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a Department of Crop Sciences, 1102 South Goodwin Avenue, University of Illinois, Urbana, IL 61801
b Department of Crop and Soil Sciences, Michigan State University, East Lansing, MI 48824-1325
* Corresponding author (daverede{at}uiuc.edu).
Received for publication October 28, 2003.
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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 • HM, high manure rate • LM, low manure rate • 1MO, first rainfall simulation (one month after treatment application) • 6MO, second rainfall simulation (six months after treatment application) • PP, particulate phosphorus • TP, total phosphorus • TSP, triple superphosphate
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Phosphorus transported by surface runoff to streams and lakes often accelerates eutrophication, thus affecting the usage of water resources for many purposes such as drinking, fishing, and recreation (Foy and Withers, 1995). The transport of P occurs in dissolved and particulate forms. Particulate phosphorus (PP) encompasses all solid-phase forms and includes P sorbed by soil particles and organic matter eroded during runoff. While dissolved P is, for the most part, immediately available for biological uptake, PP can provide a long-term source of P for aquatic plant growth. Algal-available P represents the dissolved phase and the amount of PP that is potentially available for algal uptake (Sharpley et al., 1991).
The main factors controlling P movement in surface runoff are transport (runoff and erosion) and source factors (surface soil P content and method, rate, and timing of fertilizer and animal manure applications) (Sharpley et al., 1993). High rates of P applied either as a fertilizer or manure, particularly if it is left on the soil surface, will exacerbate the potential for movement of DRP from fields (Baker and Laflen, 1982; Mueller et al., 1984). Incorporation of P materials either through tillage or through injection will generally reduce the potential for DRP runoff (Eghball and Gilley, 1999; Withers et al., 2001; Tabbara, 2003). On the other hand, tillage operations may increase the potential for TP loss, especially on highly erosive sites. Eghball and Gilley (1999) found that runoff DRP and AAP concentrations were greater for no-till than disked treatments during two consecutive simulated rainfall events on wheat (Triticum aestivum L.) residue plots with a 6% slope. In contrast, concentrations of TP and PP were greater for the disked treatments compared with the no-till plots. Cox and Hendricks (2000) reported a more than threefold increase in TP concentration in runoff from conventionally tilled compared with no-till soils for a wide range of soil P levels on 2 to 6% slopes.
Runoff transport of P from surface-applied manure increases with the application rate. Edwards and Daniel (1993) observed that DRP and TP concentration in runoff from fescue (Festuca arundinacea Schreb.) plots was directly related to swine slurry application rate. Tabbara (2003) also found a proportional increase in TP, PP, AAP, and DRP concentration and load in runoff from fallow soils when surface-applied liquid swine manure rates were doubled.
Phosphorus losses from treatments that compare inorganic versus organic amendments tend to vary among different experiments. Eghball and Gilley (1999) observed that the concentrations of DRP and AAP in runoff were significantly greater for a fertilizer treatment than two rates of beef cattle feedlot manure when all were surface-applied before an initial rainfall event. However, in a second rainfall event, increased DRP and AAP in runoff resulted from the highest manure rate. Withers et al. (2001) observed that P runoff from TSP was similar to liquid cattle manure when it was either surface-applied or incorporated with a rotovator. Tabbara (2003) found higher concentrations and load of all P forms from plots receiving broadcast P fertilizer compared with plots receiving surface-applied liquid swine manure.
Rainfall frequency and time of rainfall occurrence after the application of manures or fertilizers have also been shown to affect P runoff. Sharpley (1997) studied the effects of rainfall frequency and timing on P runoff after poultry litter had been applied to different soils. He observed decreasing concentration of P after successive rainfall events. Dissolved reactive P and AAP decreased when the rainfall event occurred 35 d compared with 1 d after the poultry litter had been applied. Similar trends were reported by Westerman and Overcash (1980) for TP runoff from swine and poultry wastes applied over fescue grass.
