December 2000 Vol. 165, No. 12 Printed in U.S.A.
0038-075C/00/ 16512-943-950 Soil Science Copyright © 2000 by Lippincott Williams & Wilkins, Inc.
USING SOIL PHOSPHORUS BEHAVIOR TO IDENTIFY ENVIRONMENTAL THRESHOLDS Peter J.A. Kleinman', Ray B. Bryant 2 , W. Shaw Reid 2 , Andrew N. Sharpley,' and David Pimentel3 Concern over the transport of phosphorus from agricultural soils to surface waters has focused attention on the role of soil phosphorus in environmental risk assessment. This study explores the existence of natural soil phosphorus thresholds as expressed by Quantity/Intensity relationships. Fifty-nine samples, collected from agricultural soils in New York's Delaware River Watershed, were analyzed for Morgan, Mehlich III, and 0.01 M CaC12 extractable P. Soil P sorption saturation was calculated as a function of oxalate extractable P, Fe, and Al. In addition, P sorption isotherms were determined for all soils. Thresholds in the relationships between CaCl2 P and Morgan P, Mehlich III P, and P sorption saturation were identified by segmented linear regression (change point analysis). Thresholds in the relationship between CaC1 2 P and Morgan P, Mehlich III P, and P sorption saturation occurred at CaCl2 P concentrations of 0.9 mg kg, suggesting a threshold for soil P that may have use in environmental risk assessment. A P sorption threshold was also identified by segmented, quadratic-linear regression of the sorption isotherms. Results described a fundamental property of soils: a nonlinear sorption of P in soils that exhibits a threshold, above which the potential for P release from soil to water increases. This threshold describes a critical point in the release of P and, therefore, may be of environmental importance in estimating the potential for soluble P loss from soil by runoff and leaching.(Soil Science 2000;165:943-950) Key words: Phosphorus, sorption, change point, threshold, isotherm.
ronmental thresholds of soil P (i.e., critical levels of soil P, above which the environmental availability of P is unacceptable) are currently determined use either agrononiic or water quality criteria. The agronomic approach establishes agronomic soil P standards as environmental thresholds and is the approach that has been commonly adopted by state agencies (Sharpley et al., 1996; Sibbeson and Sharpley, 1997; Sims, 1999). This approach is based on the rationale that soil P in excess of crop requirements is vulnerable to removal by surface runoff or leaching. Because agronomic guidelines already exist for soil P (i.e., soil test recommendations), this approach requires little investment in research and can be readily implemented. However, a major problem with equating agronomic and environmental thresholds is that the controlling processes by which plants access soil P are quite different from those that determine soil P availability for removal by runoff.
GRICULTURAL soils are a major contributor A of nutrients, particularly phosphorus (P), to
US surface waters (U.S. Environmental Protection Agency, 1996; U.S. Geological Survey, 1999). Concern about accelerated eutrophication has prompted several states to identify levels of soil P above which P-oriented management is required to control soil I' loss (Sharpley et al., 1996). As these and other P-based recommendations expand, much attention has focused on the paucity of site-specific data linking soil P with the potential for P loss (Sharpley et al., 1999). The two general approaches by which enviUSDA.ARS, Pasture Systems and Watershed Management Research Laboratory, Curtin Road, University Park, PA 16802-3702. Dr Kleinman is corresponding author. E-mail:
[email protected] t Dept. of Crop and Soil Sciences, Cornell University, Ithaca, NY 14853 'Dept. of Entomology, Cornell University, Ithaca, NY 14853. Received Feb 22, 2000; accepted Aug. 1, 2000.
