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USDA-ARS, Pasture Systems and Watershed Management Research Laboratory, Building 3702, Curtin Road, University Park, PA 16802-3702
* Corresponding author (ans3{at}psu.edu)
Received for publication January 31, 2001.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: RUSLE, Revised Universal Soil Loss Equation
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Three management options for land application of P are proposed in the new strategy: (i) managing P based upon agronomic soil P thresholds, so that P applications are based on crop needs; (ii) managing P based upon environmental soil P thresholds, by identifying a critical soil P concentration above which runoff P enrichment is unacceptable; and (iii) using a P index to limit P applications on fields at greatest risk for P loss (USDA and USEPA, 1999). Both agronomic and environmental soil P threshold approaches provide narrow, often incomplete, assessments of the risk of P loss, as variables other than soil P concentration control losses from fields and landscapes (Sharpley et al., 1996; Sims, 2000). Illustrating this point, Sharpley and Tunney (2000) found that the dissolved P concentration of surface runoff from 2-m2 plots using simulated rainfall (50 mm h-1 for 30 min; 5-yr return period) varied from 0.20 to 0.49 mg L-1 over a 2-ha field. Elsewhere, Pote et al. (1996) measured total P losses of 0.05, 0.16, 0.35 kg P ha-1 from three sites with a Mehlich-3 soil test P ranging from only 285 to 295 mg kg-1, due to varying erosion susceptibilities among sites. Thus, the USDA Natural Resources Conservation Service is recommending at a national level that the P index be used in development of P-based nutrient management plans (USDA and USEPA, 1999).
The P index accounts for transport and source factors controlling P loss (Lemunyon and Gilbert, 1993; Gburek et al., 2000). Transport factors include erosion, surface runoff, and subsurface flow, and whether the field is connected and flow contributes to stream discharge. Source factors are soil test P concentration and the form, rate, method, and timing of applied P. Overall, a field is ranked as highly vulnerable to P loss when high P availability due to soil test P concentrations and/or P application in fertilizer or manure coincides with high surface runoff or erosion potential. In some cases, where subsurface P transport is important, preferential flow through soil macropores is also considered as a factor (Leytem et al., 1999).
The P index is intended to serve as a practical screening tool for use by extension agents, watershed planners, and farmers to identify agricultural areas or management practices that have the greatest potential to accelerate eutrophication. As such, the P index identifies alternative management options available to land users, providing flexibility in developing remedial strategies.
The P index was not originally developed as a quantitative predictor of P loss from a watershed. Rather, it was designed to serve as a qualitative assessment tool that ranks site vulnerability to P loss, helping to identify and prioritize P management options. Ultimately, the P index serves as an educational tool that facilitates interaction between planners and farmers, helping to elucidate the water quality implications of management decisions.
Although there is a great deal of research that justifies the transport and source factors included in the P index, there has been little site evaluation of the index ratings. The P index has been used to assess the potential for P loss in several regions including the Delmarva Peninsula (Leytem et al., 1999; Sims, 1996), Oklahoma (Sharpley, 1995), Texas (McFarland et al., 1998), Vermont (Jokela et al., 1997), and Canada (Bolinder et al., 1998). However, few comparisons of P index ratings and measured P loss have been made. In Nebraska, Eghball and Gilley (1999) found correlation coefficients between total P loss from simulated rainfall–runoff plots and P index ratings as high as 0.84, when erosion factor weighting was increased from 1.5 to 7.5.
This paper describes a quantitative evaluation of soil P and components of a P index approach to assess site vulnerability to P loss. For each approach, estimates of site vulnerability to P loss were compared with measured losses of P in surface runoff within a mixed–land use watershed in south central Pennsylvania (the FD-36 watershed). In addition, site-specific observations were extrapolated to all fields in the watershed, enabling a watershed-scale comparison of the management implications of both approaches.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Surface Runoff Sites and Rainfall Simulation
Surface runoff sites were comprised of two abutting plots, each 1 by 2 m, with the long axis orientated down the slope (Fig. 3) . At each site, the abutting plots were considered replicate observations and data from the two plots were averaged to give a single site value. Soil slope was measured at each plot location and used in the calculation of erosion and surface runoff factors of the P index (Table 2). Metal borders were installed 5 cm above and below ground level to isolate surface runoff, which was diverted by a downslope gutter to a collection vessel. A representative surface soil sample (0–5 cm depth) was obtained after rainfall simulation by collecting 10 cores (2.5-cm diameter) from within each plot. Soil cores were air-dried and sieved (2 mm), and equal amounts were combined and thoroughly mixed to give a representative bulk soil sample for each plot.
