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Published online 2 December 2005
Published in Soil Sci Soc Am J 70:163-171 (2006)
DOI: 10.2136/sssaj2005.0102
© 2005 Soil Science Society of America
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Nutrient Management & Soil & Plant Analysis

Phosphorus Loss in Runoff from Long-term Continuous Wheat Fertility Trials

H. Zhanga, J. L. Schrodera,*, R. L. Davisc, J. J. Wangd, M. E. Paytonb, W. E. Thomasone, Y. Tangf and W. R. Rauna

a Dep. of Plant and Soil Sciences
b Dep. of Statistics, Oklahoma State Univ., Stillwater, OK 74078
c Apex Environmental Inc., Lenexa, KS 66215
d Dep. of Agronomy and Environmental Management, Louisiana State Univ., Baton Rouge, LA 70803
e Dep. of Crop and Soil Environmental Sciences, Virginia Polytechnic Institute and State Univ., Blacksburg, VA 24061
f Yangzhou Univ., Jiangsu, China

* Corresponding author (jschrod{at}okstate.edu)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Some wheat (Triticum aestivum L.) farmers in the southern Great Plains routinely apply P fertilizer without soil testing. This practice may lead to P buildup in soils, hence, increased P runoff potential, making soil P management of concern in continuous wheat production. At present, there is also debate over the nature of P loss trends, particularly whether soil P release to runoff can be described generally, across a broad range of soils, or is soil-specific. Paired 1 m by 2 m runoff plots were established on three existing long-term continuous winter wheat fertility experiments. Two experiments have received annual fertilizer P application at different rates since 1970 (0–44 kg P ha–1), and the third received a one-time P application at much higher rates in 1977 (0–587 kg P ha–1). Rainfall-runoff experiments were conducted following the National P Research Project protocol. Simulated rain (75 mm h–1) produced 30 min of runoff from plots with different soil test P levels. Soil Mehlich-3 P (M3P) ranged from 11.5 to 130 mg kg–1 and water-soluble P (WSP) ranged from 0.70 to 15.7 mg kg–1. Runoff total P and dissolved reactive P (DRP) concentrations ranged from 0.47 to 1.5 and 0.03 to 0.70 mg L–1, respectively. Dissolved reactive P in runoff was significantly related to M3P, WSP, and ammonium oxalate P saturation index (PSIox) for combined soils as well as for individual soil series. Significant differences (p < 0.05) among the slopes of the regressions for the DRP-M3P, DRP-WSP, and DRP-PSIox relationships indicate that the relationships are soil specific. This study highlights the need for soil specific management to protect water quality.

Abbreviations: Alox, ammonium oxalate extractable Al • DRP, dissolved reactive P • Feox, ammonium oxalate extractable Fe • ICP–AES, inductively coupled plasma–atomic emission spectroscopy • M3P, Mehlich-3 P • NPRP, National Phosphorus Research Project • Pox, ammonium oxalate extractable P • PP, particulate P • PSIox, ammonium oxalate P saturation index • Smax, phosphorus adsorption maximum • STP, soil test P • TSS, total suspended solids • TP, total P • USEPA, United State Environmental Protection Agency • WSP, water soluble P


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
PHOSPHORUS LOSS from intensely managed agro-ecosystems is often associated with accelerated eutrophication in lakes and other surface water bodies (Carpenter et al., 1998). Over the past 25 yr, the amount of plant-available P in some soils has increased substantially from excessive P fertilizer and manure application (Bundy et al., 2001). Elevated soil P levels can account for most of the total annual load of P in surface water (Edwards and Daniel, 1993). Consequently, controlling agricultural nonpoint source (NPS) P loss is of great concern in minimizing surface water degradation (USEPA, 1998).

While most research has focused on livestock manure management (e.g., Mueller et al., 1984; Cox and Hendricks, 2000; Sauer et al., 2000), some information also exists on tillage and cropping systems receiving commercial P fertilizers. Romkens et al. (1973) found that reduced tillage systems in corn decreased sediment nutrient losses but increased runoff dissolved P concentrations when compared with conventional tillage. Gascho et al. (1998) also worked with corn and found that runoff P losses were greatest 1 d after commercial P fertilizer application but dropped sharply (i.e., from >5 to <1 mg L–1) over 1 mo after commercial P fertilization. Sharpley et al. (1991) stated that conservation tillage reduced sediment and P transport in runoff relative to conventional tillage while working with sorghum in the Southern Plains of the USA. Douglas et al. (1998) found that runoff P was higher for continuous fallow than for both winter wheat and spring pea cropping systems.

