples were centrifuged at 1800 rpm (1310 x g) for 5 min using an MSE GF-8 centrifuge (Measuring Scientific Equipment, Beckenham, UK) to take out particles to around 10 µm, followed by filtration through a 2-µm Whatman GMF-150 glass microfiber membrane. For the filtration method, replicate fresh soils (three for method development; five for method testing) were first extracted with deionized water and the supernatant from each extract passed through a 2-µm Whatman GMF-150 membrane using exactly the same protocol described above for the extraction method; a subsample was retained in a polyethylene bottle for subsequent analysis. The remaining filtrate was passed through either a 0.45-µm CNA membrane (Millipore HAW) using a Sartorius polycarbonate filter holder, or through a 0.0003-µm cut-off filter (Amicon YM3) using an Amicon 8400 ultrafiltration cell.


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  		Table 1. Physical and chemical properties of selected agricultural soils used to investigate particulate and colloid release potential.

 
Transfer of Colloids and Associated Phosphorus in the Field
Colloid-facilitated P transfer was determined using samples collected in the field from a randomized plot experiment using an experimental design described in full in Preedy et al. (2001). Briefly, the setup comprised 24 plots with dimensions of 3 x 10 m arranged in three blocks of eight plots. Each plot was hydrologically isolated by a gravel-filled trench to an 85-cm depth at the top of the plot and plastic sheeting to a depth of 50 cm along each side. Vertical drainage below 30 cm was naturally impeded by a clay horizon of low hydraulic conductivity (approximately 0.005 m d–1). During the drainage season, discharge from the plots was recorded at 30-min intervals as a composite of surface runoff plus lateral flow to a 30-cm depth. There was sufficient rainfall to generate runoff from the plots on 2, 4, and 10 Mar. 1999 before applying fertilizer and manure treatments to 18 of the 24 plots. A single sample of this runoff, bulked over a 4-h period, was collected from the six zero-treatment plots to characterize the background concentrations of P and the pattern of particle-size transfer. The treated plots received different P fertilizer and/or manure applications in the form of four treatments replicated six times. Two replicates of each treatment were randomly positioned within each block of eight to allow for any hydrological variation along the gradient of the slope. The treatments were (i) zero P, (ii) dairy manure slurry applied at a rate of 50 m3 ha–1, (iii) inorganic fertilizer applied as triple superphosphate (TSP), and (iv) TSP plus dairy manure slurry to supply equal amounts of P. The total P applied was equivalent to 38 kg P ha–1 for all treated plots. The treatments were applied 44 d before a rainfall event on 14 Apr. 1999 when a single sample of runoff, bulked over a 4-h period, was collected from each of the 18 treated plots plus the 6 zero-treatment plots.

At the experimental site the slope is 5 to 10% and the soil is a Typic Haplaquept overlaying shales of the Crackington Formation. An Ap horizon (0- to 27-cm depth) with 37% clay, 5% organic matter, and a well-developed fine sub-angular blocky structure overlies subsoil with 40% clay and strongly developed coarse prismatic structure (Findlay et al., 1984). The soil P characteristics are described in Haygarth et al. (1998a). Soil sodium bicarbonate–extractable P (0-to 27-cm depth) is low with an average of 7 mg kg–1, although 13 mg kg–1 is recorded in the upper 2 cm of the soil. The average TP concentration for the Ap horizon is 540 mg kg–1. The sward is dominated by ryegrass (Lolium perenne L.) and is grazed between April and October with 4 steers ha–1. Phosphorus inputs on the 1-ha lysimeters are incorporated in the form of TSP fertilizer, recycling from cattle excreta during grazing, and dairy manure slurry additions (Haygarth et al., 1998b). These P inputs are typical of grassland management practice in the UK.

