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USDA-ARS, Pasture Systems and Watershed Management Research Unit, Building 3702, Curtin Road, University Park, PA 16802-3702
* Corresponding author (ans3{at}psu.edu)
Received for publication August 7, 2002.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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One approach to decreasing soil P stratification is tillage that will redistribute and mix high-P surface soil with low-P subsoil. Several studies have reported a decrease in surface soil P (as Bray-1, Olsen, and total P in 0 to 5- or 10-cm depths) and redistribution of P within the soil profile after plowing P-stratified soils (Morrison and Chichester, 1994; Pezzarossa et al., 1995; Rehm et al., 1995). For four fertilized soils (20–78 kg ha-1 yr-1 for 12 yr), Weil et al. (1988) found that dilute acid extractable P in the 0- to 4-cm depth was about three times lower under tilled (16–54 mg kg-1) than no-till corn (28–233 mg kg-1). Tilled soils were moldboard plowed to a 15- to 20-cm depth in spring.
The above decreases in surface soil–extractable P result from the combined processes of dilution and sorption when high P surface soil (approximately 0–5 cm) was mixed with a large amount of subsoil (approximately 5–30 cm) with a lower P concentration and higher P sorption capacity and binding energy (Guertal et al., 1991; Sharpley, 1999b). For instance Weill et al. (1990) found moldboard plowing of no-till corn decreased Bray-1 P in the 0- to 5-cm depth from 156 to 100 kg P ha-1 for a clay soil, with no significant change in a sandy loam soil (178 and 186 kg ha-1).
Phosphorus sorption following mixing has been found to be an important process in overland and stream flow systems, where the input of P-deficient or high P sorbing subsoil can resorb dissolved P in overland and stream flow (McDowell et al., 2001, 2002; Sharpley et al., 1981). In fact, Kunishi et al. (1972) found dissolved P concentrations of about 0.2 mg L-1 in overland flow from fertilized fields were decreased to <0.1 mg L-1 as it moved 10 km downstream in the Mahantango Creek watershed, Pennsylvania. This was attributed to sorption of P by subsoil and stream bank material during movement downstream (Taylor and Kunishi, 1971).
The objective of this study was to determine the effect of plowing on the concentration and forms of P in soils highly stratified with respect to P, as a result of continual, long-term application of manure. This was evaluated by laboratory incubation of high-P surface soil and low-P subsoil in differing ratios for 10 Oklahoma and 8 Pennsylvania soils, which had a large accumulation of P in the surface 5 cm, due to continual manure (dairy, poultry, and swine) application. In addition, the loss of P in overland flow from a P-stratified Berks channery silt loam soil (Typic Dystrochrept) before and after moldboard plowing (15- to 25-cm depth) was measured under field and simulated rainfall conditions.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Soil Mixing and Incubation
Samples of 0- to 5-cm and composited 5- to 20-cm depths were mixed in ratios of 20:0, 15:5, 10:10, 5:15, and 0:20 (gram wt. basis) to simulate varying degrees of soil mixing by plowing. Soil mixtures were moistened to field capacity (approximately 30% soil water capacity), covered to prevent dust contamination, and regularly rewetted to prevent drying during incubation for 28 d at 298 K. After incubation, soil mixtures were air-dried and Mehlich-3 extractable P, P sorption, and P sorption saturation of each mixture was determined by methods described below.
Overland Flow Studies
Site Selection and Treatment
The transport of P and sediment in overland flow before and after plowing a high-P soil was evaluated using simulated rainfall. The soil was located in the mixed land-use watershed, FD-36 (Northumberland County, south central Pennsylvania); a 39.5-ha subwatershed in the Mahantango Creek, which discharges into the Susquehanna River and ultimately the Chesapeake Bay.
