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Journal of Environmental Quality 32:1422-1429 (2003)
© 2003 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

TECHNICAL REPORTS
Surface Water Quality

Phosphorus Runoff Relationships for Louisiana Coastal Plain Soils Amended with Poultry Litter

Lewis A. Gaston*,a, Caye M. Drapchob, Soma Tapadara and John L. Kovarc

a Dep. of Agronomy, Louisiana State Univ. AgCenter, Baton Rouge, LA 70803
b Dep. of Biological and Agric. Eng., Louisiana State Univ. AgCenter, Baton Rouge, LA 70803
c USDA-ARS National Soil Tilth Lab., Ames, IA 50011

* Corresponding author (lagaston{at}agctr.lsu.edu)

Received for publication April 2, 2002.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
Long-term application of poultry (Gallus gallus domesticus) litter has built high levels of P in certain Coastal Plain soils of north Louisiana. However, soil P/runoff P relationships for soil and environmental conditions of the area have not been examined. This study measured soil P (total, Bray 1, Bray 2, Mehlich 3, resin-exchangeable, and water-extractable) and runoff P (dissolved P, DP; and total P, TP) at four pasture sites previously amended with poultry litter. Sites varied in soil P due to annual litter applications ranging from 1 to more than 20. Three replicated plots at each site were subjected to simulated rainfalls over 2 yr, and concentrations of DP and TP in runoff were measured and related to soil P. This allowed examination of soil P/runoff P relationships and their changes over time. Runoff DP was also related to DP desorbed from surface soil in a miscible displacement experiment. Among measures of soil P, only resin-exchangeable and water-extractable P showed significant decreases over 2 yr. These measures of soil P explained 54 to 64% of the variability in runoff DP data. However, the miscible displacement technique proved the best indicator of runoff DP, explaining 70% of the variability. Runoff varied among sites (decreasing with increasing years of litter application), limiting the predictive capability of the soil extraction methods. Linking runoff characteristics with miscible displacement data may be a useful predictive tool and warrants further examination.

Abbreviations: DP, dissolved phosphorus • ICAP, inductively coupled argon plasma • TP, total phosphorus


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
POULTRY PRODUCTION has been a major industry in the mid-South of the USA for many years. In Louisiana, production is concentrated in the hilly Coastal Plain, where soils are highly weathered and typically infertile. The poultry litter generated is a P-rich combination of rice (Oryza sativa L.) hull bedding and manure. It is generally applied to nearby pastures to supply P, as well as other plant nutrients and organic matter—components that have historically limited crop production in these soils (Kovar et al., 1999). However, recent concern about nonpoint-source (nutrient) pollution in the Lake D'Arbonne watershed of north Louisiana has raised questions about this practice.

Transport of P from agricultural lands to nearby water bodies may induce eutrophication (Parry, 1998). This condition is characterized by increased levels of undesirable algae, aquatic vegetation, and biological oxygen demand that degrade water quality and impair its use for fishing, recreation, and consumption (Sharpley et al., 1994). Although many soils exhibit a high capacity to sorb P, long-term application of poultry litter often results in a build-up of P, particularly near the surface (Kingery et al., 1994; Robinson et al., 1994). When the P sorption capacity of a soil is approached, the potential for P transport in runoff water (Sharpley et al., 1994) and vertical (Heckrath et al., 1995) or lateral migration of dissolved P within the soil (Walthall and Nolfe, 1998) increases. For off-site P loss, greatest concern has been with surface runoff. In general, surface soils enriched in P are a source of dissolved and particulate P (Sharpley et al., 1994).

Several studies have examined the use of routine soil P tests to predict P mobilization in runoff water, but results have been inconsistent (Pote et al., 1996; Sharpley, 1995b, 1997; Sauer et al., 2000). Sauer et al. (2000) obtained good predictions of runoff P based only on Mehlich 3–extractable P (Mehlich et al., 1984), whereas Sharpley (1995b)(1997) found that correlation between runoff P and Mehlich 3 P was soil-specific. A model based on Mehlich 3 P in conjunction with P sorption isotherms, however, was applicable to several soils (Sharpley 1995b, 1997).

