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USDA-ARS, Pasture Systems and Watershed Management Research Unit, University Park, PA 16802-3702
* Corresponding author (pjk9{at}psu.edu)
Received for publication June 4, 2002.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONIMPLICATIONS TO PHOSPHORUS SITE...REFERENCES |
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Abbreviations: DRP, dissolved reactive phosphorus • SS, suspended solids • TP, total phosphorus • WEP, water-extractable phosphorus
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONIMPLICATIONS TO PHOSPHORUS SITE...REFERENCES |
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Broadcasting manure onto soil remains a common method of manure application in the northeastern USA (Kephart, 2000; Dou et al., 2001), especially where smaller farms lack manure storage facilities, requiring daily manure application, and stony soils prevent the use of injection equipment. Broadcast manure concentrates soluble P at the soil surface where it is readily available to runoff water (Sharpley et al., 1984; Eghball and Gilley, 1999). Of existing manure application methods, broadcasting generally results in the greatest potential for soluble P losses in runoff (Romkens et al., 1973; Mueller et al., 1984; Andraski et al., 1985; Zhao et al., 2001).
The sequence and time interval between manure application to soil and runoff event plays a key role in the magnitude of observed P losses (Westerman and Overcash, 1980; Sharpley, 1997). Immediately following manure broadcasting, the potential for P loss peaks and then declines over time, as water-soluble P applied in the manure increasingly interacts with soil and is converted to recalcitrant forms (Edwards and Daniel, 1993a). Mueller et al. (1984) reported declining DRP concentrations (from 0.94–0.26 mg L-1) in runoff from no-till plots broadcast with dairy manure over two months of the growing season. Similarly, Gascho et al. (1998) observed exponential declines in DRP concentrations in successive surface runoff events (from >5 to <1 mg L-1) over a slightly shorter time period after mineral fertilizer application.
Variability in P fractions, particularly soluble P, of different manures applied to soil can affect the forms of P available to runoff water. Moore et al. (2000) reported significant differences in DRP losses from pastures amended with either alum-treated or untreated poultry litter at the same rate of TP addition. They observed concomitant decreases in the water-extractable P fraction of alum-treated poultry litter and runoff DRP losses from pasture receiving that litter. Kleinman et al. (2002a) found that the WEP concentrations of dairy, poultry, and swine manures were related to DRP and TP concentrations in runoff, when these manures were broadcast at the same rate of TP addition (100 kg TP ha-1) onto runoff boxes packed with different soils (r2 = 0.86 for DRP, r2 = 0.78 for TP). In both of these studies, DRP accounted for the majority of runoff TP (>60%), as soluble P from broadcast manures served as the primary source of P in runoff.
Manure application rate also regulates the concentration of P available to runoff water at the soil surface. Edwards and Daniel (1992) observed concentrations of 0.8, 16.2, and 34.5 mg DRP L-1 in runoff from grassed soils broadcast with poultry manure slurry at rates of 0, 76, and 304 kg TP ha-1, respectively. In a related study, Edwards and Daniel (1993c) reported runoff DRP concentrations of 0.8, 11.9, and 29.4 mg L-1 from grassed soils broadcast with swine manure at rates of 0, 19, and 38 kg TP ha-1.
In addition to its effect on P concentrations at the soil surface, the application of manure to soil can affect soil physical properties that control runoff and erosion, thereby influencing runoff P losses. In the short term, broadcasting manure may improve soil surface protection from rain drop impact and aggregate dispersion (Barthès et al., 1999). McDowell and Sharpley (2003) found the loss of particulate P and TP in runoff from a Hagerstown silt loam (fine, mixed, semiactive, mesic Typic Hapludalf) amended with up to 50 kg P ha-1 dairy manure (9.9 mg particulate P and 15.1 mg TP) was lower than from untreated soil (13.3 particulate P and 18.1 mg TP). Over the long term, however, addition of manure increases soil organic matter levels, which in turn influences porosity, aggregate stability, and infiltration, factors that affect runoff and erosion potential (Rousseva, 1989; Oades and Waters, 1991; Gilley and Risse, 2000).