943
944KLEINMAN, BRYANT, REID, SHARPLEY, AND 1IMENTEL SOIL SCIENCE The second approach to determining soil P thresholds, the water quality approach, requires correlation of soil P with measures of P in runoff or ground water so that water quality standards can be used to establish soil P thresholds (Pote et al., 1996, 1999; Sharpley et al., 1996). This approach has been used in the Netherlands, where soil P sorption saturation (P,,) serves to guide agricultural P management, and is based on research linking P,, t with groundwater P concentrations (Breeuwsma and Silva, 1992). We propose a third approach to setting environniental thresholds of soil P, a soil chemical approach. This approach identifies existing thresholds in P sorption (soil chemical behavior) that control the transfer of P from soil to water. Soil P thresholds identify the point beyond which a continued increase in soil P or in P additions poses significantly greater risks of P loss in runoff. Soil chemical thresholds may be viewed as sorption saturation phenomena, delineating a critical level of soil P loading above which a significantly larger proportion of added P is not sorbed but remains as soluble P and is readily available to transport pathways. For instance, Heckrath et al. (1995) related soil test P (Olsen I') to P in tile drain effluent from fields at Broadbalk. They identified a "change point" in the relationship. Effluent P concentrations were negligible at Olsen P levels below the change point. Above the change point, incremental increases in soil test phosphorus corresponded with large rises in effluent P concentrations. Building on this research, Hesketh and Brookes (2000) identified a change point between Olsen P and CaC1 2 extractable P (CaC1 7 P). The Olsen P values at the change points reported in both studies were approximately the same. They concluded that the Quantity/Intensity relationship presented by Olsen P (representing sorbed P, or the Quantity factor) and CaC1 2 P (representing solution P, or the Intensit y factor) could be substituted for that of Olsen P and leachate P in order to identify leaching potential. Thus, the change point represents a threshold that may be of environniental significance. Phosphorus Quantity/Intensity relationships range from P sorption isotherms that are generated under experimental conditions in the laboratory to multiple comparisons of P Quantity/ Intensity indicators from a population of soil samples. The soil properties known to affect P Quantity/Intensity relationships include, but are not limited to, pH (Barrow, 1984; Muljadi et al., 1966), mineralogy (Scheinost and Schwertmann, 1995), organic matter content (Sample et al.,
1980), and soil P additions (Indiati et al., 1995; Nair et al., 1998). The effect of these properties on P sorption isotherms has been quantified using various models ranging from Langmuir and Freundlich equations to curve splitting models (e.g., Ryden et al., 1977). This study explores the existence of natural thresholds in soil chemical behavior observable in P Quantity/Intensity relationships. The specific objective of this study was to identify this threshold via various P Quantity/Intensity relationships for soils collected from the Delaware River Watershed of New York State and to relate theni to the P isotherm, which describes P sorption behavior of soils. MATERIALS AND METHODS Study A rca The study was conducted in the Delaware River Watershed of New York State (42'21'N, 74'52'W), part of the Glaciated Allegheny Plateau and Catskill Mountain Region (Major Land Resource Area 140), a subregion of the Northeastern Forage and Forest Region (Fig. 1) (Soil Conservation Service, 1981). Land use in the watershed is dominated by forests (59% of area) and agriculture (27% of area), which is primarily dairy farming (Schneiderman et al., 1998). Sample Collection and Processi,i Fifty-nine samples obtained from agricultural soils within the watershed represent the following soil taxonomic subgroups: Typic Fragiudepts (Bath, Mardin, Lewbeach, Willowernoc, Lackawanna, Wellsboro, Willdin series); Typic Dystrudepts (Chenango, Lordstown, Tunkhannock, Oquaga series); Fluvaquenoc Dystrudepts (Basher series); Fluventic Dystrudepts (Barbour series); Aquic Fragiudepts (Onteora series); and Acne Fragiaquepts (Ontusia series). Because the parent materials are glacial deposits derived from similar
Eastern United States Delaware River ( Watershed en \ uk vILRA 14(1 Pennsylvania
Fig. 1. Map of Eastern United States identifying location of MLRA 140 and New York's Delaware River Watershed.
VOL.