Nonroutine soil P extractants or methodologies have also been examined. Magdoff et al. (1999) found that soil P extracted with a modified Morgan's solution (McIntosh, 1969) was the best indicator of both P availability and potential P desorption to runoff water. Pote et al. (1996) found that runoff P was better correlated to soil P extracted with acidified ammonium oxalate, deionized water, or recovered by iron oxide-coated paper (Sharpley, 1993) than Bray 1 (Bray and Kurtz, 1945) or Mehlich 3–extractable P. Delgado and Torrent (2001) have proposed isotopic exchange methods with a narrow (1:1) water/soil ratio (Torrent and Delgado, 2001) to quantify potential P loading in runoff.

Since the amount of P in runoff is affected by transport and source factors other than soil P content alone (Sharpley et al., 1994), measures of soil P have been jointly weighted with other factors in an attempt to better estimate expected P losses (Lemunyon and Gilbert, 1993; Sharpley, 1995a). Furthermore, soil P content has been used in various models to predict P loading into runoff (Sims et al., 2000). Regardless of its use as a direct indicator of P mobilization, as a factor in a risk-assessment index, or as a model parameter, the most convenient measure of soil P would be a routine soil test. However, a nonroutine procedure for easily desorbed P may be necessary for best predictions of P losses in surface runoff.

Heretofore, there had been no research on the reliability of soil test P as an indicator of P mobilization in runoff from Louisiana Coastal Plain soils. Therefore, it was uncertain whether correlations between soil test P and concentrations of P in runoff obtained elsewhere in the region (Pote et al., 1996; Sharpley, 1995b, 1997; Sauer et al., 2000) could be extrapolated to Louisiana. One objective of this study was to determine how well soil P extracted by several routine and nonroutine soil P test methods correlated with P in surface runoff for several Louisiana Coastal Plain soils, each with different histories of poultry litter application and different initial P levels. A second objective was to track changes in surface and subsurface soil P concentrations, and runoff P over 2 yr. Since soils included in this study were no longer being amended with poultry litter, changes in soil P and runoff P might provide insight into the short-term fate of P in soils with histories of poultry litter application.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
Study Sites
Four sites in the Lake D'Arbonne watershed of Union Parish, LA, were included. They had established common bermudagrass (Cynodon dactylon L. Pers.) cover, but varied in history of poultry litter application, ranging from a single application at Site 1 to more than 20 annual applications at Site 4 (Table 1) . Total tonnage of applications was unknown but an annual application of approximately 9 Mg ha-1 is typical. Litter was last applied to all sites in 1996 and none was applied during this study. Vegetative cover at Site 1 was sparse because the area had been cleared of forest less than 5 yr earlier. The soils were Malbis fine sandy loam (fine-loamy, siliceous, thermic Plinthic Paleudults) at Sites 1 and 2, Sacul very fine sandy loam (clayey, mixed, thermic Aquic Hapludults) at Site 3, and Darley gravelly fine sandy loam (clayey, kaolinitic, thermic Typic Hapludults) at Site 4. Slopes varied from 8 to 12% at the four sites.


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  		Table 1. History of poultry litter application and initial soil characterization data (August 1997) for the study sites.

 
Runoff Plots
In August 1997, approximately 25 m2 at each site were fenced to exclude cattle and three plots (2.1 by 2.1 m) established within the enclosures. Plots were bordered on three sides with sheet metal embedded 10 to 15 cm into the soil. Runoff collection troughs (10 cm diam PVC pipe cut in half lengthwise) were installed on the downslope side of each plot. Polyethylene tubing was attached at the lower corner of troughs and run to collection wells at the base of plots. The systems were cleaned with supply rainwater before simulations. The bermudagrass was periodically cut but not removed. Drapcho et al. (2000) provide further detail on plot design and rainfall simulations.

Soil Sampling and Analyses
Twelve random baseline soil samples at 0- to 5-cm and 5- to 15-cm depths were collected with a 2.1-cm diameter push probe within each 25-m2 study site at plot establishment in August 1997. The soil was later sieved (<2 mm) moist and stored sealed at room temperature before analysis for particle size by the hydrometer method (Gee and Bauder, 1986); available P by Bray 2 extraction (Byrnside and Sturgis, 1958); pH in water (McLean, 1982); and organic matter by Walkley-Black oxidation (Nelson and Sommers, 1982).