Additionally, earlier runoff P- soil P experiments were conducted on soils amended with P fertilizer or manure to establish a wide range of soil P levels. Often times, these fertilizer or manure amendments were applied shortly before the data were collected resulting in a relatively short time period for the amendment to react with the soil. Reddy et al. (1978) studied soil microplots amended with manure and commercial fertilizers incubated for 23 d and reported that runoff P loss increased with increased chemical P and manure applications. Edwards and Daniel (1993) applied simulated rainfall to soil plots 24 h after manure application and found that runoff P increased with increased manure application. Bundy et al. (2001) found that P in runoff collected from no-till corn production plots which received manure and biosolids for 5 yr. intensified as Bray 1 P increased. Cox and Hendricks (2000) found that runoff DRP losses increased with recent fertilizer P application in wheat and barley (Hordeum vulgare L.) production.

Numerous researchers have shown positive significant relationships between DRP and STP for soils amended with manures or a combination of manures and mineral fertilizers (Sharpley, 1995; Pote et al., 1996; Pote et al., 1999; Cox and Hendricks, 2000; Torbert et al., 2002; Daverede et al., 2003; DeLaune et al., 2004; Kleinman et al., 2004). However, conflicting information exists as to whether one regression equation or multiple regression equations best describe the DRP-STP relationship. A recent study by Vadas et al. (2005) indicated slopes for the DRP-STP relationship were not statistically different for several different soils but earlier studies have reported statistical differences among slopes (Sharpley, 1995; Pote et al., 1999, Cox and Hendricks, 2000; Andraski and Bundy, 2003; Turner et al., 2004). As a result, the question remains as to whether soil P management for water quality protection should be soil-specific or whether general management recommendations are sufficient.

Soil P saturation is a practicable tool used to evaluate the environmental fate of P and is defined as the amount of P sorbed divided by the P sorption capacity of the soil. The concept of P saturation is important as it estimates the degree to which P sorption sites have been filled and indicates the potential desorbability of soil P (Beauchemin and Simard, 1999). Phosphorus saturation has been correlated with P desorption such that P desorption increases with higher degrees of P saturation (Sibbesen and Sharpley, 1997). Researchers reported that DRP is related to soil P saturation (Sharpley, 1995; Pote et al., 1996, 1999; Davis et al., 2005). The majority of these studies have been indoor simulated rainfall studies using packed boxes or conducted on grass pastures amended with manures or a combination of manures and mineral P fertilizer. However, there are few studies that involve experimental soils or sites that have been under long-term fertilizer P application and long-term continuous winter wheat cultivation. Tarkalson and Mikkelsen (2004) investigated a single Hiwasee soil under conventional tillage fertilized approximately 9 yr earlier with a combination of broiler litter and inorganic P fertilizer. In their study, they found that runoff DRP was significantly related to M3P and to soil P saturation.

Wheat is an important crop grown in Oklahoma with over 2.5 million ha being planted in 2004 (USDA-National Agricultural Statistics Service, 2005). A free wheat soil testing and education program conducted in 1996 indicated that as few as 3.5% of fields planted in wheat in Oklahoma were soil tested (Zhang et al., 1998). Many wheat farmers in the southern Great Plains routinely apply P fertilizer without soil testing. This practice could lead to P buildup in soil and increased runoff P that could result in undesirable environmental consequences such as eutrophication. However, the relationship between soil P and runoff P in soils under long-term continuous winter wheat cultivation receiving chemical fertilizer has not been thoroughly researched.

The objectives of this study were to (1) evaluate the relationship between runoff DRP and soil P on three Oklahoma soils under continuous wheat production that have received inorganic fertilizer P annually for over 30 yr; (2) to determine the relationships between runoff DRP and soil P saturation for the same soils; and (3) to determine whether one regression equation or multiple regression equations best describe the DRP-STP relationship. This study will also provide useful information for site-specific nutrient management using P risk indices such as those described by Lemunyon and Gilbert (1993).