The objectives of this study were to (i) determine the effect of placement of P-containing materials on the concentration and load of three P forms (DRP, AAP, and TP); (ii) determine the effect of P source and rate on P in runoff; (iii) determine the relationship between soil test P levels and P in runoff; and (iv) evaluate P in runoff 1 and 6 mo after the treatment application.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Plot Establishment and Treatment Application
To obtain four categories of soil P levels ranging from 30 to 300 mg kg–1, each 9- by 6-m main plot was soil sampled from 0 to 2.5 cm on 3 May 1999 and sent to a commercial lab (for rapidity), to be analyzed by the Bray and Kurtz P1 soil extraction method. Triple superphosphate was broadcast to every main plot based on the soil test and every treatment combination was then randomly assigned to each soil P level category. A field cultivator was used to mix and prepare the soil that was going to be used for Year 1, and soybean was planted on 19 May 1999 at 38-cm row spacing. Meanwhile, the adjacent field that was going to have soybean planted in 2000 to repeat the experiment was being planted with corn (Zea mays L.), having being tilled with a field cultivator to incorporate the phosphorus fertilizer.
In early October 1999, after the soybean crop was harvested, soil samples were collected from the outside perimeter of the microplots of Year 1 to be analyzed for Bray P1 soil extraction levels and by a water-extractable P method. Simulated rainfall collection microplots 2 by 1.5 m 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. This field had residue from soybean that had been planted on no-till at 38-cm row spacing. Before the first rainfall simulation, soil samples were collected from the outside perimeter of the Year 2 microplots and were later analyzed for Bray P1 soil extraction levels and by a water-extractable P method. The range of Bray P1 soil extraction values for both years was 27 to 1248 mg kg–1, which was many times greater than the range sought originally. We found out that the commercial lab had not been diluting the samples with high P levels so many of the Bray P1 extraction values from May 1999 were extremely underestimated. The corrected Bray P1 extraction values for each category are found in Table 1.
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After the P amendments were applied, each microplot was isolated with three plastic frames: the 2-m-long and 20-cm-wide frames were set along the slope and the 155-cm-long and 15-cm-wide frame was set across the slope and at the top side of the microplot. A 155-cm-wide by 76.2-cm-long collection triangle was attached at the downhill side above a 50.2-cm-diameter by 76.2-cm-high cylindrical plastic container that had been inserted into a hole augered into the soil. The barrel was uncovered during rainfall simulation, but the collection triangle was always covered to prevent rainfall simulation water from drifting onto it and flowing into the barrel. The plastic frames (1.3-cm-thick) were inserted 5 cm into the soil. An extra 7 cm at the top of the collection triangle (adjacent to the lower part of the microplot) was bent 90 degrees, and this part was inserted into the soil to prevent water from flowing under the triangle. Residue-cover percentage was determined subsequently by the line-transect method (Shelton et al., 1992). The collection equipment was left in place until the following rainfall simulation (6MO). In November 1999, the microplots were brought to field capacity 24 h before rainfall simulation using a hose connected to a water tank. This was done because soils were very dry due to lack of natural rainfall and we sought to minimize the effect of soil moisture on runoff.
Rainfall-Runoff Simulation and Sample Collection
Rainfall simulations were conducted at each of the microplots in mid-November 1999 and in 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 (TeeJet 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. 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 6MO 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 19 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. The concentrations were weighted according to each runoff volume to collect one composite sample per experimental unit per time. 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
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 (Billerica, 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). This was done by adding two drops of phenolphthalein indicator solution to the filtered acid sample and subsequently adding drops of 10 M NaOH while swirling the bottle until the solution turned light pink. 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 P concentration. Runoff water sediments were measured by drying 10 mL of unfiltered water sample at 110°C until constant weight.