Eight push probe subsamples (0–5 cm and 5–15 cm) were taken immediately outside the perimeter of each runoff plot before each of three simulated rainfalls in June 1998, June 1999, and December 1999. Subsamples were combined (resulting in 4 sites x 3 replicate plots x 3 times = 36 samples, each depth) and prepared as above. Total P (Walker and Adams, 1958), soil test P (Bray 1, Bray 2, and Mehlich 3), water-extractable P (Self-Davis et al., 2000), and labile inorganic P extracted by a chloride-saturated anion exchange resin method (Amer et al., 1955; as modified by Kovar and Barber, 1988) were measured. Phosphorus in extracts was measured by inductively coupled argon plasma (ICAP) spectroscopy. However, for extracts with P concentrations that approached the lower detection limit (0.1 mg P L-1) of the instrument, P was determined colorimetrically (Murphy and Riley, 1962).

Rainfall Simulations
Rainfall simulations were conducted in June 1998, June 1999, and December 1999. Rainfall was applied to all plots at every site on each date except Site 4 in December 1999, due to equipment problems. Simulated rainwater was prepared by filtering local well water through an ion exchange column to reduce electrical conductivity to 2.0 x 10-4 dS m-1. An oscillating-nozzle rainfall simulator (Meyer and Harmon, 1979) was used to apply rainfall at an average intensity of 74 ± 8 mm h-1 (measured using five gauges; intensity of a 30-min, 1-yr frequency rain) from a height of 3.0 m above the soil surface. The simulator was enclosed in a plastic tarp to minimize drift. Simulations were conducted from early morning to evening. Rain was applied to an area of approximately 2.1 by 1.8 m. A 2.1 by 0.3 m area directly above the collection trough was excluded to avoid direct application of rainfall into the trough. The duration of rainfall was controlled so that at least 3 L but less than 30 L of runoff was collected. Due to variation in infiltration rates among simulation dates and sites, total rainfall applied ranged from about 20 to 170 mm (or 15–140 min). During a rainfall simulation, runoff from each plot was pumped from the collection well into a polyurethane carboy. The entire runoff volume was then mixed using a magnetic stir plate. Three 0.5-L aliquots of the runoff were pumped into polyethylene sample bottles for TP analysis, and 1.5 L of runoff water was pumped into a large whirl-pak bag for subsequent filtration and DP analysis. Rainwater collected in rain gauges was analyzed in the field for pH and conductivity, and a subsample was transferred to a polyethylene bottle for TP and DP analyses. Samples were stored on ice until delivered to the laboratory.

Runoff Analyses
Water samples collected for DP analysis were filtered (0.45 µm pore size) and then frozen. Unfiltered and filtered samples were analyzed for TP and DP, respectively, with a sulfuric–nitric acid digestion procedure (Method 4500-P B; APHA, 1995) for conversion of organic P forms to inorganic orthophosphate, followed by colorimetric determination of the orthophosphate with the stannous chloride method (Method 4500-P D; APHA, 1995). Two laboratory replicates of each sample were analyzed. Total suspended solids, pH, and specific conductivity were also determined for each sample (Drapcho et al., 2000).

Short Column Miscible Displacement for Desorbable Phosphorus
In addition to batch extraction methods, a miscible displacement approach was used to estimate potential P desorption and mobilization in surface runoff. Twenty g (oven-dry equivalent) of each surface (0–5 cm) soil sample for which runoff P was determined (33) were packed into fritted glass funnels (4.0 cm diam. disc) to a depth of 1.0 cm. Deionized water was then pumped through a sprinkler head onto the soil surface at 40 mm h-1 (lower than field application rate to avoid ponding) and effluent collected in fractions. The depth of water applied to each short column varied depending on the depth of rainfall for the corresponding field site, plot, and rainfall simulation. Effluent fractions were filtered (0.45 µm), digested with sulfuric–nitric acid, and analyzed as earlier described for soil extracts.

This approach assumes that time-dependent runoff rate and P concentration in the soil solution–runoff mixing zone can be used to estimate mean P concentration in runoff. In particular, mean P concentration is given by the ratio

[1]
where Q(t) is runoff rate (mm h-1), C(t) is P concentration in runoff (µg cm-3), QTotal is total runoff for an event (mm), t is time (h), and the integration is over the duration of the rainfall event, T (h). For a rainfall of constant intensity, cumulative rainfall depth at any time, i (mm), is directly proportional to time so that

[2]
where I is total rainfall (mm), is equivalent to Eq. [1]. This approach next assumes that P concentration in short column effluent approximates C(I) in the surface soil solution before onset of runoff and in the soil solution–runoff mixing zone during runoff. Thus, if Q(I) was known and C(I) so approximated (over a depth of effluent equal to depth of rainfall), mean runoff P concentration could be predicted.