The objective of this study was to contribute to the calibration of P site assessment indices by quantifying the differential effects of manure type and application rate on runoff P concentrations over successive rainfall events. An experiment was conducted in which different manures were broadcast to soils packed into runoff boxes and subjected to a series of simulated rainfall events. Broadcast dairy, poultry, and swine manures were applied at a broad range of rates, and successive rain simulations were conducted over a 24-d period following manure application.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONIMPLICATIONS TO PHOSPHORUS SITE...REFERENCES |
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Soil P sorption saturation was determined by shaking 0.25 g of soil in 10 mL of acid oxalate solution [0.1 M (NH4)2C2O4·H2O + 0.1 M H2C2O4·2H2O] for 4 h in the dark, centrifuging (510 x g for 20 min), and then filtering (0.45 µm) the extract. Oxalate-extractable P, iron (Fe), and aluminum (Al) concentrations were determined by inductively coupled plasma, and molar concentrations (mmol kg-1) of each element used to calculate soil P sorption saturation were determined as follows (Kleinman and Sharpley, 2002):
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Soil pH was determined by mixing 5 g air-dry soil with 5 mL distilled water. Particle size analysis was conducted by the pipette method (Day, 1965).
Manures
Manures were selected to represent a range of types, dry matter contents, and P solubilities. Dairy manure, layer poultry manure, and swine slurry were collected and stored at 4°C for a maximum of 2 wk before analysis. Dairy manure and swine slurry were sampled from the Pennsylvania State University Dairy and Swine Centers, respectively, at University Park, PA. The dairy manure was from lactating Friesian-style dairy cows and was scraped from a free stall barn. Swine slurry was from finishing sows that was washed into a holding tank and agitated before sampling. Poultry manure was from a laying operation in Northumberland County, PA, and was collected directly from the layer house.
Manure was analyzed for TP by the modified semimicro-Kjeldahl procedure (Bremner, 1996). Water-extractable phosphorus was analyzed by the method of Kleinman et al. (2002b). One gram dry-weight equivalent fresh manure was shaken with 200 mL of distilled water on an end-over-end shaker for 60 min. The mixture was then centrifuged (2900 x g for 20 min to facilitate filtration) and filtered through a Whatman1 (Maidstone, UK) #1 filter paper, before being analyzed for P colorimetrically. Manure pH was determined after mixing 1 g (equivalent dry weight) fresh manure with 100 mL of distilled water. Dry matter content of all manures was determined gravimetrically after oven-drying manures at 70°C for 48 h.
Rain Simulation Experiment
An experiment was designed to assess interactions between soil, manure type, P application rate, and timing and sequence of runoff event on runoff P losses using the National Phosphorus Research Project indoor runoff box protocol (National Phosphorus Research Project, 2001). The protocol employed stainless steel runoff boxes (1 m long, 20 cm wide, and 5 cm deep) with back walls 2.5 cm higher than the soil surface, and 5-mm drainage holes in the base (Kleinman et al., 2002a). Cheese cloth was placed on the bottom of the box. Buchanan and Lewbeach soils were packed into runoff boxes to achieve a bulk density of 1.3 to 1.5 g cm-3. Dairy manure, poultry manure, and swine slurry were broadcast onto individual runoff boxes at five rates corresponding to 10, 50, 75, 100, and 150 kg TP ha-1. A control treatment (zero manure application) was left for comparison. Each treatment was conducted in duplicate.
Runoff was generated by applying artificial rainfall on inclined (3%) soil runoff boxes using a TeeJet 1/2 HH SS 50 WSQ nozzle (Spraying Systems Co., Wheaton, IL) placed approximately 3.1 m above the soil surface. At this height, simulated rainfall achieves >90% terminal velocity. Rainfall was delivered at approximately 70 mm h-1, and had a coefficient of uniformity of >0.83 within the 2- x 2-m area directly below the nozzle. Rainfall event intensity and duration corresponded to an approximate 10-yr return period for the areas of Pennsylvania and New York where the Buchanan and Lewbeach soils were collected. Runoff was collected via a gutter, equipped with a canopy to exclude direct input of rainfall and inserted at the lowest edge of the runoff box.
Rainfall-runoff simulations were performed at three time intervals (3, 10, and 24 d) after manure application, and 30 min of runoff was collected in 2-L plastic pails for each simulation.
Following each simulation, runoff water was thoroughly stirred to resuspend settled particles and a subsample was immediately filtered (0.45 µm). Dissolved reactive P was determined on the filtered sample by colorimetric P determination (Murphy and Riley, 1962) within 24 h of collection. Total P was measured on unfiltered runoff water with the modified semimicro-Kjeldahl procedure following Bremner (1996). Suspended solids (SS) were determined gravimetrically from 200 mL of unfiltered runoff water.
