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a Dep. of Crop Sciences, 1102 S. Goodwin Ave., Univ. of Illinois, Urbana, IL 61801
b Dep. of Crop and Soil Sciences, Michigan State Univ., East Lansing, MI 48824-1325
* Corresponding author (daverede{at}uiuc.edu)
Received for publication June 21, 2002.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: AAP, algal-available phosphorus • DRP, dissolved reactive phosphorus • TP, total phosphorus
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The load of P loss depends on the runoff volume, which in turn is related to climatic, edaphic, and agronomic factors. Pote et al. (1999) studied the effects of runoff volume on the DRP load in runoff, and since the volumes were highly variable in some soils, so were the mass losses. However, their work did show linear relationships between DRP loads and soil test levels.
The transport of AAP in surface runoff and sediments is dependent on the erosion potential and the surface soil P content (Sibbensen and Sharpley, 1997). The transport of eroded material in surface runoff is particle-size selective and hence is highly effective at transporting P adsorbed to organic-rich clay and silt-sized soil fractions (Heathwaite, 1997). Pote et al. (1996) found that the AAP concentration in runoff from fescue (Festuca spp.) pastures was only slightly higher than that of DRP, and both were linearly correlated with Mehlich-III and Bray–Kurtz soil test P levels. Cox and Hendricks (2000) found that TP was highly dependent on the sediment concentration in runoff. They also observed that increasing soil P levels increased the concentrations of TP, but that this relationship varied among the different soils used.
No-till has been widely adopted for highly erodible soils of U.S. Midwest. Studies have shown that TP and AAP losses decrease with no-till practices, compared with conventional tillage practices (Andraski et al., 1985; Chichester and Richardson, 1992; Sharpley and Smith, 1994). On the other hand, other studies have shown that DRP concentrations and losses increase in no-till fields, even when P fertilizer has been incorporated into the soil (Gaynor and Findlay, 1995). Crop residues contribute significant quantities of soluble plant nutrients to agricultural runoff (Schreiber and McDowell, 1985; Power and Legg, 1978).
Much of the recent research associated with P runoff has been conducted on pasturelands. There is a lack of information concerning the loss of P associated with row crop agriculture, and how typical tillage methods relate to P runoff, particularly on soils where long-term application of manure to agricultural fields has led to extremely high soil P levels.
The objectives of this study were to (i) determine the effect of soil P level on the concentration and loads of DRP, AAP, and TP loss in runoff; (ii) compare the effects of no-till and chisel-plow on the concentration and loads of DRP, AAP, and TP loss; and (iii) evaluate the P loss associated with rainfall simulation immediately following soil tillage in the fall and in the following spring.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The experimental design was a randomized complete block with two replications. Each block contained eight 9- by 6-m main plots, with 5.5% mean slope. Treatments consisted of two tillage methods (chisel-plow and no-till), and a desired range of soil P levels (15–150 mg kg-1).
To obtain a range of soil P levels, each 9- by 6-m main plot was soil sampled from 0 to 2.5 cm in May 1999. Triple superphosphate was broadcast to every main plot based on the soil test, to try to establish soil P levels that ranged from 15 to 150 mg kg-1. Then, a field cultivator was used to mix and prepare the soil, and soybean was planted.
In early October 1999, after the soybean crop was harvested, 2- by 1.5-m simulated rainfall collection microplots were delimited by flags at the center and lower part of the 9- by 6-m main plots. Simulated rainfall took place only on the 2- by 1.5-m microplots. The shorter sides of microplots and main plots were perpendicular to the slope. The same experimental design was set up again in late September 2000 on an adjacent site to repeat the experiment. Soil samples were collected from the outside perimeter of the microplots and analyzed for Bray P1 soil extraction. The range of P test levels was 27 to 1248 mg kg-1, which was around nine times greater than the range sought originally. Soil P levels were divided into four categories (Table 1) and treatment combinations were randomly assigned within each of the four soil P level categories.