Analytical Techniques
After filtration, the individual size fractions selected for the filtration protocol described above were analyzed for operationally defined forms of P. Reactive phosphorus (RP) was determined colorimetrically using the standard Murphy and Riley (1962) molybdenum-blue complex method measured using flow injection analysis (FIA). The procedure for measurement of RP is described in full in Haygarth et al. (1997). Total P was also determined colorimetrically by FIA following sulfuric acid–persulfate digestion using a method optimized from Eisenreich et al. (1975). Absorbance was calibrated against standard solutions of KH2PO4 in the range 0 to 500 µg P L–1 using six standards that were prepared on the day of analysis. Samples outside of the range of the highest KH2PO4 standard were diluted with Milli-Q ultrapure water. Accuracy was maintained using in-house quality control and Aquacheck (Bury, UK) samples. Only analytical batches where quality control samples fell within ±5% of the known value were accepted. Unreactive P was calculated by difference (TP – RP). This fraction is generally considered to represent organic P compounds such as inositol phosphates and P held within nucleic acids.

A Hannah Instruments (Woonsocket, RI) LP2000 turbidity meter, calibrated immediately before measurement, was used to measure turbidity in each supernatant generated from the extraction and filtration methods used to develop the soil colloidal P test. Because the turbid nature of the samples had the potential to interfere with the colorimetric analysis of P, the method of Standard Addition was undertaken on the 2-µm filtrates of each soil using addition of 140, 280, and 400 µg P L–1 as dihydrogen orthophosphate (KH2PO4). The P determinations are expressed on a dry weight equivalent basis.

All statistical analyses were conducted using Genstat 6.1 (Payne and Arnold, 2002). Simple linear regression was used to examine the relationship between extractable turbidity and soil TP for different size fractions and between percentage clay and colloidal P. One-way analysis of variance (ANOVA) was used to test for the following: (i) variation in turbidity determined by extraction or filtration for a range of agricultural soils and also to test for the variation in TP (mg P kg–1 dry weight) determined by extraction or filtration for a range of agricultural soils; (ii) variation between the different size fractions for both TP and RP for the zero-P treatments, and also to see if there was a significant difference between the 2, 4, and 10 March sample dates; and (iii) variation between the different P treatments (six replicates of each treatment) and between the control (zero-P treatment) and the P treatments.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Validation of the Sequential Filtration Protocol
The procedure used for sample collection, storage, and fractionation of particles and colloids can interfere with the determination of colloidal matter within a solution (Chen and Buffle, 1996; Haygarth et al., 1997). In particular, the choice of filters and filtration techniques may affect results because of sorption, electrostatic repulsion, and clogging (Buffle et al., 1978; Nanny et al., 1994). The potential interference of the filtration protocol was checked by comparing recovery across the 2- to 0.1-µm filtration range of two model P compounds under three different test solutions: water and 0.01 and 0.1 M NaCl. There was no significant difference (p = 0.05) between the water, 0.01, and 0.1 M NaCl test solutions. Recovery coefficients of 1.00 ± 0.03 were recorded, and the variation in replicate filtration was low (Table 2). However, for the 0.001- to 0.0003-µm ultrafiltration range there was significant variation in the concentrations of P recorded after ultrafiltration in water compared with the 0.01 and 0.1 M NaCl test solutions. Less than 10% P was recovered as orthophosphate in water from the 0.001-µm membrane compared with >98% recovery in the 0.1 M NaCl test solution (Table 2). The recovery of P in the 0.01 M NaCl test solution ranged from 53 to 71%. Retention during ultrafiltration may result from surface interactions between the negatively charged membrane and the P ions. Increasing the salt concentration of the solution from 0.01 to 0.1 M NaCl considerably improved the recovery coefficient by reducing these surface repulsive interactions so that the difference in concentrations of P was reduced to less than 0.13 µm following filtration in 0.1 M NaCl. The 0.0003-µm membrane performed better than the 0.001-µm membrane, with 50 to 52% recovery as orthophosphate in water, but salt was still needed to increase the recovery to 97 to 99% by preventing P retention on the membrane. Although this may cause changes in the concentrations of the P fractions over time, immediate fractionation after salt addition was considered to have little effect.