Statistical Methods
Differences in soil P with time for each site were established using analysis of variance with time (rainfall simulation number) as treatment and plot as replicate. Relationships between runoff and soil P were determined by regression. Analyses were performed using the Statistical Analysis System (SAS Inst., 1996) and all statistical comparisons were reported at {alpha} = 0.05.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
SUMMARY AND CONCLUSIONS
 REFERENCES
 
Initial Soil Properties
Table 1 gives baseline soil characterization data at plot establishment, and shows that organic matter and Bray 2 P in surface soil generally increased from Sites 1 to 4, consistent with increased years of litter application. Movement of P into subsurface soil occurred at Sites 3 and 4, where litter had been applied >10 yr. Walthall and Nolfe (1998) reported similar results.

Temporal Changes in Soil Phosphorus
Total P in surface soil did not change with time except at Site 2 (Table 2) . Lack of statistical differences elsewhere, despite large numerical differences over time, suggests high spatial variability in total P. Changes in Bray 1, Bray 2, and Mehlich 3–extractable P in the surface soils (Table 2) were either inconsistent across sites (Bray 1) or failed to indicate any change over the course of this study (Bray 2 and Mehlich 3). Corresponding measures of soil P in the subsurface (Table 3) also failed to show consistent trends. As with the surface soil, these results were likely due to short-range spatial variability.


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  		Table 2. Temporal changes in total and extractable soil P in the surface 0 to 5 cm at the four study sites.

 

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  		Table 3. Temporal changes in total and extractable soil P in the subsurface 5 to 15 cm at the four study sites.

 
However, resin-exchangeable P in the surface soil at Sites 2, 3, and 4 was higher initially and decreased during the course of the study (Table 2). Resin-exchangeable P in subsurface soil at three of the four sites also decreased (Table 3). These short-term decreases in resin-exchangeable P are consistent with expected behavior of soluble P, which decreases with time in litter-amended soils (Shreve et al., 1996). This suggests that resin-exchangeable P may be a suitable indicator of potential P mobilization in runoff from Louisiana Coastal Plain soils. Similarly, average surface and subsurface water–extractable P numerically decreased at all sites (Tables 2 and 3) over the course of this study. However, variability among plots was high, and decreases in water-extractable P were significant only for the 0- to 5-cm depth at Sites 3 and 4 and the 5- to 15-cm depth at Site 4.

Temporal Changes in Runoff Phosphorus
Rainfall required to generate runoff and runoff produced from individual plots within a site varied (data not shown), but differences were greater among than within sites. Mean rainfall needed to generate runoff increased from Sites 1 to 4 (Table 4) . Increased soil organic matter content (Table 1) and improved vegetative cover due to longer history of litter application likely led to greater infiltration and water retention. Thus, the effect of long-term litter application on increased runoff P may be partially offset by improved infiltration.


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  		Table 4. Average rainfall depth, runoff depth, and volume-averaged concentrations of total P (TP) and dissolved P (DP) in runoff for rainfall simulations.

 
Phosphorus concentrations in runoff generally increased from Sites 1 to 4 (Table 4), corresponding with increased years of litter application. Mean TP concentration in runoff for all rainfall simulations increased from Site 1 to Site 4 (2.73 mg L-1 at Site 1, 3.60 mg L-1 at Site 2, 5.17 mg L-1 at Site 3, and 6.59 mg L-1 at Site 4). Concentrations of TP in runoff decreased with successive rainfall simulations, being significant at Sites 1 and 2 (Table 4). Comparing June 1998 and December 1999 (June 1999, for Site 4), percentage decreases in runoff TP concentrations were greater at lower levels of surface soil P (Table 2). This likely reflected a smaller P reservoir and slower infiltration at Sites 1 and 2 than at Sites 3 and 4. Similar decreases in DP concentrations occurred (Table 4). Decreasing P in runoff with time is consistent with results of Pierson et al. (2001), who found that dissolved reactive P concentrations in runoff slowly decreased in the months following the last application of litter to an Ultisol in Georgia. Clearly, decreases in DP and TP concentrations reflect losses of surface soil P not only due to simulated rainfall but also natural rainfall (approximately 2000 mm between June 1998 and December 1999; LSU AgCenter, Calhoun Res. Stn., Calhoun, LA).