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Rainfall Simulation Sites
Three sites (Lahoma, Stillwater, and Haskell Agricultural Research Stations) were chosen from existing long-term fertility research plots located across Oklahoma to evaluate the relationship between runoff P and soil P (Table 1). The Lahoma site was established on a Grant silt loam (fine-silty, mixed, superactive, thermic Udic Argiustoll) in 1970 (Raun et al., 2000) with five fertilizer P application rates (0–39 kg P ha–1) on an annual basis. The Stillwater site was established on a Kirkland loam (fine, mixed, superactive, thermic Udertic Paleustoll) in 1969 (Raun et al., 2000) with four fertilizer P application rates (0–44 kg P ha–1) on an annual basis. Both sites have been under continuous winter wheat cultivation since their establishment. The P fertilization for both sites was triple super phosphate (TSP, 0–46–0) and the experimental design for the sites was a randomized complete block with four replications. The last P fertilization was about 1 yr before rainfall simulation took place on both sites.


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  		Table 1. Soil classification and chemical and physical characteristics (top 15 cm) of three soil series evaluated.

 
The Haskell site was established in the fall of 1977 on a Taloka silt loam (fine, mixed, active, thermic Mollic Albaqualf) (Raun et al., 2000) with four, one-time fertilizer P application rates (0–587 kg P ha–1). Winter wheat has been planted every year at the site since its establishment. The P fertilization for the site was TSP and the experimental design for the site was a randomized complete block with four replications. To alleviate soil acidity problems, lime was applied for two consecutive years (July 1998 and July 1999) at a rate of 1600 kg ECCE ha–1 (Raun et al., 2000).

Runoff Plots
Rainfall simulation plots were established according to the protocol recommended by the National Phosphorus Research Project (NPRP, 2002) at each of the three sites. Two sets of paired runoff plots were established on each P rate treatment to give four replications per treatment. Each site was disked in late July in accordance with normal wheat management, approximately 14 d before rainfall simulation. Following disking, 1 m by 2 m runoff plots were established using 0.2 cm by 15 cm metal strips installed to a depth of 10 cm on approximately 5% sloped fertility plots. The sites represent the worse case scenario when runoff P potential should be the greatest (i.e., an intense rainfall event in the summer with no crop cover). Virtually no surface cover was present on the sites except for small amounts of wheat stubble (<5%). Gutters were installed on the down-slope end of the plot to collect runoff. In addition, paraffin wax was used to seal around the gutters to prevent runoff loss along the edges. Plexi-glass strips were placed over guttering to prevent rainfall from falling directly into runoff gutter while still allowing runoff collection. The plots were saturated using the rainfall simulator (75 mm h–1 until ponding was observed, approximately 10 min) and excess water drained naturally 24 h before rainfall simulation.

Rainfall Simulation
Rainfall simulations were conducted at the Stillwater and Lahoma sites during August 2000 and the Haskell rainfall simulation was conducted during August 2001. Simulated rainfall was applied using a portable, solenoid-operated, variable intensity rainfall simulator based on the design of Miller (1987) with one TeeJet 1/2 HH-SS50WSQ nozzle (Spraying Systems Co, Wheaton, IL) placed in the center of the 3 m height by 2.8 m length by 2.3 m width aluminum frame. The rainfall simulator was calibrated to deliver 75 mm h–1 (the National Phosphorus Research Project) and equivalent to a 10-yr storm event in north-central Oklahoma (U.S. Department of Commerce, 1961). The intensity of the rainfall was controlled by the on-off (1.3 s on; 0.4 s off) spraying times of the nozzle. Uniformity was measured and the resulting Christiansen's uniformity coefficient was >0.85% (Cuneca, 1989). Water was supplied to the simulator from a 1893-L water tank filled from the respective research station's potable water source. The source waters for the rainfall simulations contained an average of 0.07 mg L–1 DRP.

Simulated rainfall was applied to runoff plots at each experimental site until 30 min of measurable runoff was collected. Peristaltic pumps were used to transfer runoff collected in each gutter to 35-L collection containers. Runoff volume was recorded every 5 min for 30 min. At the end of the event, collected runoff was manually agitated to homogenously resuspend sediment before one representative runoff sample (500 mL) was obtained from each runoff plot and stored at 4°C until lab analyses were performed.

Soil Analyses
Soil samples (ten random cores per plot at a depth of 0–15 cm) were collected approximately 24 h after rainfall simulations for characterization and correlation with runoff P. Collected soil samples were air-dried, sieved to pass a 2-mm sieve and analyzed for M3 P (Mehlich, 1984), water soluble P (WSP, Self-Davis et al., 2000), P sorption maxima (Smax), ammonium oxalate extractable P (Pox), Al (Alox), and Fe (Feox), texture by the hydrometer method (Gee and Bauder, 1986), soil organic matter by loss on ignition (SOM, Ben-Dor and Banin, 1989), and pH in a 1:1 soil/deionized water suspension (Thomas, 1996). Duplicate analyses were conducted on each sample.