165 ' No. 12 ENVIRONMENTAL THESHOLDS OF SOIL PHOSPHORUS 945 TABLE 1 Selected properties of soils collected from Delaware River Watershed P sorption threshold
Soil series Texture pH SOM Morgan's P Mehlich III P CaCI, P Sorbed P Solution P (55) (rug kg- ') (mg kg 1) (mg kg ') (55) (mg kg ') (ing kg ') Barbour Loam 3.8 6 1 Barbour Loam 6.1 4 4 Barbour Loam 5.8 5 27 Barbour Loam 5.9 5 30 Barbour Loam 5.9 7 38 Basher Silt Loans 6.4 3 9 Basher Silt Loam 6.2 4 10 Basher Silt Loans 6.4 5 II) Bath Silt Loans 5.4 6 1 Bath Silt Loam 5.4 6 1 Bath Silt Loam 5.9 6 1 Chenango Silt Loam 6.4 5 2 Lackawanna Silt Loam 5.4 6 1 Lackawanna Silt Loam 5.3 8 1 Lewbeach Silt Loam 5.8 5 1 Lewbeach Silt Loans 5.2 7 1 Lewbeach Silt Loans 5.5 7 1 Lewbeach Silt Loans 6.2 8 3 Lewbeach Silt Loam 6.3 7 3 Lewbeach Silt Loam 5.6 7 13 Lewbeach Silt Loam 6.2 4 19 Lordstown Silt Loam 3.5 6 2 Mardin Silt Loam 6.3 7 7 Mardin Silt Loam 6.7 6 36 Onteora Silt Loam 5.3 6 1 Onteora Silt Loam 6.5 3 5 Ontusia Silt Loam 6.1 5 22 Oquaga Silt Loam 6.6 5 12 Tunkannock Loam 6.2 4 5 Tunkannock Loam 6.3 4 11 Tunkannock Loam 6.6 4 16 Tunkannock Loam 6.6 3 19 Tunkarinock Loans 6.1 7 21 Tunkannock Loam 6.4 4 31 Tunkannock Loam 6.6 5 38 Tunkannock Learn 6.3 4 39 Tunkannock Loam 6.9 3 45 Tunkannock Loam 6.4 6 57 Wellsboro Silt Loam 6.0 7 5 Welisboro Silt Loam 6.8 6 9 Wellsboro Silt Loam 5.3 6 11 Welisboro Silt Loam 6.6 6 12 Wellsboro Silt Loam 6.4 4 22 Wellsboro Silt Loans 7.0 5 23 Welisboro Silt Loam 6.6 4 31 Wellsboro Silt Loans 6.4 7 60 Willdin Silt Loam 5.8 7 1 Willowemoc Silt Loam 5.5 7 1 Willowenioc Silt Loam 3.5 7 1 Willowemoc Silt Loam 5.8 7 1 Willowemoc Silt Loam 5.8 7 2 Willowemoc Silt Loam 5.3 5 2 Willowensoc Silt Loam 5.3 6 3
2 0.04 4 460 1.6 20 0.29 12 211 2.8 216 6.14 39 77 8.6 210 6.46 43 77 8.9 290 6.52 38 107 4.7 44 0.35 10 86 5.5 79 0.49 27 116 4.8 77 0.26 18 172 5.6 3 0.20 4 432 2.2 4 0.06 6 657 8.4 6 0.19 7 328 4.0 9 0.05 9 489 4.4 6 0.07 5 362 2.3 5 0.09 6 528 1.7 12 0.05 4 531 2.0 3 0.09 6 420 1.2 6 0.11 3 441 1.0 10 0.19 9 282 2.5 10 0.13 7 442 5.3 105 1.16 22 214 4.0 176 0.40 32 113 0.9 10 0.07 7 433 3.9 31 0.31 14 209 2.9 145 1.32 26 137 7.6 5 0.30 5 409 3.9 19 0.14 14 121 2.0 129 1.82 25 174 7.2 55 0.57 19 186 2.0 27 0.07 14 179 13.7 89 0.65 21 133 10.1 92 0.51 20 191 16.0 113 0.96 20 137 7.6 116 0.73 25 276 5.0 197 3.46 30 78 8.5 207 4.36 35 63 4.0 221 4.84 30 72 4.0 227 3.76 41 108 22.6 266 3.85 34 105 3.2 24 0.38 10 230 8.7 41 0.09 15 275 4.7 130 1.43 27 221 11.3 53 0.80 19 342 6.4 152 1.32 32 259 18.5 88 0.33 13 352 9.5 125 1.53 27 59 5.8 265 8.81 41 76 7.9 11 0.05 5 468 1.0 4 0.05 5 538 2.1 6 0.34 5 631 12.6 6 0.11 6 474 1.2 10 0.27 7 306 1.7 20 0.07 12 405 2.9 14 0.02 8 533 3.8 continued
946
KLEINMAN, BRYANT, REID, SHARPLEY, AND PIMENTEL
SOIL SCIENCE
TABLE 1-Continued Selected properties of soils collected from Delaware River Watershed P sorption threshold Soil series Texture pH SOM Morgan's P Mehlich Ill P CaC1, P P, 0 Sorbed P Solution P (%) (nig kg 1) (mg kg- 1 ) (nig kg - 1 ) (%) (mg kg ) (mg kg') Willowemoc Silt Loam 6.2 5 4 Willowemoc Silt Loam 6.2 6 5 Willowemoc Silt Loam 5.7 5 10 Willowemoc Silt Loam 6.2 5 11 Willowemoc Silt Loam 6.2 5 21 Willowemoc Silt Loam 5.4 6 28