In late March 2000, three overland flow plots described below, were set up in FD-36 on the high-P Berks loam (495 mg P kg-1 in the 0- to 5-cm depth) that was in no-till corn and received poultry manure for the last 8 yr (Table 1). Two simulated rainfalls, each of 6.5 cm h-1 for 30 min of runoff, were applied on consecutive days and overland flow volume, P concentration (dissolved, particulate, and total P), and sediment discharge measured, as described below. Rainfall and overland flow collection was repeated 1, 3, and 6 wk later. Metal plates delineating the overland flow plots were then removed and the field chisel plowed to a depth of about 25 cm, disked to prepare seedbed surface, and orchardgrass planted in mid-May 2000. Overland flow plots were then returned to the same landscape location with a survey transect and kept in place for the remainder of the study. Rainfall and overland flow collection, as described above, was then conducted 1, 2, 4, 8, 16, 20, 44, and 52 wk after plowing.
Three overland flow plots were installed on a high P Berks loam (411 mg kg-1) in no-till corn that was not plowed. In addition, three plots were set up in a low P Berks loam (25 mg kg-1) under orchardgrass in a field adjacent to the high P soil. Simulated rainfall was applied to the unplowed low P plots on the same day as the plowed high P Berks plots, but during the same period on the unplowed high P Berks plots. The low P plots were not plowed and served as a control, with plot borders remaining in place throughout the study. Data presented in this paper represent the average of flow-weighted concentrations for each of the two events and three plots for both plowed and unplowed soil.
Soils were sampled before and after plowing by taking six 5-cm diameter cores to a depth of 100 cm, air-dried, composited in depth increments as described earlier, sieved (2 mm), and stored for analysis.
Overland Flow Plots and Rainfall Simulation
Overland flow plots, each 1 by 2 m, with the long axis orientated down the slope, were delineated by metal borders installed 5 cm above and below ground level to isolate overland flow. The slope of all plots was between 4 and 5%. Rainfall was applied to each plot with one TeeJet 2HH-SS50WSQ1 nozzle approximately 2.5 m above the soil to achieve terminal velocity (Sharpley et al., 2002). The nozzle, associated plumbing, in-line filter, pressure gauge, and electrical wiring are mounted on a 3 by 3 by 3 m aluminum frame, fitted with canvas walls to provide a windscreen. A coefficient of uniformity of 85% was obtained for rainfall over a 2-m2 footprint, which encompasses one pair of abutting plots. A rainfall intensity of 6.5 cm h-1 was applied to the plots until 30 min of runoff was obtained. This rainfall intensity and duration has an approximate 10-yr return frequency in south central Pennsylvania. Local ground water was used as the water source for the simulator, and had a dissolved reactive P concentration of <0.01 mg L-1, total P of <0.02 mg L-1, nitrate-N of 3.1 mg L-1, and pH of 5.7.
Overland flow was collected in metal gutters at the downslope edge of each plot and pumped to 200-L (50-gallon) plastic containers. Total overland flow was measured by weighing the containers. A runoff sample was collected from each container after thorough mixing and agitation, and a subsample was immediately filtered (0.45 µm) and stored at 277 K. Filtered samples were analyzed within 24 h of collection and unfiltered samples no more than 7 d after the completion of the rainfall simulation. All methods used in plot design and installation, rainfall simulation and runoff collection, and analysis, follow protocols detailed in the National Phosphorus Research Project (National Phosphorus Research Project, 2002).
Chemical Analyses
Soil Samples
All analyses were conducted on air-dried and sieved (2 mm) samples. Soil sand, silt, and clay contents were determined by hydrometer after dispersion with sodium hexametaphosphate (Gee and Bauder, 1986). Organic C was determined by combustion using a Leco C/N analyzer and pH using a glass electrode at a 1:2.5 soil/water ratio (w/w). Values of these properties for the 0- to 5- and 5- to 20-cm depths are given in Table 2
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Phosphorus sorption isotherms were constructed using the procedure of Nair et al. (1984). One gram of the soil mixture was shaken with various additions of P (0–500 mg kg-1 added as KH2PO4) in 25 mL of 0.01 M CaCl2 on an end-over-end shaker at 298 K. After 24 h, the soil suspensions were centrifuged and filtered (0.45 µm) and the solution P concentration (C) determined. The amount of P sorbed (X) is the difference between P added and P remaining in solution. Using the Langmuir sorption equation, soil P sorption maximum was calculated as the reciprocal of the slope of the plot C/X vs. C and binding energy as slope/intercept of the same plot (Syers et al., 1973). The sorption saturation of each soil was calculated as the percentage of soil P sorption maximum as Mehlich-3 extractable P (Kleinman and Sharpley, 2002) as below:
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Phosphorus in all filtrates and neutralized extracts and digests was measured by the colorimetric method of Murphy and Riley (1962).