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Rainfall Runoff Simulation
Rainfall simulations were conducted at each of the microplots in mid-November 1999 and mid-May 2000. The trial was repeated in late October 2000 and early May 2001. Four rainfall simulators (Humphry et al., 2002), each equipped with one nozzle (TeeJetTM 1/2HH-SS50WSQ; Spraying Systems, Wheaton, IL) placed 3 m above the soil surface, were used to simulate a 95 ± 12 mm h-1 intensity rainfall. This rainfall intensity is equivalent to a storm with a 10-yr return period in western Illinois (Huff and Angel, 1989). Rainfall intensity was measured by placing rain gauges on the microplots during the rainfall simulations. The aluminum frame supporting the nozzle was fitted with tarpaulin sheets to provide a windscreen. The duration of simulated rainfall varied from microplot to microplot, but was sufficient to provide water for a 30-min runoff event. The water used for rainfall simulation came from a 76-m-deep aquifer near Monmouth, IL. This water was stored in a tank, and the DRP value of this water ranged from 0.02 to 0.12 mg L-1, depending on the day of supply. In spring 2001, while sampling the last block, the hose used to transfer water from the main storage tank to the container used for the experiment was contaminated with high levels of P. All P runoff data obtained from the subsequent rain simulations were discarded.
Runoff samples were collected from each microplot at 2.5, 7.5, 17.5, and 27.5 min after the onset of runoff. These numbers represented the midpoints of the first, second, fourth, and sixth 5-min periods of collection. This was done to get a more intense sampling of the first 10 min, as in some cases, P concentrations in runoff have been found to be higher immediately after the start of runoff and to decrease exponentially thereafter (Laflen and Tabatabai, 1984). However, no significant differences between P concentrations and different sampling times were detected, so the concentrations were weighted according to each runoff volume to collect one composite sample per experimental unit. Runoff volumes were recorded by measuring the depth of water in the bucket at each sampling time (including Time 0) and after 30 min.
Water and Soil Analysis
Composite samples were analyzed for DRP, AAP, and TP concentration. Phosphorus load (kg ha-1) was calculated by multiplying the total volume of runoff in 30 min by the composite sample concentration. "Rainwater" DRP concentration was subtracted from the runoff concentrations.
Within 12 h after sample collection, portions of the runoff samples for DRP analysis were filtered through Whatman (Maidstone, UK) no. 1 filter paper and then vacuum-filtered through a 0.45-µm Millipore (Bedford, MA) filter paper. After filtering, samples were stored at 4°C and were analyzed within 24 h for DRP using the ascorbic acid method (American Public Health Association, 1995).
Unfiltered portions of samples were stored at 4°C until analysis for AAP. Algal-available P was measured on unfiltered runoff samples using the iron oxide strip method (Sharpley, 1993). Unfiltered samples were also analyzed for TP by a Kjeldahl digestion method (Patton and Truitt, 1992). Samples analyzed for both AAP and TP were neutralized before using the ascorbic acid method (American Public Health Association, 1995).
Sediments were measured by drying 10 mL of unfiltered water sample at 110°C until a constant weight had been attained. The Bray and Kurtz P-1 test for extracting soil P was used (Frank et al., 1998). Eight subsamples from around the microplot were collected for each soil sample, which was subsequently air-dried, crushed, and sieved to pass a 2-mm sieve. Clay content was determined by the hydrometer method (Klute, 1986) on 10 samples.
The ascorbic acid method was used to carry out color development for determination of Bray P1 soil extraction values. When the transmittance exceeded the standard curve, the extractant was diluted as needed. Soil organic matter was estimated as the weight loss on ignition (Combs and Nathan, 1998). Soil pH was measured on a 1:1 soil and water slurry (Watson and Brown, 1998).
Statistical Analysis
Results were analyzed using SAS (SAS Institute, 1999). Non-normally distributed data were log-transformed. PROC GLM was used to analyze the effects of year, season, and tillage method on runoff volumes, sediment concentrations and loads, time to runoff, and runoff concentrations and loads (kg ha-1) of DRP, AAP, and TP.