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  		Table 2. Recovery rates for two model phosphorus compounds following sequential filtration.{dagger}

 
For soil water samples collected as either leachate from the gravity-drained lysimeter or as surface runoff from the undrained 1-ha lysimeters, the concentrations of P recorded in solution following nonsequential filtration showed no significant difference (p = 0.05) to those filtered sequentially (Table 3). Importantly, the sequential filtration procedure appeared to prevent clogging and so decreased the filtering time significantly. This meant that soil water samples could be immediately fractionated after collection, which reduced the potential for coagulation and microbial growth in the samples. We note that the use of this method is limited to negatively charged soils.


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  		Table 3. Comparison of the effect of sequential and nonsequential filtration on the mean phosphorus concentration recorded in the leachate from a sandy (Typic Haplaqualf) and clayey (Typic Haplaquept) soil.{dagger}

 
The Soil Colloidal Phosphorus Test
We developed a rapid extraction technique to determine the colloidal P content of a soil and compared this technique with a filtration procedure, which is more time consuming, to check for consistency in terms of colloid release. The results are shown in Tables 4, 5, and 6. Table 4 shows the turbidity and soil TP across the different size fractions for replicate samples of a Typic Haplaquept soil (UK Soil Series: Hallsworth). The turbidity recorded in the <2-µm size fraction was highest after extraction with deionized water (642 formazin turbidity units [FTU]; Table 4); introducing Ca2+ prevented desorption of virtually all the colloids (3 FTU; Table 4). Filtration through a 0.45-µm membrane reduced particle release across all extracts, inferring that the majority of particles were in the 0.45- to 2-µm size range. No particles were detected in supernatants filtered through the 0.0003-µm membrane. Because the composition of mobile in situ colloids is similar to the fine fraction generally present in soil (Kretzschmar et al., 1999), and the fine clay fraction of the test soil used here comprised only 9% of the total clay fraction (Table 1), this result could be expected. The concentration of TP recorded in the supernatants showed a similar pattern to that of the turbidity data, with most P (19.4 mg P kg–1 dry wt.) recorded after extraction with deionized water in the <2-µm fraction (Table 4). The relationship between turbidity and TP was significant (r2 = 0.996, p < 0.001) for all replicate samples and filter sizes.


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  		Table 4. Turbidity and total phosphorus (TP) in soil extracts for a Typic Haplaquept soil used to develop the extraction method for the colloidal phosphorus test.{dagger}

 

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  		Table 5. Variation in turbidity determined either by extraction or filtration for a range of agricultural soils.{dagger}

 

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  		Table 6. Variation in total phosphorus (TP) in the colloidal fraction determined by extraction or filtration for a range of agricultural soils.{dagger}

 
We tested the viability of the extraction method as a rapid indicator of the variation in turbidity for the range of agricultural soils shown in Table 5. We used only the deionized water and 0.01 M CaC12 extracts because the ratio of NaCl extractant to soil used in Table 4 appeared to suppress dispersion and hence release of colloids, and so was not a good indicator of turbidity. In Table 5, turbidity using the extraction method is given by the difference between the deionized water and 0.01 M CaC12 extracts; for the filtration method turbidity is given by the difference between the <2- and <0.0003-µm filtrates. There was no significant difference (p < 0.001) in turbidity between the extraction and filtration methods for particulate material of <2 µm (filtration method) or extractable particles in water (extraction method). However, there was significant variation (p < 0.001) in turbidity for the different soils tested. The noncalcareous clay (USDA classification: Haplaquept [Suborder Aquents]; UK Soil Series: Denchworth) produced the most turbid supernatant (3678 FTU) and the sand (USDA classification: Typic Quartzipsamment; UK Soil Series: Bridgnorth) the least turbid (314 FTU) supernatant. The Haplaquept (Suborder Aquents) had the highest percentage of 64% clay particles relative to the Typic Quartzipsamment with 7% clay (Table 1).