Phosphorus adsorption isotherms were determined according to the method of Graetz and Nair (2000). Phosphorus adsorption maximum (Smax) was determined with the linearized form of the Langmuir equation.

Acidified Pox, Alox, Feox were determined by shaking 1.5-g samples of soil with 30 mL of 0.5 M (COONH4)2 · H2O at pH 3.0 in 50-mL centrifuge tubes (Schoumans, 2000) for 2 h, in the dark, on an end-to-end shaker at 150 opm and centrifuged for 10 min at 5211 x g. The supernatant was decanted, filtered (Fisherbrand P4 filter paper), and analyzed for P, Al, and Fe using inductively coupled plasma–atomic emission spectroscopy (ICP–AES). PSIOX was computed using the P, Al, and Fe contents (mmol kg–1) according to Eq. [1] (Schoumans, 2000):

[1]

Runoff Water Analysis
Immediately after rainfall simulation, an aliquot of runoff water sample was filtered (0.45 µm) and analyzed colorimetrically (Murphy and Riley, 1962) to determine DRP (Pote and Daniel, 2000). Total P (TP) from each runoff sample was determined by digesting 25 mL of runoff at 175°C with 1 mL of concentrated H2SO4 and 5 mL of concentrated HNO3 until a total volume of 1 mL remained (Pote and Daniel, 2000). All digested samples were neutralized and analyzed for P colorimetrically. Particulate P (PP) was calculated by subtracting DRP from TP. Because DRP and dissolved P determined by ICP–AES on a subset of the data were nearly the same, we considered DRP to be nearly equivalent to total dissolved P in runoff, justifying the inference that TP – DRP = PP. Total suspended solids (TSS) were determined for all runoff water samples by vacuum filtering (0.45 µm) 50 mL of well-mixed runoff water sample and drying the vacuum filter cup and filter paper at 95°C.

Statistical Analysis
All statistical analyses were performed with the use of PC SAS Version 8.2 (SAS Institute, 2001). Simple linear correlation coefficients between all possible combinations of soil extractions and saturation indexes were calculated with PROC CORR for the soils separately and combined. Simple linear regressions were performed for M3P and WSP against P added, and for DRP against each of M3P, WSP, and Psat. These regressions were performed for each soil separately, and the slopes for a particular model for the soils compared with an analysis of covariance model in PROC GLM. The independent variable in question was used as a covariate in the GLM model, with SOIL as a classification variable. The slopes were compared globally using an interaction term for the covariate and SOIL in the model statement. If significance at a 0.05 level was attained with this test, then slopes were pair-wise compared using a dummy variable model in PROC REG. Means for M3P, WSP, PSIox, DRP, TP, TP Load, PP, TSS, and Volume were compared among treatments and within the individual soils using analysis of variance techniques in PROC GLM. If treatment differences were found in the analysis of variance, pairwise t tests (Fisher's Protected LSD) were performed. Due to the varying levels of P for the treatments nested within soil, control plots for the soils were compared for all the aforementioned response variables.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Soil Characteristics
Soil pH of the study soils ranged from 5.1 to 7.0, clay content from 190 to 270 g kg–1, and organic C from 16.0 to 23.0 g kg–1 (Table 1). The P sorption data were described by the linearized Langmuir equation with correlation coefficients ranging from 0.96 to 0.98. Phosphorus sorption maximum (Smax) determined by Langmuir adsorption isotherms ranged from 175 to 263 mg P kg–1 soil (Table 1). In addition, the annual application of commercial P fertilizers on the Kirkland and Grant soils and the large one-time application of P fertilizer on the Taloka soil resulted in a wide range (11.5–130 mg kg–1) of soil M3P and WSP (0.70–15.70 mg kg–1) (Table 2). As soil WSP generally constituted a small portion of soil M3P (<12%), a minimal amount of free P existed in soil. Currently, Oklahoma considers a M3P concentration of 32.5 mg kg–1 as 100% sufficient for wheat production (Johnson et al., 2000). All long-term P application rates resulted in M3P concentrations greater than that required for wheat production for the Grant and Kirkland soils. However, only the greatest single application rate of P (i.e., 587 kg ha–1) on the Taloka soil produced M3P concentrations greater than that required for wheat production. Overall long-term P application increased WSP and PSIox with PSIox ranging from 5.03 to 15.9% (Table 2). Our results are consistent with other researches that have shown long-term P additions to soils increased soil P saturation (Sharpley, 1996; Börling et al., 2004; Allen, 2004).