lithology, the mineralogy (dominantly illite with lesser concentrations of chlorite, smectite, kaolinite, and sesquiox.ides) and texture (loam and silt loam) of the surface horizons are relatively uniform across the sampled soils. These soils are considered to represent an association of soils which, in their native condition, had very similar chemical behavior with respect to P sorption. Soil P levels, however, vary widely as a result of diverse management histories: current soil P levels reflect the frequency and intensity of past dairy manure additions (Table I). At each of the 59 sampling locations, 15 to 20 soil cores were extracted from the soil surface (0-20 cm) with a 2.5-cm diameter stainless steel soil probe. These cores were mixed thoroughly to provide a composite sample representative of average soil conditions for each location. Samples were air dried, ground, and sieved (2 mm) before laboratory analysis. Laboratory Analysis
Morgan's extractable P, used commonly in New York for agronomic recommendations, was determined by shaking 10 g of soil in 50 rnL of Morgan's solution (1 N NaOAC buffered to pH 4.8) for 15 mm (Lathwell and Peech, 1964). Mehlich III extractable P, also used commonly for agronomic recommendations, was determined by shaking 2.5 g of soil in 25 mL of Mehlich III solution (0.2 NCH 3COOH + 0.25 NNH4NO3 + 0.015 NNH 4F + 0.013 NHNO 3 + 0.001 M EDTA) for 15 mm (Mehlich, 1984). Calcium chloride extractable P (CaC1 2 P) was determined by shaking 5 g of soil in 20 mL of 0.01 M CaC17 solution overnight. All extractions were run in duplicate. Mehlich III and Morgan's P concentrations were measured by rapid flow colorimetnc analyzer following molybdate complexation using stannous chloride as a reducing agent (Olsen and Sommers, 1982). Calcium chloride extract P concentrations were measured by the colorimetnc method of Murphy and Riley (1962). Soil P sorption saturation was nieasurcd for ill
31 0.22 13 318 8.8 30 0.08 13 386 7.6 72 0.23 20 272 3.3 74 1.15 13 161 2.6 138 0.59 28 167 2.9 191 5.66 30 85 4.6
samples. The role of P535 as an indicator of P loss potential derives from the observation that P 535 is strongly correlated to P desorption, such that P desorption increases at higher degrees of P53 (Sibbeson and Sharpley, 1997). To estimate P, duplicate subsamples were ground, sieved (150 Inn), and subjected to acid amnionium oxalate extraction following Ross and Wang (1993). Subsamples weighing 0.25 g were agitated in a reciprocal shaker in the dark for 4 h with 10 ml. of acid ammonium oxalate solution [0.1 M (NH4) 2C204.H20 + 0.1 M l-I,C 704.2H90]. The extracts were centrifuged (510y for 20 mm) and filtered (0.45 pm). Iron and Al concentrations were determined by atomic adsorption spectroscopy, whereas P concentrations were determined by the stannous chloride method described above (Olsen and Sommers, 1982). Molar concentrations of acid ammonium oxalate extractable P (P,), aluminum (Al0 .5), and iron (Fe0) were related in the equation, P 1 = ° X 100. (1) 0.5 (Fe., + Al_) An at of 0.5 was selected to allow direct comparison of P,35 estimates from this project with the equation used in the Netherlands (Breeuwsma and Silva, 1992). Phosphorus sorption isotherms were determined for each soil. Standard P solutions from 0 to 300 mg P L 1 were obtained by dissolving KH2PO4 in 0.01 M CaC13 . Each soil was equilibrated in a minimum of five standard solutions. Duplicate subsamples weighing 5 g were shaken for 24 h in 20 niL of standard solution (soil:solution = 1:4), allowed to settle overnight, and then filtered through Whatman no. 42 paper. Phosphorus concentrations in the filtrate were measured by the colorirnetric method of Murphy and Riley (1962). Statistical Analysis