The concentration of TP in the colloidal fraction for the different agricultural soils tested is shown in Table 6. A comparison of the extraction (CaCl2) and the filtration (<0.0003 µm) methods is standardized against the water-extractable TP for each soil determined by shaking a 1:30 ratio of soil to deionized water then filtering through a 0.2-µm filter. The results indicate the viability of the extraction method to determine colloidal P as an alternative to filtration. The actual size of the material within the <2- to 0.0003-µm range was not measured, so it is not possible to say whether the TP measured was linked to an increase in the proportion of fine colloidal material, only that it was linked to the turbidity of the water. The colloidal P fraction determined by the extraction method as a proportion of the TP of the bulk soil given in Table 1 varied from 3 to 19%, with three soils having less than 5% TP in the colloidal fraction (Typic Haplaquept, Dystric Eutrochrept, Typic Rendoll) and two soils having >18% TP in the colloidal fraction (Haplaquept [Suborder Aquents], Typic Ochraqualf). The high Ca content of the Upton calcareous loam may explain the small P release from this soil, but the large P release from the Hanslope calcareous clay does not support this. Using linear regression, a strong positive relationship (r2 = 0.86, p < 0.001) was recorded between "colloidal P" (defined by the difference between the H2O and CaCl2 extracts) and the turbidity of the water extracts (Fig. 1) . Linear regression of the proportion of fine clay (<2 µm) for each soil type (Table 1) against the (H2O–CaCl2) colloidal P fraction (Table 6) gave a weak positive relationship (r2 = 0.38, p = 0.082). A relationship might have been anticipated because the P enrichment ratio predicts that finer-sized particles would be richer in P relative to the bulk soil (Haygarth et al., 1998a). Further work is needed to calibrate the colloidal P test across a wider range of soil types.



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  		Fig. 1. Linear regression of the relationship between water-extractable supernatant turbidity (formazin turbidity units, FTU) and colloidal P release across the particle-size range 2 to 0.0003 µm for different soils.

 
Hydrological Pathways of Colloidal Phosphorus Transfer
Figure 2 shows the variation in TP and RP for the six 30-m2 zero-P plots forming part of the randomized field plot experiment described earlier. Phosphorus concentrations in near-surface lateral flow to a 30-cm depth are shown across the <0.001- to 2-µm particle-size range. A single sample of runoff, bulked over a 4-h period, was collected on 2, 4, and 10 Mar. 1999 (Fig. 2) when 70 mm of rainfall in the 10 d before sampling was sufficient to generate runoff from the plots. Discharge was recorded at 30-min intervals. The average discharge from the plots on each sample date was 0.2 mm (SE = 0.01), 0.01 mm (SE = 0.004), and 0.1 mm (SE = 0.03), respectively. For all sample dates, there was a polarization of TP transfer with most TP transferred either in association with the 2-µm fraction or with the 0.001-µm or smaller fractions. The particle-size fractions measured in runoff from the plots showed significantly different (p = 0.05) concentrations of P in each size fraction for both TP and RP for the 2 and 4 Mar. 1999 sample dates but not for the 10 Mar. 1999 date where missing data constrained the statistical significance of the results. A repeated measures analysis of variance on the three sample dates showed that the filter-size fractions were significantly different (p = <0.001) and there was some effect of time (p = 0.047) in the pattern of TP concentration in association with the different filter-size fractions. The apparent decrease in the concentration of TP in the >2-µm fraction over the 8-d sampling period was not significant (p = 0.05). It is perhaps surprising that relatively large particles were recorded in subsurface flow from the plots at the low discharges measured; this may be evidence of preferential flow pathways and mobilization of coarse clay fractions, organic detritus, and bacteria. More importantly, the results shown in Fig. 2 suggest that the >2-µm fraction and the fine clay fraction are potentially important carriers of P. It is difficult to discern a trend in the ratio of RP to TP across the size fractions or through time in Fig. 2. For the >2-µm fraction, RP as a percentage of TP ranges from 21 to 52% (Fig. 2), while RP as a percentage of TP for the 0.8- to 0.22-µm particle-size range is generally >35%. For the smallest size fractions, RP as a percentage of TP ranges from <10 to >90%.