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  		Table 2. Total amount of fertilizer P applied, mean extractable soil P and mean ammonium oxalate P saturation index for each treatment (n = 4) of long-term fertility plots used for rainfall simulation.

 
Relationship Between Long-Term Fertilizer P Addition and Extractable Soil P
Significant relationships (r2 > 0.90, p < 0.001) existed between M3P and fertilizer P added for all three soils with slopes for the relationships ranging from 0.05 to 0.09 (Fig. 1 ). A significant relationship (r2 = 0.80, p < 0.001) existed between M3P and fertilizer P added for the soils combined (Fig. 1). The slope for the soils combined of 0.07 indicates that approximately 14 kg ha–1 of fertilizer P would be required to increase M3P by 1.0 mg kg–1 under normal wheat production practices. Our results are similar to those reported by other researchers. Allen (2004) examined Iowa soils that had received annual P rate treatments for greater than 20 yr as TSP and found significant relationships (r2 > 0.93) between M3P and cumulative P addition with slopes ranging from 0.06 to 0.11. Significant relationships (p < 0.001) also existed between WSP and P added for each soil with coefficients of determination ranging from 0.83 to 0.88 and slopes ranging from 0.004 to 0.02 (data not shown). Combination of data from all three soils resulted in a weaker but still significant relationship between WSP and P added (r2 = 0.52, p < 0.001) (data not shown).



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  		Fig. 1. The relationships between Mehlich-3 P and P added for the three individual study soils and all soils combined. ***p < 0.001.

 
Relationships Between Extractable Soil P and Saturation Index
Extractable P values (M3P or WSP) and PSIox were correlated among themselves (r2 > 0.75, p < 0.001) for individual soils (data not shown). For all soils combined, M3P was related (r2 = 0.94, p < 0.001) to WSP. Our results concur with those of several other researchers who reported significant relationships between WSP and M3P (McDowell and Sharpley, 2001; Burt et al., 2002; Sims et al., 2002; Fuhrman et al., 2005). Additionally, M3P was correlated with PSIox (r2 = 0.90, p < 0.001) for all soils combined (data not shown). Similarly, WSP was related to PSIox (r2 = 0.85, p < 0.001) for the combined soils (data not shown). Our results for PSIox are similar to those found by several other researchers. Tarkalson and Mikkelsen (2004) reported a significant relationship between PSIox and M3P while Sims et al. (2002) found that PSIox was correlated with both WSP and M3P. Similarly, in an indoor rainfall simulation study using box plots and soils spiked with different levels of inorganic P and allowed to age for approximately 210 d, Davis et al. (2005) reported PSIox was correlated with both M3P and WSP for three individual soils and for the soils combined.

Runoff Sample Characteristics
Runoff volumes did not vary (p > 0.05) within treatments for each soil series (Table 3). Total suspended solids were similar (p > 0.05) among treatments for the Grant soil, while only slight significant differences (p < 0.05) were observed among treatments for the Kirkland and Taloka soils (Table 3). Dissolved reactive P varied within treatments (p < 0.05) for each individual soil series and increased with increasing P addition (Table 3). Runoff DRP ranged from 0.03 to 0.70 mg L–1 for all treatments (Table 3) which is similar to levels reported by other researchers for simulated rainfall studies on conventionally tilled fields (Cox and Hendricks, 2000; Sharpley and Kleinman, 2003; Tarkalson and Mikkelsen, 2004). Phosphorus concentrations as low as 0.020 to 0.023 mg L–1 may cause eutrophication in lakes and rivers (USEPA, 2001a, 2001b). In our study, the lowest level of treatment for the Grant and Kirkland soils increased M3P to the 100% sufficient level for wheat production. However, these same treatments resulted in DRP concentrations considerably greater than those that may cause eutrophication in surface water bodies. No treatment effects within soils were observed for PP (p < 0.05) with the Grant and Taloka soils but significant differences existed (p < 0.05) for the Kirkland soil. Similar to PP, no treatment effects within soils were observed for TP (p < 0.05) with the Grant and Taloka soils but significant differences existed between treatments (p < 0.05) for the Kirkland soil (Table 3).