Phosphorus in runoff was predominately DP (avg. 96% of TP), a typical result for P in runoff from pastures (Edwards and Daniel, 1993; Sauer et al., 2000). Moreover, the amount of DP relative to TP in runoff increased from an average of 95% at Sites 1 and 2 to 98 and 99% at Sites 3 and 4. Visual inspection suggested that vegetation density increased with increasing years of litter application, perhaps explaining the slight increase in the fraction of DP (or decrease in particle-bound P).

Relation of Runoff Phosphorus to Extractable Soil Phosphorus
There was little difference among total, Bray 1, Bray 2, or Mehlich 3 P in ability to predict P concentrations in runoff (Fig. 1a–d) , with each method explaining <50% of the variability in DP data. The R2 values ranged from 0.35 for Bray 1 P to 0.46 for Bray 2 P. Pote et al. (1996) and Sauer et al. (2000) obtained R2 values >0.70 for relationships between dissolved reactive P and Bray 1 or Mehlich 3 P. However, only one soil series (Pote et al., 1996) or soils with a narrow range in soil P (Sauer et al., 2000) were used. Regressions of runoff TP with soil P parameters showed lower R2 values than DP, ranging from 0.33 for Bray 1 P to 0.44 for Bray 2 P (data not shown). Resin-exchangeable (Fig. 2a) and water-extractable soil P (Fig. 2b) showed better relationships with DP in runoff, and explained 54 and 64% of the variation in runoff DP concentrations. Unlike the acidity of Bray 1, Bray 2 and Mehlich 3 P extractants, the Cl-–saturated exchange resin does not appreciably alter pH during extraction. Thus, P displaced by Cl- from the exchange resin may more closely approximate P that is desorbable by rainfall and runoff than P displaced by more severe extractants. Extraction with water perhaps best simulates the chemical milieu of rainfall and runoff.



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  		Fig. 1. Relationships of volume-averaged concentrations of dissolved P (DP) in plot runoff from the four study sites with (a) total, (b) Bray 1, (c) Bray 2, and (d) Mehlich 3 P in surface 0 to 5 cm soil (n = 11).

 


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  		Fig. 2. Relationships of volume-averaged concentrations of dissolved P (DP) in plot runoff from the four study sites with (a) resin-exchangeable and (b) water-extractable P in surface 0 to 5 cm soil (n = 11).

 
High concentrations of DP in runoff from Sites 1 and 2 during the first (June 1998) rainfall simulation (Table 4), despite low concentrations of soil P (Table 2), account for some of the scatter in runoff DP vs. soil P data (Fig. 1 and 2). These high concentrations in runoff DP may be explained in part by the relatively low infiltration at these sites. At Site 1, an average of 19 mm of rainfall was applied to each plot and an average of 6 mm of runoff was collected (Table 2). Thus, average concentration of DP in runoff from Site 1 reflected DP concentration with infiltration of only 13 mm of rainfall. In contrast, an average of 98 mm of water was applied to Site 3 and only 4 mm of runoff collected. Average concentration of DP in runoff from Site 3, therefore, reflected P concentration after infiltration of >90 mm of rainfall. Assuming that DP concentration in the surface soil decreased as infiltration increased, the much smaller depth of water that infiltrated Site 1 plots before initiation of runoff helps explain the surprisingly high concentration of DP in runoff. Furthermore, if P desorption kinetics were slow (Lookman et al., 1995), concentrations of DP would be expected to decrease sharply with increasing cumulative infiltration. Thus, the high concentration of DP in runoff from Site 1 plots was likely due to lower infiltration rate. Similar calculations for infiltration at Site 2 during June 1998 account for an average DP concentration in runoff that was nearly equal to that from Site 4 and higher than that from Site 3 (Table 4). Thus, besides reducing litter rates to soil with high P (e.g., Sites 3 and 4), results for Sites 1 and 2 emphasize the need to reduce rates where infiltration is slow. Results for Sites 1 and 2 also demonstrate an inherent limitation to batch methods for predicting DP in runoff as soil extraction methods cannot account for the effect of infiltration.