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Water pressure at the nozzle was regulated to 28 kPa (4.1 psi) to establish a water flow rate of 126 mL s-1 through the nozzle. Shelton et al. (1985) found that this pressure gave the best coefficient of uniformity and produced drops with size, velocity, and impact energies approximating natural rainfall. For the simulator used in the present study, a coefficient of uniformity of 85% was obtained for rainfall over the 2-m2 footprint of the abutting plots. A rainfall intensity of 70 mm h-1 for 30 min after runoff initiation was used. This rainfall intensity and duration has an approximate 10-yr return frequency in southcentral Pennsylvania. Three simulated rainfall events were applied, with data presented in this paper representing the average of flow-weighted concentrations for each of the three events.
Surface runoff was collected in metal gutters at the downslope edge of each plot and pumped to 200-L (50-gallon) plastic containers. Total surface runoff was measured by weighing the containers. A runoff sample was collected from each container after thorough mixing and agitation, and a subsample immediately filtered (0.45 µm) and stored at 4°C. 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 (2001).
Rainfall–surface runoff simulations were conducted between April and November 1999. For sites in fields where manure had been applied, simulations were conducted within three weeks of manure application. All other simulations were conducted in fields where no P had been applied in the last six months. The timing of the simulations was designed to distinguish between the effects of soil P and applied manure on runoff P, as well as the ability of the two site assessment approaches (soil P and P index) to represent these effects.
Chemical Analyses
Mehlich-3 soil P concentration was determined by extraction of 1 g soil with 10 mL of 0.2 M CH3COOH, 0.25 M NH4NO3, 0.015 M NH4F, 0.013 M HNO3, and 0.001 M EDTA for 5 min (Mehlich, 1984). The concentration of dissolved reactive P (subsequently referred to dissolved P) in surface runoff was determined for a 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 semimicro Kjeldahl procedure (Bremner and Mulvaney, 1982). Phosphorus in all filtrates and neutralized extracts and 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 surface runoff event was measured in duplicate as the difference in weight of 250-mL aliquots of unfiltered and filtered runoff samples after evaporation (105°C) to dryness.
The Phosphorus Index
Site vulnerability to P loss in runoff is assessed by selecting rating values for a variety of transport (Table 3) and source factors (Table 4). We derived site vulnerability ratings by applying a P index developed for Pennsylvania to runoff plots in FD-36 based upon the following transport and source factors.
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Surface runoff class was assigned from the relationship between soil permeability class and slope detailed in the Soil Survey Manual (Soil Survey Staff, 1993) (Tables 2 and 5). Subsurface transport of P in FD-36 was assumed to be small relative to transport in surface runoff (Sharpley et al., 1999b), and index values for leaching potential were set at low (i.e., zero, Table 3). As the connectivity to the stream channel could not be assessed for surface runoff generated by the rainfall simulator, we assumed that all sites were connected to the channel and contributed equally to stream flow (i.e., connectivity factor was set at 8 for all sites, Table 3).
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Source Factors
Calculation of source factors of the P index are based on the Mehlich-3 P concentration of surface soil samples collected at each site and P application as fertilizer or manure as determined from annual farmer surveys (Tables 1 and 4). The correction factor of 0.2 for soil test P is based on field data from the FD-36 watershed, which showed a fivefold greater concentration of dissolved P in surface runoff with an increase in fertilizer or manure addition compared with an equivalent increase in Mehlich-3 P (Sharpley and Tunney, 2000).
Phosphorus Index Value of a Site
A P index value, representing cumulative site vulnerability to P loss from each site, is obtained by multiplying summed transport and source factors (Table 6). Phosphorus index values are then normalized so that the division between high and very high categories is 100. This is done by calculating a site P index value, with all transport and source factors set as high. Specifically, erosion was set at 7 Mg ha-1, considered a high value for Pennsylvania, and soil test P was set at 200 mg Mehlich-3 P kg-1, proposed as a non–site specific threshold for Pennsylvania (Beegle, 1999; McDowell and Sharpley, 2001; also see Fig. 4)
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Statistical Analyses
The relationship and change point between the dissolved P concentration of surface runoff and Mehlich-3 soil P concentration were determined using a split-line model that describes two linear relationships whose slopes are significantly different from each other (at p < 0.05) on either side of a threshold (McDowell et al., 2001; McDowell and Sharpley, 2001). Below the threshold:
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The similar relationship between dissolved and total P concentration of surface runoff and Mehlich-3 soil P concentration (Fig. 4) is due to the fact that dissolved P represented the major proportion of total P transported in surface runoff from most sites (an average 64%). This was the case even though measured erosion ranged from 2 to 10 Mg ha-1 and RUSLE estimated that 20% of the sites were above the high P index value of 7 Mg ha-1. The above differences in erosion among sites contributed to a lower correlation coefficient for Mehlich-3 P concentration and total P (R2 = 0.80) than dissolved P (R2 = 0.86) (curvilinear model of Fig. 4).