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  		Fig. 2. Variation in the concentration of total phosphorus (TP) and reactive phosphorus (RP) for different particle-size fractions in near-surface flow to 30 cm from untreated 30-m2 plots for three consecutive sampling periods: (a) 2 Mar. 1999, (b) 4 Mar. 1999, and (c) 10 Mar. 1999. The data are means of six replicate plots; the error bars represent 1 SE. The variation in TP and RP for different particle-size fractions is significant (p = 0.05).

 
Figure 3 shows the variation in TP transfer in runoff for different particle-size fractions following the application of dairy cattle manure and inorganic fertilizer in a randomized experiment using the 30-m2 plots described earlier. The results for the zero-treatment plots are also shown. The data shown are for a single 28-mm rainfall event on 14 Apr. 1999. Not surprisingly, the TP concentrations recorded in runoff were higher from the plots receiving P applications compared with the zero-P plots; however, these differences were only significant (p = 0.05) for the >0.45-µm particle-size fractions. The highest TP concentration (277 µg L–1) for a single size fraction was recorded for the >2-µm TSP treatment. There was no significant difference (p = 0.05) in TP concentration between the different treated plots, although a decrease in TP transfer in runoff from larger- to smaller-sized particles is shown in Fig. 3 for these plots. This pattern was not recorded for the control (zero-P) plots. The pattern of RP transfer in runoff from the plots for the same rainfall event is shown in Fig. 4 . There was no significant difference (p = 0.05) in the concentration of RP in runoff between the different treated plots. Unlike TP, there was no significant difference (p = 0.05) between the treated plots and the control (zero-P), although the pattern of P transfer in runoff is similar to that of TP with the larger-sized particles appearing to be more important carriers of P. The results for these field plot experiments highlight the capacity of different colloidal fractions to transport P, and rather surprisingly suggest that the larger-sized particles are relatively more important compared with the fine clay-sized particles. These results appear to contradict the generally held assumption that finer-sized particles are richer in P relative to the bulk soil. Unfortunately, no records of the effects of the P treatments on the turbidity of the runoff from the plots were made so these data cannot be related directly to the soil colloidal P test.



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  		Fig. 3. Variation in the concentration of total phosphorus (TP) for different size fractions collected in near-surface flow to 30 cm from 30-m2 replicated plots for each of four treatments: (i) untreated plots (zero P), and treated plots receiving (ii) the equivalent of 38 kg P ha–1 in the form of triple superphosphate (TSP), (iii) TSP + dairy manure slurry, and (iv) dairy manure slurry, for a single storm on 14 Apr. 1999. The data are means of six replicated plots; the error bars represent 1 SE. The difference in TP in runoff for zero-P vs. P-treated plots is significant (p = 0.05) but only for the >0.45-µm particle-size fractions. There was no significant difference in TP concentration between the P-treated plots (p = 0.05).