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  		Table 3. Comparison of mean runoff water sample characteristics (n = 4) within the study soils.

 
Comparison of control soils (i.e., plots that did not receive P fertilization) showed that the Grant and Taloka soils exhibited similar infiltration rates and runoff volumes (p > 0.05) while the Kirkland had the least average infiltration rates (data not shown) and greatest average runoff volume (p < 0.05) of the soils (Table 3). The differences in runoff volumes were not fully explained by soil physicochemical properties. The higher runoff volume of the Kirkland soil was probably due to its having a greater clay content and a smaller sand content as compared with the other soils. However, the similar runoff volumes between the Grant and Taloka soils were not adequately explained by soil particle-size distribution possibly due to high variability in runoff among replicates. Similarly, the differences in runoff volumes are not fully explained by soil hydrologic group. Both the Kirkland and Taloka soils are classified in the soil hydrologic group D but the Kirkland soil had higher runoff volumes than the Taloka soil. Statistical analysis indicated only small differences in M3P (p < 0.05) among the control soils. Means for M3P in the control soils (in parenthesis) were Grant (19.1 mg kg–1) > Kirkland (15.3 mg kg–1) > Taloka (11.5 mg kg–1) (Table 2). Differences were also observed for WSP in control soils. The Grant and Kirkland control soils had similar levels of WSP while the Taloka control soil contained less WSP as compared with the other two soils. As might be expected because of different levels of STP in the control soils, DRP was statistically different (p < 0.05) among the control soils. Means for DRP in the control soils (in parenthesis) were Grant (0.14 mg kg–1) > Taloka (0.07 mg kg–1) > Kirkland (0.03 mg kg–1) (Table 3). Phosphorus sorption maximum (Smax) is the maximum amount of P that a particular soil may hold. It appears that Smax may explain the differences in DRP for the control plots. For example, the Grant soil had a smallest Smax (175 mg ha–1) and produced the greatest DRP while the Kirkland soil had the largest Smax (263 mg ha–1) and produced the smallest DRP. Differences were not found (p > 0.05) between total P load in control soils but different levels of PP, TP, and TSS were found among the control soils with the Kirkland soil having smallest concentration of these parameters (Table 3).

Relationships between Runoff P and Extractable Soil P
Total P in the runoff ranged from 0.56 to 1.52 mg L–1 for all treatments (Table 3). Total runoff P was not significantly correlated (p > 0.05) with WSP, M3P, and PSIox for the Grant and Taloka soil series but was significantly correlated (p < 0.001) with soil M3P (r2 = 0.66), WSP (r2 = 0.61), and PSIox (r2 = 0.72) for the Kirkland soil series (Table 4). Total P in runoff was well correlated (r2 = 0.88; p < 0.001) to runoff TSS for the Kirkland soil, but not the Taloka (r2 = 0.21, p > 0.05) and Grant soils (r2 = 0.18. p > 0.05) (data not shown). Similar to runoff TP, PP was not significantly correlated (p > 0.05) with soil WSP, M3P, and PSIox for the Grant and Taloka soil series but was significantly correlated (p < 0.001) with soil M3P (r2 = 0.61), WSP (r2 = 0.57), and PSIox (r2 = 0.67) for the Kirkland soil series (Table 4). This suggests the TP-WSP, TP-M3P, TP-PSIox, PP-WSP, PP-M3P, and PP-PSIox relationships are soil specific. Particulate P constituted most of TP (47 to 95%) for all treatments although the percentage of PP decreased with an increase in soil P (Table 3). Sharpley and Kleinman (2003) investigated a Berks conventionally tilled soil that contained 320 mg kg–1 M3P found that PP averaged approximately 90% of the TP. Tarkalson and Mikkelsen (2004) found that PP composed 94% of the TP for a Hiwasee soil (M3P range of 1.0 to 325 mg kg–1) that was under conventional tillage. Similarly in our study, PP was 82% of TP when averaged over all treatments. Particulate P may serve as a P source in streams and reservoirs and become available to aquatic organisms over time, thus increasing eutrophication (Wendt and Corey, 1980; Tarkalson and Mikkelsen (2004). Our study clearly demonstrates that PP constitutes the majority of PP from these tilled wheat soils, thus management practices should consider reducing both DRP and PP for the protection of water quality.