The Bray and Kurtz P-1 test for extracting soil P was used (Frank et al., 1998). Water-extractable P was determined by slightly modifying the method of Pote et al. (1996) by mixing 1 g of soil with 25 mL of distilled water, shaking for 1 h, and syringe-filtering through a 0.45-µm Millipore filter paper. The ascorbic acid method procedure was used for the color development of Bray P1 and water-extractable P. 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). Total P in manure was analyzed by the inductively coupled plasma atomic emission spectroscopy method SW846-6010B (USEPA, 1992). Soil pH was measured in a 1:1 soil to water slurry (Watson and Brown, 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.
Data Analysis
The mixed model analysis for repeated measures was performed using the MIXED procedure of SAS (Littell et al., 2000; SAS Institute, 2001). Bray P1 extraction level was used as a covariate. The variance–covariance matrix was modeled with the unstructured option in SAS. Year and block within year were considered random variables. Time (1MO and 6MO), P source (HM, LM, TSP, and control), and two application methods (chisel plow or injection, and surface application or no-till) were considered fixed variables. The model included all possible interactions between time, P source and application method, and Bray P1 as a covariate. The repeating subject was the microplot nested in year x P source x application method. Means comparisons were performed using the Scheffé method (Scheffé, 1953) because it provides a conservative experimentwise error protection for any number of contrasts. P values <0.1 were considered significant when comparing means.
The incorporated data were analyzed by regression procedures using PROC REG (SAS Institute, 2001) with the stepwise selection method to select the independent variables that significantly affected the dependent variables (P = 0.05). Bray P1 soil extraction value, residue cover, and sediment concentration and load were used in the regression model as independent variables for DRP, AAP, and TP concentration and load. The Type II sums of squares were taken into account when assessing the relative contribution of each term in explaining the dependent variable.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Residue cover was only measured before 1MO, and the interaction P source x application method was significant (P < 0.0001). The highest residue cover was observed in the no-till plots (surface-applied amendments and control) with an average of 92%, followed by the injected manure plots with an average of 61%, and the least residue cover was observed in the chisel-plowed plots, averaging 37% (Table 3). Residue cover was negatively correlated to sediment concentration (r = –0.41, P < 0.0001), and positively correlated to runoff volume (r = 0.54, P < 0.0001). The positive correlation of residue cover with runoff volume is most probably due to the relationship between residue cover percentage and application method since the highest residue cover percentage was measured in the no-till, surface-applied plots, which had the highest runoff volumes.
Soil Phosphorus
The relationship between Bray P1 soil extraction (mg kg–1) and water-extractable soil P (mg kg–1) in 2.5-cm-deep soil samples was linear, and the following equation was found:
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Dissolved Reactive Phosphorus
Time x P source x application method interaction was significant for DRP concentration and load in runoff (Table 4). High DRP concentrations and loads were observed in runoff from plots that had been amended with surface-applied TSP and manure one month earlier (1MO) (Fig. 2 and 3). When these amendments were incorporated, DRP concentration and load were greatly reduced in 1MO, showing no difference with the control plots. The differences in DRP concentration and load between surface-applied and incorporated treatments were only significant for HM at 6MO (Fig. 2 and 3). Eghball and Gilley (1999), working on wheat and sorghum [Sorghum bicolor (L.) Moench.] residues, also found that DRP concentrations in runoff from surface-applied cattle manure and inorganic fertilizer were significantly greater than those from incorporated treatments. This application method effect was observed again in a second rainfall simulation 24 h after the first one, but as occurred in the 6MO event in our study, the differences among the tillage treatments in the second rainfall simulation were smaller. Concentration and load of DRP in runoff from surface-applied HM were not significantly higher than those from surface-applied LM (Fig. 2 and 3). Dissolved reactive P concentration from surface-applied TSP was smaller than for surface-applied HM (P < 0.01). Withers et al. (2001) surface-applied TSP and liquid cattle manure at rates of 60 kg ha–1 P on a growing crop of winter wheat. The first 25 mm of natural rainfall occurred 3 wk after the treatment application and the DRP concentrations in runoff were 6.5 and 3.8 mg L–1 for TSP and liquid cattle manure, respectively. In our study, the average DRP concentrations for the surface-applied HM, LM, and TSP were 10.3, 7.6, and 5.6 mg L–1, respectively. The TSP concentration was very similar to Withers et al. (2001), and the differences between the manure treatments are probably due to the higher content of water soluble P in the swine manure compared with cattle manure. In our 6MO simulation event, we observed DRP concentration in runoff to be around 1.3 mg L–1 from TSP and manure treatments. These results were very similar to the ones observed by Withers et al. (2001) after two subsequent runoff events.