Statistical Analysis
Runoff P concentrations (DRP and TP) were log-transformed to conform with the assumptions of normality and equal error variances. As many of the P concentrations were <1 mg L-1, data were transformed by adding 1 to the P concentration and determining the logarithm of that sum so that no negative values were obtained (Neter et al., 1996). Treatment effects (manure application rate, manure type, and runoff event timing and sequence) were evaluated by a general linear model with Tukey's pairwise comparisons. Relationships between application rate and runoff DRP, TP, and SS were quantified by least squares regression. Soil-related differences in runoff P and SS concentrations were evaluated by Student's t test. All analyses were performed using Minitab's statistical software, Release 13 (Minitab, 2001).
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONIMPLICATIONS TO PHOSPHORUS SITE...REFERENCES |
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Differences in experimental conditions probably contributed to different regression equations for swine slurry. First, the initial runoff event in Edwards and Daniel (1993c) occurred 24 h after manure application, as opposed to 72 h in the present study. Although Edwards and Daniel (1993a) concluded that drying interval between manure broadcasting and runoff event did not influence runoff P concentration, Westerman and Overcash (1980) found that drying time interval did affect runoff P. Another factor contributing to the large difference in regression slopes could be the narrow range of application rates (0–38 kg TP ha-1) in Edwards and Daniel (1993c) compared with this study (0–150 kg TP ha-1), possibly biasing the regression equation. Finally, in Edwards and Daniel (1993c) the swine slurry TP concentration on a dry matter equivalency basis was 39 g kg-1 and dissolved P concentration (colorimetric determination of P on slurry sample) was 14 g kg-1. In the present study, concentrations of TP and WEP in swine slurry were 32 and 8 g kg-1 (Table 2). The greater concentration of dissolved P and TP in the swine slurry used by Edwards and Daniel (1993c) probably contributed to the greater concentrations of runoff DRP and TP they observed.
Trends with Successive Runoff Events
Results confirm the importance of the timing and sequence of runoff event relative to manure application on runoff P concentrations, as illustrated by DRP, TP, and SS concentrations in runoff from the Buchanan soil broadcast with 100 kg TP ha-1 (Fig. 2)
. For all soils broadcast with manure, DRP and TP concentrations decreased with successive runoff events, although statistically significant (p < 0.05) declines in runoff P concentrations were only observed in soils broadcast at rates of 
50 kg TP ha-1. For those treatments, the number of days between manure application and runoff event was negatively related to DRP and TP, as evidenced by regression slopes in Fig. 2. Similar trends in runoff DRP over successive runoff events have been reported by Sharpley (1997), Gascho et al. (1998), and Walter et al. (2001). No statistically significant differences in DRP and TP concentrations were detected over time in runoff from the unamended, control soils (p = 1.00).
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Table 5 presents mass balance information for soils broadcast with manure at a rate equivalent to 100 kg TP ha-1. The percentage of TP applied in manure that was removed in runoff over the three simulated rainfall events ranged widely, with largest cumulative losses observed from soils broadcast with poultry manure (19% for Buchanan, 9% for Lewbeach), followed by swine slurry (12% for Buchanan, 7% for Lewbeach) and dairy manure (3% for Buchanan, 5% for Lewbeach). Proportional losses of WEP from manure as DRP in runoff were even greater than for TP, with the largest cumulative losses in poultry manure (27% for Buchanan, 21% for Lewbeach), followed by swine slurry (26% for Buchanan, 14% for Lewbeach) and dairy manure (4% for Buchanan, 13% for Lewbeach). Even so, there was a large proportional decline in runoff P concentrations (Table 3) with successive rainfall events (up to 90% between 3- and 24-d events). Thus, the depletion of P from the soil surface by runoff is unlikely to be the sole cause of the temporal trends observed in this study.
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Effect of Manure Type
Manure type clearly affected DRP and TP concentrations in runoff during the earlier runoff events (Table 3). Differences in runoff DRP and TP concentrations between manures were statistically significant (p < 0.05) at higher manure application rates (>50 kg TP ha-1) for the first two runoff events for the Buchanan soil and the first runoff event for the Lewbeach soil. Given the different relationships between TP application rate and P concentration in runoff across the three manure types (Table 4), differences in P concentration related to manure type are amplified by application rate.