 


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  		Fig. 4. Variation in the concentration of reactive phosphorus (RP) for different size fractions collected in near-surface flow to 30 cm from 30-m2 replicated plots for each of four treatments: (i) untreated plots (zero P), and treated plots receiving (ii) the equivalent of 38 kg P ha–1 in the form of triple superphosphate (TSP), (iii) TSP + dairy manure slurry, and (iv) dairy manure slurry, for a single storm on 14 Apr. 1999. The data are means of six replicated plots; the error bars represent 1 SE. There is no significant difference (p = 0.05) in the concentration of RP in runoff between the different P-treated plots or between the P-treated plots and the zero-P plots.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
Phosphorus delivery from land to water is complex because it encompasses transfer in both soluble and particulate phases and is controlled by a set of processes that interact with one another in time and space. Although water flux through and over the soil surface acts as the driver for P transfer (Haygarth et al., 2000), soil characteristics control both the source and form of transferred P. Soil processes such as microbially mediated mineralization or immobilization and chemically controlled sorption–desorption and precipitation reactions are important factors in determining the propensity for any soil to retain or release P. The soil colloidal P test (Table 4) showed that particle release from soils is related to the degree of cation saturation. Consequently, saturation with multivalent cations such as Ca2+ released virtually no particles. Conversely, water had a relatively high capacity to initiate the release of colloids and may be an important driver for subsurface P transfer in association with specific particle-size fractions.

Current P fractionation methodologies that commonly differentiate "dissolved" from particulate P at 0.45 µm may not adequately account for the role of colloids in assisting P delivery to water. A number of studies (Smith and Degueldre, 1993; Kretzschmar et al., 1997) have highlighted the importance of colloid-facilitated transport in subsurface hydrological pathways as a mechanism for the delivery of strongly sorbing contaminants to surface waters. The results described in this paper highlight the potential for agricultural runoff to mobilize colloids and associated P during rainfall events, and that significant P transfer may occur in subsurface pathways where a range of colloid sizes act as carriers of P. The fractionation procedure described by the colloidal P test provides a straightforward technique to assess the colloid contributing potential of different soils from which to infer the significance of this route of P transfer in agricultural systems. The colloidal P test is significant because relative to solutes, particle tracking studies have shown that colloidal particles are able to travel both rapidly and over significant distances in subsurface hydrological pathways (Cumbie and McKay, 1999). This apparent contradiction occurs because soluble contaminants in soil solution diffuse into soil micropores where they are retained. The larger size of colloidal particles means they are excluded from soil micropores and as a result are relatively more mobile within larger soil pores. Cox et al. (2002), for example, found that in some Australian soils P was attached to mobile illitic colloids rather than being present in soluble form. Thus, the quantity of P transferred attached to colloids per unit time per unit area or slope length may be greater than that for solutes. Without a test such as the colloid P test, however, we are unable to elucidate which colloid-size fractions are important. In the work reported here it appears likely that the form of P (i.e., colloidal or soluble) affects transport rates with both the <0.001-µm and >2-µm particle sizes being important for TP transfer in successive runoff events from zero-treated plots (Fig. 2). The contribution of the 2-µm fraction to TP delivery in subsurface flow was surprising but may reflect the low clay content of the soils represented by the field experiments (Table 1). For the treated plots, an alternative explanation may be that the P delivery attached to the 2-µm particles represents the mobilization and transfer of surface-applied P through the soil, possibly in preferential flow pathways that are represented in these clay soils (Armstrong and Garwood, 1991). We found that the application of P in the form of dairy manure slurry or TSP to the surface of the replicated plots shifted the balance in the pattern of P transfer in association with different-sized colloids, whereby larger particle sizes (>0.45-µm) delivered most TP (Fig. 3). This pattern was not recorded for RP (Fig. 4). The field results may be evidence of surface applications of P in both organic and inorganic forms moving through the soil profile in near-surface lateral flow in association with relatively large colloid fractions. Similar evidence of nutrient translocation from plots receiving inorganic and organic fertilizer applications was recorded by Heathwaite et al. (1998) for grassland systems, although no attempt was made in their experiments to look at the colloidal signature of the nutrient transfer. While speed of transport was not measured in the results reported here, it is likely that P transport rates will vary between different hydrological pathways (Haygarth et al., 2000). Consequently, particle size alone cannot account for the large differences in the transport rate observed between soluble and colloidal fractions in some field experiments. McKay et al. (2000), for example, report colloid transport rates up to 500 times faster than solute transfer in fractured shales. Such differences in solute and colloid transfer rates may be due to transport through preferential flow pathways, such as macropores and fractures within the soil (Toran and Palumbo, 1992; McGechan, 2002). The significance of P loss in drainflow has been reported by researchers elsewhere (Dils and Heathwaite, 1999).