Relation of Runoff Phosphorus to Miscible Displacement Phosphorus
An example short column DP elution curve from Site 3 is shown in Fig. 3 . Decreasing DP concentration with increasing depth of effluent was characteristic of all soils. Assuming that effluent concentrations approximate DP concentrations in the runoff–surface soil mixing zone, such data are a measure of DP loading into runoff as a function of cumulative rainfall depth (or cumulative infiltration + runoff depths). Thus, average DP concentration in runoff could be estimated as in Eq. [2] but using short column effluent DP concentration (at a depth of effluent to depth of rainfall) instead of C(I).



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  		Fig. 3. Dissolved P (DP) concentrations (filled circles) in short column effluent for soil collected at Plot 1, Site 3 before the December 1999 rainfall simulation. Average DP concentration was approximated as area under the curve divided by effluent depth equal to depth of simulated rainfall.

 
Since there typically is a lag between onset of steady rainfall and runoff, followed by increasing runoff as infiltration decreases (Sauer et al., 2000), detailed runoff data are needed to fully evaluate the predictive capability of this approach. In this study, however, only total runoff was measured, and tentative assessment of the miscible displacement approach was based on assumed constant runoff throughout rainfall events. Average DP concentration in runoff was estimated as the average DP concentration in short column effluent up to a depth of effluent equal to the depth of rainfall (e.g., 49 mm of rainfall, Fig. 3). Average DP concentrations from replicate short columns were then volume-averaged to give the data in Table 5 .


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  		Table 5. Short column miscible displacement results given as volume-averaged DP concentrations.

 
When field runoff DP concentrations (Table 4) were regressed on the data in Table 5, DP from the short soil columns gave better predictions of runoff DP concentrations (Fig. 4) than any of the soil batch extraction methods, explaining 70% of the variation in the data. This result was not surprising since the displacement method for predicting DP mobilization in runoff more closely simulated field conditions.



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  		Fig. 4. Relationship of volume-averaged concentrations of dissolved P (DP) in plot runoff from the four study sites with volume-averaged concentrations of DP in effluent from short columns of 0 to 5 cm soil (n = 11). A 1:1 line (dashed) is also shown.

 
Although the assumption of constant runoff for the duration of a rain event gave excessive weight to earlier, higher concentrations of DP in short column effluent, runoff DP was not overestimated. Instead, runoff DP was generally greater than estimated by the miscible displacement approach (Fig. 4). Lower P in bulk samples of 0 to 5 cm soil than in the runoff–soil solution mixing zone in the field may have led to underestimation of runoff DP.


   SUMMARY AND CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
REFERENCES
 
This 2-yr study measured soil P (total, Bray 1, Bray 2, Mehlich 3, resin-exchangeable, and water-extractable) and runoff P during rainfall simulations at four pasture sites with previous applications of litter. Concentrations of P in runoff generally increased with increasing levels of extractable soil P, except for high concentrations of P in runoff during the first rainfall simulation at the two field sites with the lowest soil P. Infiltration was lower and runoff higher at these sites compared with sites with longer histories of litter application and denser vegetative cover (emphasizing the importance of factors besides soil P in controlling P runoff). Average P concentrations in runoff decreased during the course of this study. Among measures of soil P examined, only resin-exchangeable and water-extractable P generally decreased with time. These were also the best predictors of DP losses in runoff, explaining 54 and 64% of the variability in runoff DP. A short column miscible displacement technique for approximating P desorption into runoff explained even more (70%) of the variability. Coupling the latter approach with models for infiltration and overland flow, together with a better understanding of the spatial variability of soil P (Sauer and Meek, 2000), might broaden its applicability. The resin-exchange and water-extraction methods are straightforward and could be used as routine soil tests for potential P runoff losses. However, these batch extraction methods cannot account for the effect of infiltration on P loss in runoff. Similar routine use of the miscible displacement method would require refinement and simplification.


   ACKNOWLEDGMENTS
 
This work was conducted as part of the project, Methodology to Quantify Impact of Poultry Litter Application on Surface Water Quality, funded by the Louisiana Department of Environmental Quality, contract no. 513920.


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


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