Water Samples
The concentration of dissolved reactive P (subsequently referred to as dissolved P) in overland flow was determined for 0.45 µm filtered sample. The concentration of both total dissolved and total P was determined on filtered and unfiltered runoff samples, respectively, following digestion with a semi–micro-Kjeldahl procedure (Bremner and Mulvaney, 1982). Phosphorus in all filtrates and neutralized digests was measured by the colorimetric method of Murphy and Riley (1962). Particulate P was calculated as the difference between total P and total dissolved P. The suspended sediment concentration of each overland flow event was measured in duplicate as the difference in weight of 250 mL aliquots of unfiltered and filtered (0.45 µm) runoff samples after evaporation (378 K) to dryness.
Statistical Analysis
All soil incubations, extractions, and sorption isotherms were conducted in duplicate, with means presented in this paper. In all cases, standard deviations were <2.5% of the mean value. Results of the overland flow studies are presented as means of triplicate treatments, with standard deviations of individual data <5% of mean values. Statistical analyses (t tests, means, and standard errors) were performed with SPSS v10.0 (SPSS, 1999). All r2 values given are significant at the P < 0.05 level.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Because of the pronounced surface stratification of Mehlich-3 P concentration and P sorption properties in manured soils, it is possible that mixing surface and subsoil material by profile inversion or plowing, may dilute and/or sorb P from high P surface layers. Thus, surface 5 cm and composite 5- to 20-cm soil samples were mixed and incubated in varying proportions to assess the potential for plowing to minimize soil P stratification and overland flow P enrichment.
Sample Mixing and Soil Phosphorus
Results for mixing soil from 0- to 5- and 5- to 20-cm depths in a 1:3 ratio are discussed in more detail than other combinations, as this ratio attempts to simulate the degree of horizon mixing during plowing to an approximate depth of 20 cm.
Mehlich–3 Extractable Soil Phosphorus
Mixing 0- to 5- and 5- to 20-cm samples decreased Mehlich-3 soil P compared with surface soil concentrations (Fig. 2)
. As expected, this decrease was greater with an increase in the proportion of subsoil mixed with surface soil. Similar trends were obtained with the other soils, which are not shown. There was an average decrease in surface soil Mehlich-3 P of 309 mg kg-1 (80% decline) when P-rich surface (0–5 cm) and low-P subsoil were mixed in a 1:3 ratio (Table 3).
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The Mehlich-3 P concentration of surface and subsoil mixture (1:3 ratio) was related to the weighted mean of premixed surface and subsoils (r2 = 0.87; Fig. 3)
. The weighted mean of these two depth samples was calculated as:
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Phosphorus Sorption Properties
Phosphorus sorption consistently increased with an increase in the proportion of subsoil mixed with surface soil (Fig. 5)
. All soils exhibited the same effect of soil–mixture composition on P sorption isotherms, as shown by Hagerstown soil (fine, mixed, semiactive, mesic Typic Hapludalfs) and Ruston soil (fine-loamy, siliceous, semiactive, thermic Typic Paleudults) in Fig. 5. The effects of surface-subsoil mixing (1:3 ratio) was reflected in significantly greater (P < 0.01) P sorption maxima (avg. 457 mg kg-1) and binding energy of sorption (1.05 L kg-1) than of surface soil (223 mg kg-1 and 0.08 L kg-1, respectively; Table 3). Averaged across all soils, there was a 49% increase in P sorption maximum and 1914% increase in binding energy relative to P-stratified surface soil, after mixing to approximate plowing. As a result of the changes in Mehlich-3 P and P sorption with mixing, the P sorption saturation of mixed surface and subsoil (avg. 15%) was significantly lower than surface soil (avg. 60%) (P < 0.01; Table 3).