Kretzschmar et al. (1999) found that the composition of mobile in situ colloids is dominated by the fine clay fraction of a soil. For P, Beckett and Chittleborough (1994) also found that fine material (<0.1-µm) dominated P transfer in soil leachate with transport occurring as fine colloidal organo–P complexes. We found some evidence of TP transfer in near-surface flow in association with fine-colloidal material. In particular, the <0.001-µm fraction was often associated with high TP concentrations (Fig. 2). However, our results also suggest that the >2-µm fraction is important in near-surface flow when the soil is saturated, probably as a consequence of preferential flow pathways being activated (Preedy et al., 2001). Hens and Merckx (2002) found that only dissolved P species were present at significant concentrations in the <0.025-µm fraction of several different soil solutions. For both arable and grassland soils, the authors suggest that 40 to 58% of RP and at least 85% of unreactive P was associated with colloidal-sized material. In the research reported here, for untreated grassland soils RP is important in P transfer associated with the <0.001-µm fraction (Fig. 2), but the relative contribution of RP to TP is not consistent. The Typic Haplaquept soil used in the field experiments reported here typically has a fine clay fraction around 10% of the total clay fraction, which explains why filtration through a 0.45-µm membrane removed most of the particles for all extracts, as the majority of particles were in the 0.45- to 2-µm size range.

There is little research on the relative contribution of inorganic versus organic P applications to agricultural land on P transfer associated with different particle-size fractions in runoff. Recently, McGechan et al. (2002) proposed that P transfer in association with organic colloids may be an important mechanism of P delivery from livestock systems to receiving waters. The initial work reported here suggests that although different particle-size fractions were important for P transfer, there was no clear signal associated with organic compared with inorganic P applications at the soil surface (Fig. 3 and 4). The significance of larger-sized particles as vehicles of P transfer was reported by Heathwaite et al. (1998), where P losses in surface runoff from grassland receiving inorganic fertilizer exceeded that from solid dairy manure (which includes straw) or liquid dairy manure slurry treatments. The authors found that inorganic fertilizer applied under controlled experimental conditions was rapidly mobilized and often remained in pellet form as it was transferred, which accounted for the significance of the larger particles; whereas the slurry and manure applications remained on the soil surface for some time after application, forming a longer-term diffuse pollutant source.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
This paper shows how the addition of a sequential filtration step to standard water-extractable P methods may yield information on the colloidal P content of soils. The soil colloidal P test developed by combining sequential filtration with extractants of differing ionic strengths and cation content is a simple and straightforward technique for assessing the propensity for colloid release from agricultural soils. We show that colloidal material falling within the notional "dissolved" (<0.45-µm) size fraction is important in accounting for the transfer of P from soils. Furthermore, application of P to land in the form of inorganic fertilizers or organic manures produces a different colloidal P signature compared with that of untreated land. Colloidal P mobilization may represent a significant vehicle for subsurface P transfer from soil solution or in association with surface applications of fertilizers or manures moving through the soil during rainfall events. This finding is important because colloidal P transfer is a physical mode of transport that may operate independent of the factors that control the chemical mobility of P in terms of precipitation and chemical complex mechanisms.


   ACKNOWLEDGMENTS
 
This research was funded by Biotechnology and Biological Sciences Research Council (BBSRC) Grant A08080 awarded jointly to the BBSRC Institute of Grassland and Environmental Research (IGER) and the University of Sheffield. We wish to acknowledge Alan House (CEH) and Ben Turner (USDA-ARS) for valuable discussions regarding methods for separation and soil colloids testing, respectively.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 


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