Years, and blocks nested in years, were considered random in the model. Interactions between factors and blocks were pooled into the error when the P value exceeded 0.25 (Bozivich et al., 1956). When no significant effects were found for years or seasons (DRP concentration and load), or for years, seasons, and tillage method (TP and AAP concentration and load), the data were analyzed together by the PROC REG procedure along with the stepwise selection method to select the independent variables that were significantly related to the dependent variables. Bray P1, residue cover, and sediment concentration were used as independent variables for DRP, AAP, and TP concentrations and loads. The PROC RSREG procedure was used to estimate the response surface. Significant terms along with their corresponding lower-order terms were included in a revised model, which was again run as multiple regression.
A logistic model was fit with PROC NLIN to relate DRP concentration to Bray P1 soil extraction values for no-till microplots:
 | [1] |
The change point in the split-line model (McDowell and Sharpley, 2001) was estimated by nonlinear regression.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Total sediment load had a CV of 131%, so that no differences between no-till and chisel-plow were statistically significant. The higher runoff volumes and lower sediment concentrations of no-till plots were matched by the low runoff volumes and high sediment concentrations of the chisel-plow plots (Table 3). The average amount of sediment loss was 224 g ha-1. Gaynor and Findlay (1995), working on slopes less than 1%, observed that sediment concentration was about two times larger in conventional tillage compared with ridge or zero tillage, but the amount of sediment loss in surface runoff did not differ among the three different tillage treatments in any of the three years of study. Seta et al. (1993), working on 9% slopes, reported higher sediment concentrations and amounts in runoff from chisel-plowed plots compared with no-till plots, because, contrary to our study, runoff volume from chisel-plowed plots was greater than runoff volume from no-till plots. Seta et al. (1993) worked on long-term no-till plots, which probably had higher infiltration rates than the short-term no-till plots used in our study.
Dissolved reactive P, TP, and AAP concentration and load in runoff did not differ significantly between years nor between fall and spring rainfall simulation events (P = 0.1). Therefore, the data from both years and seasons were combined to give a total of 64 data points.
Dissolved Reactive Phosphorus
Dissolved reactive P concentrations in runoff from no-till plots (0.40 mg L-1) were greater (P = 0.001) than from chisel-plow plots (0.24 mg L-1). Loads of DRP in runoff were similarly higher from no-till (0.05 kg ha-1) than from chisel-plow plots (0.02 kg ha-1). These results are very similar to those obtained by Laflen and Tabatabai (1984) for a sandy loam in a soybean and corn rotation, and by Gaynor and Findlay (1995) working with a clay loam.
A logistic function (Eq. [1]) best explained the relationship between DRP concentrations from no-till plots and Bray P1 extraction values (Fig. 1) . No other independent variable explained DRP concentrations in runoff. A better fit was obtained when using 2.5-cm-deep soil samples (r2 = 0.87), as compared with 17-cm-deep soil samples (r2 = 0.78). Therefore, only 2.5-cm-deep soil test values are shown. None of the DRP values surpassed 1 mg L-1, and predicted concentrations reached a maximum of 0.77 mg L-1, which is the parameter a of the logistic function (Fig. 1). The plateau may represent the maximum P that diffused into the solution in the time frame of the rainfall simulation.
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The logistic function showed a marked increase in the rate of DRP concentration in runoff as the Bray P1 extraction values surpassed 120 mg kg-1. McDowell and Sharpley (2001) fitted split-line models to the relationship between DRP concentration in runoff and Mehlich-III soil test values (where the latter were less than 400 mg kg-1) to determine a change point that separates the relationship between soil test levels and DRP into two sections. For the purpose of comparing our work with McDowell and Sharpley (2001), a split-line model was fitted and 126 mg kg-1 was determined as the change point for no-till plots. Only the data under 360 mg kg-1 Bray P1 were used for fitting this model, since at higher values a plateau was observed.
The load of DRP in runoff was highly variable due to the extensive variability in runoff volumes (Fig. 2) . The logarithmic model reached a plateau of 0.11 kg ha-1 DRP. Pote et al. (1999) found a linear relationship between DRP loads and water-extractable P. They also found extensive variability in DRP loads due to variable runoff volumes.