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Plowing and Overland Flow Phosphorus
The dissolved P concentration of overland flow from the high P Berks soil averaged 2.9 mg L-1 for three rainfalls in 8 wk before plowing. During these flow events, particulate P averaged 0.5 mg L-1 and total P averaged 3.4 mg L-1. Overland flow P from the no till plots was mostly dissolved P (85%). The concentration of P from the high P soil was appreciably greater (P < 0.01) than from the low P plots, which averaged 0.02, 0.07, and 0.09 mg L-1 as dissolved, particulate, and total P, respectively (Fig. 6)
. Overland flow volume and sediment concentration were not significantly different (P > 0.05) from high and low P Berks plots (Table 4)
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There was no change in dissolved P concentration of the first rainfall–overland flow event after plowing (2.86 mg L-1) compared with before plowing (2.90 mg L-1) (Fig. 6). However, particulate P increased dramatically after plowing. This increase can be attributed to a 30-fold rise in sediment concentration of overland flow (0.22–6.58 g L-1). The greater sediment and associated P discharge occurred in spite of a lower overland flow volume after plowing (17 L) than before (31 L) (Table 4), likely due to a greater infiltration rate afforded by plowing (Gaynor and Findlay, 1995; Soileau et al., 1994).
With time after plowing, there was a steady decline in overland flow P and sediment concentration as orchardgrass became established, protecting the soil surface from P release and soil detachment and entrainment (Fig. 6). In contrast, there was no decrease in the concentration of P in overland flow from the unplowed high P Berks soil (Fig. 6). One year after plowing, dissolved P (0.07 mg L-1) and particulate P (0.28 mg L-1) concentrations were much lower than before plowing (2.90 and 0.50 mg L-1) and from the unplowed high P Berks plot (2.40 and 0.52 mg L-1), but elevated compared with low-P plot concentrations (0.025 and 0.076 mg L-1). Further, overland flow volumes were similar to preplowing and unplowed plot volumes about 20 wk after plowing (28 wk time in Table 4). The loss of P and sediment in overland flow showed similar trends to concentration (Table 4).
The effect of plowing on particulate P entrainment in overland flow and the erosional process was evaluated by calculating the P enrichment ratio (PER) as the P concentration of sediment discharged (mg particulate P kg sediment-1) divided by that of source soil (mg total P kg soil-1). Phosphorus ERs were greater for overland flow from the high-P plots before plowing (avg. 2.71) than from the low-P unplowed plots (avg. 1.76; Table 4). This is consistent with a general increase in PER with increased soil P concentration (Sharpley, 1985). Immediately following plowing, PER decreased to 2.12, reflecting the greater propensity for sheet erosion with little particle size–sorting during transport (Table 4). With time after plowing, however, there was a gradual increase in PER as vegetative cover increased along with the selective entrainment and transport of finer, P-enriched particles.
For all plots, PER decreased with an increase in sediment discharge of each overland flow event (Fig. 7) . This relationship reflects the degree to which particle-size selection and preferential transport occurs with overland flow. As sediment discharge increases, there is less particle-size sorting by overland flow, less clay-sized particles are transported in proportion to total soil loss, and P enrichment, thus, decreases. Clearly, plowing a P-stratified soil and then establishing a grass cover to reduce the risk of P loss in overland flow, influences the amount and P enrichment of eroding particles as well as the release of P from surface soil. Although the increase in subsoil P may enhance downward movement of P, the disruption of contiguous macropores during tillage has been shown to decrease percolation (McGregor et al., 1999; Rhoton et al., 2002).
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Over time, plowing a P-stratified Berks soil decreased total P loss in overland flow compared with an unplowed high P soil. Even so, the potential long-term benefits of plowing P-stratified soils on overland flow P should be considered along with the near-term exacerbated risk of erosion and associated P loss. Once the vegetative cover became established (orchardgrass in this study) and erosion minimized, particulate P in overland flow was also decreased. The potential of plowing a given P-stratified soil on overland flow P may be determined from the weighted mean Mehlich-3 P concentration of surface and subsoil samples.
This combined laboratory incubation and field study demonstrates the potential benefits of plowing P-stratified soils, as long as erosion is minimized after plowing. Thus, plowing P-stratified soils may reduce the long-term loss of P in overland flow and provide landowners an additional option in keeping these soils in production under P-based nutrient management strategies.
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NOTES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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