Change points were assessed for P Quantitv/liitcnsitv rclstioiiships by split-litic iiiodc]
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12 ENVIRONMENTAL THESHOLDS OF SOIL PHOSPHORUS
following Heckrath et al., (1995). Associations between variables were assessed by least-squares regression (Neter et al., 1996). SAS's RESTRICT procedure was used to force regressions through expected points (SAS Institute, 1988). P sorption isotherms were interpreted by segmented, quadratic-linear regression using a modified version of SAS's nonlinear segmented regression (NUN). The quadratic and linear models were evaluated by least squares regression. The NUN procedure requires initial estimation of quadratic (a + bx + cx2) and linear (d + ex) model parameters (a, b, c, d, and e), and solves for the threshold between the quadratic and linear regressions by iterative re-evaluation of the equation (SAS Institute, 1988). As part of the NLIN modification, a positively sloped linear model was assumed. This assumption matches our observation that isotherm data do not plateau but exhibit a linear trend of shallow positive slope. RESULTS AND DISCUSSION Table 1 presents soil P data. In the following discussion, P,at, Morgan's P, and Mehlich III P are treated as indicators of P Quantity, whereas CaC12 P is used as an indicator of P Intensity. P Quantity/Intensity Change Paints A change point analysis was conducted on the relationship between Psat and CaC12 P (Fig. 2a). The two variables were positively related, and a change point was identified at P, t = 25% and CaC12 P = 0.9 mg kg' (R2 = 0.79, SE = 1.8). The 25% P sat value corresponds with the P value identified by the Dutch as a critical limit for soil P, which was established to protect water quality (Breeuwsma and Silva, 1992). Two agronomic soil tests, Morgan's P and Mehlich III P, were compared with CaCI 7 P to determine whether change points could be identified from the corresponding Quantity/ Intensity relationships. The change point for Morgan's P was identified at 16 mg kg t , with a corresponding CaC17 P of 0.9 mg kg - t (Fig. 2b; R2 = 0.70, SE = 8.1). In New York State, the agronomic classes of Morgan's P are as follows: Very Low < 0.5 mg kg'; Low = 0.5-1.5 mg kg'; Medium = 1.5-4mg kg- t ; High = 4-20 mg kg- I ; Very High > 20 mg kg (Anonymous, 1999). Thus, the change point for Morgan's P occurs at P levels near the upper end of crop requirements (i.e., the boundary between High and Very High). Similarly, a change point is clearly visible for Mehlich III P at 141 mg kg 1 , with a corresponding CaC19 P of 0.9 mg kg- 1 (Fig. 2c; R2 = 0.61, SE = 12.8). Although New York State does ,
947
10 Point P,, = 25%
Change 'no
cad 2 P
07,4
0.9 mg kg'
• a
V.
C-)
C-) 2 .. •+•.