The concentration of dissolved and total P in surface runoff was not related to Mehlich-3 soil P for fields that had received P as fertilizer or manure within three weeks of rainfall simulation (p > 0.05; Fig. 4). As expected, both dissolved and total P concentrations in runoff increased with increasing rates of applied P. The average dissolved P concentration in runoff was 1.06 mg L-1 from sites receiving 56 kg P ha-1 (as triple superphosphate), 1.76 mg L-1 P from sites receiving 112 kg P ha-1 (as swine slurry), and 2.42 mg L-1 from sites receiving 150 kg P ha-1 (as poultry manure). Average total P concentrations from these sites were 1.67, 2.56, and 3.36 mg L-1, respectively (Fig. 4).
As with P concentrations, the loss of dissolved and total P in surface runoff from sites not recently fertilized or manured was related to Mehlich-3 soil P concentration (data not presented). However, correlation coefficients were consistently lower for P loss than for concentration, due to the variability in surface runoff volume among sites (8–70 L event-1). For example, correlation coefficients for Mehlich-3 P and dissolved and total P concentration were 0.86
and 0.80 
, respectively, while coefficients for dissolved and total P loss were 0.60 
and 0.55 
, respectively (Fig. 4).
Ranking Site Vulnerability to Phosphorus Loss
Phosphorus index ratings for each site were closely related to both concentration and loss of dissolved and total P in surface runoff from all sites (Fig. 5 and 6)
. Consideration of site potential for runoff and erosion, P application rate and method, and Mehlich-3 soil P concentration, effectively described surface runoff P concentration and loss from recently fertilized and manured fields, as well as from untreated fields. Clearly, the P index described P loss potential from a wider range of land management conditions than did Mehlich-3 soil P alone (Fig. 4).
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Management Implications
Of the 57 surface runoff sites evaluated in FD-36, 32% were ranked as having a low vulnerability to P loss, 21% were medium, 18% were high, and 29% were very high. From these vulnerabilities, implications for nutrient management in FD-36 can be made considering the Animal Feeding Operations strategy established for Pennsylvania (Beegle and Sharpley, 1999). In this strategy, nutrient management recommendations are established from either P index or soil test P threshold options, as outlined in Table 7. In the case of the P index, sites ranked at low and medium vulnerability to P loss could continue to receive manure applications based on crop N requirements. Sites ranked as high could receive manure applications, but on a crop P removal basis only, and sites ranked as very high would not be eligible to receive manure additions (Table 7). When using the soil P threshold option, manure application rates are based on crop removal of P above a Mehlich-3 soil P threshold of 200 mg kg-1 (see Fig. 4).
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Differences in the implications of applying the two P management options are apparent. For example, Field 23 was ranked as very highly vulnerable to P loss due to high surface runoff and erosion potential and a recent history of P application (112 kg P ha-1 swine slurry). However, because Mehlich-3 soil P concentration was only 65 mg kg-1, the soil P threshold option recommended N-based manure applications (Table 8). Indeed, mean dissolved P and total P runoff concentrations from sites in this field were 2.42 and 3.36 mg L-1, respectively.
Overall, manure applications would be restricted on fewer fields using the P index (five fields totaling 3.96 ha) than the soil P threshold option (eight fields totaling 7.6 ha). For instance, manure applications to those fields that were part of this evaluation were projected based on the index and soil P threshold options of the Pennsylvania nutrient management strategy (Table 7). It was assumed that on a N basis, poultry manure would be applied to meet the N requirements of a 3 Mg ha-1 corn crop on all fields except 9. On Field 9 (pasture), a locally representative manure application rate of 9.6 Mg ha-1 (4 tons acre-1) was assumed (Beegle, 1999). Overall, however, the total amount of poultry manure that was projected to be applied to the 11 fields studied was 71 Mg with the P index option and 39 Mg with the soil P threshold option (Table 8).
The smaller area of the watershed targeted for P management by the P index (23%) compared with soil P threshold option (51%) is consistent with measured P loss from FD-36. For example, the mean annual flow-weighted concentration of dissolved and total P in stream flow from FD-36 for 1996 to 1999 was 0.05 and 0.075 mg L-1, respectively (Pionke et al., 1999; Sharpley et al., 1999b). These levels are below eutrophic criteria (0.1 mg L-1 as total P) established for streams or other flowing waters not discharging directly into lakes or impoundments (Dodds et al., 1998; USEPA, 1994). Based on the level of water quality impairment of FD-36, in terms of P loss criteria, there is little justification for major changes in P management at a watershed scale at the present time. Thus, a P index strategy may be the most prudent management approach, given the relatively low concentration of P in stream flow, as long as targeted conservation measures reduce the potential for P loss during high-risk periods (e.g., storm flow and after land application of manure or fertilizer).
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Clearly, the P index recently adopted by the Natural Resources Conservation Service's P-based nutrient management strategy accurately described site vulnerability to P loss. Further, based on the relatively small export of P from the studied watershed, manure management projections using a P index were more defensible than the restrictive projections derived from a soil P threshold.
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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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