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  		Table 4. Runoff total phosphorus (TP) or particulate P (PP) correlated with five types of soil phosphorus values in long-term fertility plots.

 
A significant relationship (r2 = 0.54), p < 0.001) existed between runoff DRP and M3P for the soils combined (Fig. 2A ). A more thorough examination revealed that DRP was also related with M3P (p < 0.001) for the Grant (r2 = 0.69), Kirkland (r2 = 0.57), and Taloka (r2 = 0.59) soil series (Fig. 2A). Our results agree well with those of Tarkalson and Mikkelsen (2004) who reported a significant relationship (r2 = 0.70) between DRP and M3P for a conventionally tilled Hiwasee soil. Our results differ slightly from those of Cox and Hendricks (2000) who reported coefficients of determination were greater than 0.90 for a Coastal Plain and a Piedmont soil under conventional tillage. Perhaps the difference in our study and the one by Cox and Hendricks (2002) was that they used 3 yr averages for DRP and M3P in their regression analysis while our regression analysis used individual replicates. Overall, this would have the effect of minimizing the variability of the regression and increasing the coefficient of determination.



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  		Fig. 2 The relationships between runoff dissolved reactive P and (A) Mehlich3 P for the three individual study soils and all soils combined or (B) water soluble P for the three individual study soils and all soils combined. ***p < 0.001.

 
Slopes for the regressions between runoff DRP and soil test P concentrations are referred to as extraction coefficients and are typically used as model inputs for P transport models and P site assessment indices (Sharpley et al., 2002; Kleinman et al., 2004). Extraction coefficients for our study ranged from 0.002 to 0.005 with an extraction coefficient for the combined soils of 0.005. The results of our study agree with those of Cox and Hendricks (2000) who reported extraction coefficients of 0.001 and 0.004 for conventionally tilled Piedmont and Coastal Plain soils, respectively. Similarly, Tarkalson and Mikkelsen (2004) reported an extraction coefficient of 0.005 for a conventionally tilled Hiwasee soil.

Conflicting information exists as to whether one regression equation or multiple regression equations best describe DRP-STP relationships. A recent study by Vadas et al. (2005) indicated slopes for the DRP-M3P relationship were not statistically different for several different soils with widely varying physicochemical properties and management conditions including soil packed boxes and tilled, no-tilled, and grass field plots. For our study statistical analysis showed differences in the slopes of regressions (p < 0.05) for the DRP-M3P relationship for the three soils. Perhaps the difference in our study and the one by Vadas et al. (2005) is that the large variation in their data set masked any statistical differences that were present. Other things that may be responsible for the difference in our study and one by Vadas et al. (2005) include soil P, soil type, soil particle size, organic matter content, land and crop management, fertilizer or waste characteristics and application techniques, degree of interaction between soil and water, and P sorption capacities. Our results are consistent with those of other researchers who found similar differences existed between different soils (Cox and Hendricks, 2000; Andraski and Bundy, 2003; Turner et al., 2004).

Combination of the soils resulted in a significant relationship (r2 = 0.69, p < 0.001) between DRP and WSP (Fig. 2B). Dissolved reactive P was related to WSP (p < 0.001) for the individual soil series (Fig. 2B). Coefficients of determination for the individual soil series were: Grant (r2 = 0.69), Kirkland (r2 = 0.48), and Taloka (r2 = 0.59). The slope for the DRP-WSP relationship combined soils was 0.04 (Fig. 2B) while slopes for the individual soil series ranged from 0.02 to 0.04 (Fig. 2B). Statistical analysis showed differences in the slopes of regressions (p < 0.05) for the DRP-WSP relationship for the three soils. Again, our results conflict with those of Vadas et al. (2005) but agree well with those of other researchers who have reported significantly different slopes among soils for the DRP-WSP relationship (Cox and Hendricks, 2000; Andraski and Bundy, 2003; Turner et al., 2004).

Relationships between Runoff P and P Saturation Indexes
A significant relationship (r2 = 0.58, p < 0.001) was found between DRP and PSIox when the soils were combined (Fig. 3 ). Significant relationships were observed between DRP and PSIox for the individual soil series (r2 > 0.50, p < 0.001) (Fig. 3). The results of our study are similar to those of Tarkalson and Mikkelsen (2004) who reported a significant relationship (r2 = 0.62) between DRP and PSIox for a conventionally tilled Hiwasee soil.