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The load of DRP in runoff from tilled plots, when regressed on Bray P1 extraction values, showed a linear relationship, but with a poor fit (Fig. 4) . Pote et al. (1999) observed a correlation between DRP concentrations and runoff volumes for each soil P level. When they divided DRP concentrations by runoff volumes and related these numbers to the soil P levels, they obtained high correlations (r = 0.87–0.92). In our study, no correlations were observed between DRP concentrations and runoff volumes for each soil P level category, and the regression of DRP concentration per mm runoff across soil P levels had a significant (P = 0.01) but relatively poor fit (R2 = 0.40).
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The following regression was fitted for the TP load:
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Andraski et al. (1985) observed that AAP concentrations were generally higher for tilled than for no-till treatments, although the differences were not always significant. They also concluded that AAP concentrations were primarily related to particulate P because concentrations of DRP were about 30% of AAP, and P extracted in excess of DRP was attributed to inorganic P desorbed from sediment in the runoff suspensions. In our study, DRP accounted for 85% of AAP for no-till plots and 27% for chisel-plow plots, and therefore only AAP concentrations in runoff from tilled plots were related to sediment concentrations. Runoff from no-till plots was primarily related to Bray P1 soil extraction values, and the quadratic function was very similar to the logistic function observed when relating DRP concentration to Bray P1 extraction values.
Algal-available P load had a CV of 113%, more than twice the CV for AAP concentration. This high variability was also attributed to the inclusion of the runoff volumes to calculate runoff loads. When an equation was fitted with multiple regression, the quadratic term for soil P level was no longer significant and the R2 was 0.42 (P = 0.001):
 | [5] |
The terms B1 and B1 x SED had the largest sums of squares. The sediment P enrichment that was found for AAP concentrations in runoff was observed for AAP loads with Bray P1 extraction values higher than 100 mg kg-1. Higher runoff volumes from no-till plots were associated with slightly higher AAP loads in runoff than for chisel-plow plots (Fig. 8) .
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Dissolved reactive P, TP, and AAP concentrations and loads in runoff were similar in the fall and spring rainfall-runoff simulation events. Dissolved reactive P concentration and load in runoff were related to surface soil Bray P1 extraction levels. In no-till plots, DRP concentration and load across Bray P1 extraction values were described by a logistic function, with plateaus of 0.77 mg L-1 and 0.11 kg ha-1 at 360 mg kg-1 of Bray P1 extraction values, respectively. In chisel-plowed plots, DRP concentration responded linearly to Bray P1 extraction values, reaching 0.77 mg L-1 at about 960 mg kg-1 Bray P1. The highest DRP load from chisel-plowed plots was around 0.06 kg ha-1.
Total P concentrations and loads were highly related to sediment concentrations, and chisel-plowed plots had greater sediment concentrations in runoff than no-till plots. Bray P1 extraction values were also related to TP concentrations through the DRP fraction, which was 38% of TP in runoff from no-till treatments and only 7% in runoff from chisel-plow treatments.
Algal-available P concentrations and loads were related to Bray P1 extraction values and sediment concentrations in runoff. In no-till treatments, DRP accounted for 85% of AAP, so AAP was mainly related to Bray P1 extraction values. In chisel-plow treatments, only 27% of AAP was composed of DRP, so AAP was primarily related to sediment concentrations. These sediment concentrations were enriched with P when Bray P1 extraction values were higher than 100 mg kg-1.
Algal-available P poses a threat to water bodies since it is the form of P available to algae. Total P and DRP have been used throughout North America as a basis for setting trophic state criteria, and TP is one of the most likely trophic state candidates for the nutrient criteria that are being currently developed by USEPA (2000). When seeking measures to reduce DRP, AAP, or TP runoff from agricultural fields, tillage practices must be taken into account. In no-till fields, it is very important to keep Bray P1 extraction values lower than about 120 mg kg-1 and to increase infiltration as much as possible so as to increase the time to runoff and decrease runoff volume. In chisel-plowed fields, it is important to reduce sediment concentrations in runoff and maintain high residue covers that protect the soil from the impact of raindrops. Bray P1 extraction values should not surpass 100 mg kg-1 to avoid P enrichment in sediments.
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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indicates chisel-plowed plots, 
indicates no-till plots. The TP concentration = -0.08 + 0.003B1 + 0.53SED - 2 x 10-6 B12; R2 = 0.77, P = 0.001.