0
0% 10% 20% 30% 40% 50%
to
change Morgan's
CaC1
'OO
Point
P = 16 mg kg'
2 P
0.9 mg kg`
07,4 C-)
2
0 10 20 30 40 50 60 70 Morgan's P (mg kg') Ii Changi_Point 'OO
. C
Mehlich Iii P = 1 41 mg kg'
cad 2 P = 0.9 mg kg'
07,4 C-)
0
* * ^Z'
0 50 tOO 150 200 250 300 350 Meh!ich Hi P (mg kg')
Fig. 2. Quantity/Intensity relationships for 'sat' Morgan's P, and Mehlich III P (Quantity factors) and CaCl 2 P (Intensity factor) with corresponding change points.
not currently provide agronomic recommendations based on Mehlich III P, Pennsylvania identifies Mehlich III P values > 50 ng kg - 1 as having no plant yield response (Beegle, 1999). Thus, the change point identified with Mehlich III P suggests the presence of a threshold P level in excess of crop requirements. All three of the change points occurred at CaC12 P values of 0.9 ng kg'. The consistency in the CaC17 P values of the change points bolsters the inference that this point represents a singular characteristic threshold in the behavior of soil P chemistry. Differences in the agronomic interpretation of the Morgan's P and Meblich III P change point values (High for Morgan's P, Excessive for Mehlich III P) may reflect soil-specific variability in soil P availability that is not represented adequately by single, state-wide agronomic thresholds. Even so, Quantity/ Intensity change points for both Morgan's P and Mehlich III P seem to occur near the agronomic threshold.
948
KLEINMAN, BRYANT, REID, SHARPLEY, AND PIMENTEL SOIL SCIENCE
P Sorption Isotherm Analysis 1000 P a We propose a threshold interpretation of P sorption isotherms that describes fundamental tion threshold =63mPkg" soil chemistry and is relevant to estimation of po250 tential for P loss through surface runoff or leaching. The basis for this approach is the assumption 1000 that a fundamental property of a soil with a low bI - Psa,=20% is a standard, Lor H-type Q/I curve (see 750 1sat sorption threshold = 272 mg P kg' McBride, 1994). Figure 3 illustrates such a curve a. 500 divided roughly into two integral stages: (i) an . 250 initial, steeply-sloped, fixation stage in which the proportion of soluble P additions that are sorbed 1000 is high; and (u) a saturated stage of gradual slope in which the proportion of soluble P additions 750 that are sorbed is significantly lower relative to 500 the first stage. Eventually, soluble P additions will 250 / n threshold =468mgPkg' no longer be sorbed, and the line slope will approach zero. 10 20 30 The sorption threshold for an "unsaturated" soil (i.e., a soil with both fixation and saturated Solution P (mg L') stages) is estimated by determining the point that joins the fixation and saturated stages of the sorp- Fig. 4. Sorption thresholds of three soils with varying detion isotherm. The ordinate value of this join grees of P sorption saturation. point, the sorbed P value, represents the remaining capacity of a soil to sorb soluble P before the surface runoff or leaching because of the lowered P sorption efficiency declines significantly. Figure 4 illustrates P sorption isotherms for P sorption efficiency. Although the P sorption three soils representing low (Fig. 4c), intermedi- threshold concept has important environmental ate (Fig. 4b), and high P,at (Fig. 4a). For all sorp- uses and implications, it does not assure that soltion isotherms presented in Fig. 4, 12 exceeded uble P loss standards will be achieved. Note that 0.95. Notably, as P, , t increases, presumably due to solution P concentrations (1.5-4.0 mg L 1 ) may increased P loading, the magnitude of the fixa- be considered excessive from the standpoint of tion stage (i.e., the sorption threshold value) de- eutrophication for all three sorption thresholds in creases. Sorption isotherms of soils with high Fig. 4. For instance, total P concentrations greater than 0.002 mg L 1 have been used to identify eudegrees of sat (Fig. 4a) manifest near-linear relationships (i.e., only the saturated portion of the trophic water bodies (Novotny and 01cm, 1994), sorption isotherm) and, therefore, possess low P although no critical concentration has yet been sorption efficiencies. Soluble P added to these P- identified for agricultural runoff. Phosphorus sorption thresholds were related saturated soils is more likely to be removed by to CaCl7 P, from which a Quantity/ Intensity change point was determined (Fig. 5). The change 500 point was identified at a P sorption threshold of sorption threshold 141 mg kg - 1 and a CaCl2 P of 0.8 mg kg - ' (R2 = 0.60, SE = 12.8). Again, the CaC1 2 P value of this change point (0.8 mg kg- 1 ) is similar to the CaC12 of the previously discussed change points (0.9 mg kg). This implies that the P sorption threshold derived from the segmented isotherm represents the same chemical behavior threshold that explains the change points observed in the P 0 Quantity/Intensity analyses. Solution P (mg L') Phosphorus sorption saturation was related to the sorption thresholds of the subset of unsaturated Fig. 3. Description of P sorption isotherms by segsoils (i.e., those with segmented quadratic and linmented regression. ear isotherms) by forcing a regression through the sorp
( ^
"saturated stage" 200 E "fixation stage" described by linear described by model
VOL. 165 No. 12 ENVIRONMENTAL THESHOLDS OF SOIL PHOSPHORUS
40% 30%
c%ngnJm
200 300 400 500
a
20% •.••....•• . .