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  		Fig. 3 The relationships between runoff dissolved reactive P and phosphorus saturation index calculated with acid ammonium oxalate extractable data (PSIox) for the three individual study soils and all soils combined. ***p < 0.001.

 
Statistical analysis revealed that the slopes of the regressions between DRP and PSIox relationship were similar (p > 0.05) for the Kirkland and Taloka soils but the slope of the regression for the Grant was statistically different (p < 0.05) from the other two soils. Our results agree well with those of Davis et al. (2005) who reported differences existed in the slopes of the DRP-PSIox relationship for three Oklahoma benchmark soils. Our results contrast with those of Vadas et al. (2005) who reported the relationship between DRP and PSIox was the same for ten different soils. Comparison of our data set with the data set from the study by Vadas et al. (2005) shows that our PSIox values ranged from 5.86 to 15.9% while the values used by Vadas et al. (2005) ranged from 1 to 82%. Perhaps the difference in the studies is that smaller variability in our data allowed us to detect smaller statistical differences as compared with the study by Vadas et al. (2005).

Relationships between P Load and Soil P
The DRP load-M3P relationship was statistically significant (r2 = 0.58, p < 0.001) (Fig. 4A ) for the combined soils. Dissolved reactive P load was also related to M3P (p < 0.001) for the Grant (r2 = 0.55) and Kirkland (r2 = 0.66) soils but was only weakly related for the Taloka soil (r2 = 0.26. p < 0.05) (Fig. 4A). Combination of the soils produced a significant relationship between DRP load and WSP (r2 = 0.63, p < 0.001) (Fig. 4B). Similar relationships were found between DRP load and WSP for the Grant (r2 = 0.54, p < 0.001) and the Kirkland soils (r2 = 0.59, p < 0.001) with the Taloka soil being less correlated (r2 = 0.24, p < 0.05) than the other two soils (Fig. 4B). Our results for the DRP load-M3P relationships agree well with those reported by Tarkalson and Mikkelsen (2004) who reported a significant relationship (r2 = 0.59) between DRP load and M3P for a conventionally tilled Hiwasee soil. Pote et al. (1999) found that DRP load in runoff was correlated with water extractable P for three different Ultisols in a pasture situation.



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  		Fig. 4 The relationships between DRP load and (A) Mehlich3 P for the three individual study soils and all soils combined or (B) water soluble P for the three individual study soils and all soils combined. ***p < 0.001, *p < 0.05.

 

   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Due to commercial P fertilizer application, a wide range of soil P levels was observed in the three long-term continuous wheat fertility trials evaluated. The addition of fertilizer P to the three soils increased M3P and WSP and these P levels were well correlated with P fertilizer application rate at all three sites evaluated. Even the lowest long-term P application rates resulted in M3P concentrations greater than that required for wheat production for the Grant and Kirkland soils.

Runoff volumes did not vary with total amount of P applied for individual soil series but runoff volumes were different among the control soils with the Kirkland control soil having a greater runoff volume as compared with the other two soils. Soil physicochemical properties do not fully explain the differences in runoff volumes between the soils. The higher clay content and smaller sand content explains why the Kirkland soil displayed the highest runoff volume. However, soil particles-size analysis does not adequately explain the similar runoff volumes between the Grant and Taloka soils. The Kirkland soil also produced less TSS, TP, and PP as compared with the other soils. Because TSS is reported on a concentration basis, this may be explained by a dilution effect due to the higher runoff volume of the Kirkland soil.

Runoff DRP increased with increasing P addition within treatments for the soils and was related to M3P and WSP for the individual soils. The difference between the slopes of the regressions for both the DRP-M3P relationship and DRP-WSP relationship for the different study soils indicate the relationships are probably soil specific under similar management practices. The Smax property of soil seemed to be responsible for the observed variation in the relationship between runoff DRP and soil M3P among different soils. For example, the soil with the lowest Smax (the Grant soil) released the highest concentration of runoff DRP per unit soil P while the soil with the highest Smax (the Kirkland soil) released the lowest concentration of runoff DRP per unit soil P.

Ammonium oxalate P saturation index increased with increasing P addition. Significant relationships were found between runoff DRP and soil PSIox. Similar to DRP-STP relationships, statistical analyses revealed significant differences for the slopes of the DRP-PSIox regression for the different soils, indicating the relationships will probably have to be soil specific to be useful in management decisions.

Received for publication March 31, 2005.


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





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