The absence of significant differences in runoff P concentrations between manure types at lower application rates can be explained by the low contribution of soluble P from the manures to runoff TP at these rates. As described above, the contribution of DRP to TP concentrations in runoff increased with dry matter application rate (Fig. 1). Consequently, the absence of significant differences in runoff P concentrations between manure types at lower application rates stems from the fact that soluble P losses are not the dominant factor in runoff P concentrations. At higher manure application rates, however, DRP accounts for the majority of TP concentration in runoff.
When comparing runoff from soils receiving manure at the same rate of TP addition, DRP and TP concentrations were significantly greater from poultry and swine manures than from dairy manure (p < 0.05), and generally not significantly different between poultry and swine manures (e.g., Fig. 2). Observed differences in runoff P concentrations were consistent with differences in the WEP concentration (mg WEP kg-1) of individual manures (Table 2), corresponding with the findings of Kleinman et al. (2002a). Water-extractable P application rate (kg WEP ha-1) was not an effective predictor of runoff P concentrations across manure types. Even though broadcast dairy manure consistently resulted in the lowest runoff DRP concentrations at a given P application rate, a higher percentage of dairy manure TP was composed of WEP (29%) than poultry manure (27%) and swine slurry (25%).
Effect of Soil Type
Runoff TP concentrations were significantly greater from the Buchanan soil than from the Lewbeach soil (p = 0.01), but runoff DRP concentrations were not significantly different between soils (p = 0.15). The significant differences in runoff TP concentrations follow trends in erosion (SS), which was significantly greater from the Buchanan soil than Lewbeach soil (p < 0.01) (Table 3). The specific source of SS in runoff cannot be directly ascertained from this study (i.e., eroded manure vs. eroded soil), as specific sediment characteristics that would allow such an inference were not analyzed (e.g., organic vs. inorganic solids content). Indirect evidence, however, does shed some light on the sources of P in runoff.
Soil-related differences in runoff TP concentration (Buchanan > Lewbeach) were the opposite of Mehlich-3 P concentrations in the two soils (Buchanan < Lewbeach, Table 1), ruling out differential contributions of soil-derived particulate P to runoff TP concentrations. Moreover, the highly erodible clay- and silt-sized particle fractions of the two soils were practically identical (58 vs. 59%).
Pote et al. (2001), in examining the effect of swine slurry application rate on runoff properties following simulated rainfall, reported significant relationships between infiltration rate and runoff DRP concentrations, which could also contribute to differences between soils with dissimilar hydrologic characteristics. Even though runoff volumes from Buchanan and Lewbeach soils did not differ significantly (p = 0.66), runoff TP load (TP concentration x runoff volume) differed sufficiently between soils to result in significantly more removal of applied TP from the Buchanan soil than Lewbeach soil (Table 5). Thus, differential removal of applied P probably accounts for observed differences in runoff TP concentrations between the two soils.
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IMPLICATIONS TO PHOSPHORUS SITE ASSESSMENT INDICES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONIMPLICATIONS TO PHOSPHORUS SITE...REFERENCES |
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Results of this study show that differences in runoff P losses related to soluble P contributions from broadcast manure were diminished by repeated runoff events. Experimental conditions of this study favored high rates of erosion (high rainfall intensity, low soil infiltration capacity due to the 5-cm-deep runoff boxes), which may have caused larger cumulative removal of applied P by runoff than would a series of events with low intensity on soils with low erodibility. Even so, the combined mechanisms of removal of manure P and soluble P translocation into the soil clearly limited the duration that broadcast manure influences runoff P concentrations. Site assessment indices generally do not account for the rapid reduction of P availability to runoff by these mechanisms, or for their effect in diminishing the influence of manure WEP concentrations. Rather, seasonal variation in runoff potential is used to weigh index values. As a result, it is possible that differences in P loss potential related to soluble P are exaggerated by existing indices. Clearly, further research is needed in this area.
Finally, findings of this study suggest that differential erosion of broadcast manure can be an important contributor to the variation in runoff TP concentrations among soils. In many areas, manure is broadcast onto no-till and grassed soils that have low erosion rates. While soil erosion is included as a transport factor in all site assessment indices (Weld et al., 2000), it is possible that erosion of applied manure is not adequately represented.
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ACKNOWLEDGMENTS |
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NOTES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONIMPLICATIONS TO PHOSPHORUS SITE...REFERENCES |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONIMPLICATIONS TO PHOSPHORUS SITE...REFERENCES |
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