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Total Phosphorus
Time x P source x application method interaction was significant for TP concentration and load in runoff (P < 0.001; Table 4). In 1MO, surface-applied manure produced greater TP concentration and load in runoff compared with injected manure (Fig. 5 and 6). In 6MO, no differences were found for TP concentration or load in runoff between surface and incorporated treatments.
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No differences were found for TP concentration between surface-applied and chisel-plowed TSP in 1MO (Fig. 5). This was mainly caused by the high TP concentration from the chisel-plow treatments that equaled the TP from the surface-applied TSP. Ninety percent of the TP from chisel-plow plots was PP, and only 33% of the TP from surface-applied TSP was PP. So evidently what caused the high TP concentration in runoff was the erosion coming off the chisel-plow treatments. However, if we take into account that the time to runoff for the chisel-plowed TSP treatment was in average 126 min compared with 14 min for the surface-applied TSP (Table 3), it is clear that incorporating TSP is the preferred practice to reduce P runoff. The TP load was much lower in the chisel-plowed TSP compared with the surface-applied TSP (Fig. 6). This was caused by the very low runoff volumes coming off chisel-plow plots, which were about one-fifth the runoff volumes from no-till plots.
Total P concentration and load in runoff did not differ between the two surface-applied manure rates (Fig. 5 and 6). Higher TP concentration was observed in HM compared with the TSP treatment in 1MO, whereas no differences were observed among the surface-applied amendments for TP load (P < 0.1).
Total P concentration and load in runoff from incorporated treatments showed no differences between times or P sources. However, sediment concentration and Bray P1 soil extraction level were related to TP concentration from all the incorporated treatments including the control (Fig. 7). The following equation was found:
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Concentration and load of DRP, TP, and AAP for the high surface-applied manure rate were not significantly different compared with the low surface-applied manure rate. The high rate of surface-applied manure produced generally more DRP, TP, and AAP concentration in runoff than the inorganic fertilizer P, whereas the P concentration in runoff from the latter was similar to the low rate of surface-applied manure. Runoff volumes introduced variability when calculating P load in runoff, and no significant differences were observed between surface-applied P amendments.
Phosphorus losses one month after surface amendment applications were greater than the P losses after six months, when only small or no differences were observed between surface-applied and incorporated treatments for DRP, TP, and AAP concentration and load. Therefore, the residual effect of the surface-applied amendments for this scenario was very small.
Soil test P levels and sediment content in runoff influenced P loss from incorporated treatments. Only Bray P1 soil extraction value influenced DRP concentration and load in runoff from injected manure and chisel-plowed TSP and control, and the relationship was linear. Bray P1 soil extraction value and sediment concentration and load in runoff from the incorporated treatments were significantly related to TP and AAP concentration and load. Sediment concentration and load were the most important variables in explaining TP concentration and load in runoff. In contrast, AAP concentration was highly associated with Bray P1 soil extraction value, and to a much lesser extent with sediment concentration. The interaction between sediment load and Bray P1 soil extraction value was most important in explaining AAP load in runoff.
Incorporating organic and inorganic amendments was shown to be an acceptable technique to reduce P losses from agricultural fields. Injecting manure and chisel-plowing inorganic P fertilizer on the contour was not only effective in reducing P losses, but it also increased the time to runoff and decreased the runoff volumes. However, this practice should be accompanied by Bray P1 soil extraction levels below 100 mg kg–1 (0–2.5 cm) and by keeping residue cover on the field to prevent sediment losses.
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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