P sop0on threshold = 141 rng kg Cad, P = 0.8 mg kg
0 100
y 0.25e'° 00285 C 0.58
949
10%
r'no
P sorption threshold (mg kg)
Fig. 5. Relationship of P sorption threshold with CaCl 2 P and corresponding change point.
15
50
a
50
10
05 00
160
P value on the y axis corresponding to the previously identified change point (1,,t = 25%) (Fig. 120 a 6). Forcing the regression through the change 80 point value is justified based on the observation that the change point value and the isotherm join40 point value both correspond to a single soil chemical threshold. Similar regressions were established 0 I "•. 0 100 200 300 400 500 600 700 for the relationships between Morgan's P and P sorption thresholds as well as Mehlich III P and the Sorption threshold (nog kg') P sorption threshold (Fig. 6). These regressions establish the relationship between an extract and the Fig. 6. Forced regressions relating P sorption threshold remaining capacity of a soil to sorb additional P in to P 081, Morgan's P, and Mehlich Ill P. the fixation stage. Being able to estimate this remaining capacity is essential for making mass balance calculations with respect to future P loading before reaching the P sorption threshold. By relating the fixation stage of sorption isotherms to soil test P values it is possible to esCONCLUSION timate the soil capacity remaining to sorb addiThe observed change points and P sorption tional P. Although data that relates water quality thresholds describe a fundamental property of to soil P are required for validation of environthe soils included in this study, a nonlinear chem- mental soil P thresholds, soil tests related to natuical behavior of P in soils that exhibits a thresh- rally occurring thresholds in soil chemical behavold. This threshold describes a critical point, be- ior are an improvement over current efforts that yond which the potential for P release from soils simply convert agronomic standards to environincreases dramatically. Coincidentally, these thresh- mental thresholds. olds occur near a CaCl2 P value of 0.9 mg kg1 - ACKNOWLEDGMENTS pointing to a common threshold value regardless of the extract used to estimate P Quantity. The authors thank Harry Pionke and Richard In our study, an association of soils having McDowell for critical reviews of this manuscript. similar surface horizon characteristics, hence com- Thanks are also extended to Lei Guo and Megan mon P sorption characteristics, were used collec- Marshall for laboratory analysis and to Steven tively to determine critical P levels for a water- Schwager for assistance in developing the segshed. Other soils are expected to have different mented regression model used to identify the P thresholds and require additional studies similar sorption threshold. to the analyses presented here. The degree to REFERENCES which the thresholds of other soils differ from those determined in this study will depend on Anonymous. 1999. 1999 Cornell Recommends for contributing properties such as texture, mineral- Integrated Field Crop Management. Cornell Cooperative Extension Publication, Ithaca, NY. ogy, organic matter, and pH.
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KLEINMAN, BRYANT, REID, SHARPLEY, AND I'IMENTEL SOIL SCIENCE
Barrow, N. J . 1984. Modelling the effects of pH on Pote, I). H., T. C. Daniel, D.J. Nichols, A. N. Sharpley, phosphate sorption by soils.J. Soil Sci. 35:283-297. P. A. Moore, Jr., I). M. Miller, and I). R. Edwards. Beegle, I). B. 1999. Soil fertility management. In Penn 1999. Relationship between phosphorus levels in State University, The Agronomy Guide, 1999-2000. three ultisols and phosphorus concentrations in Penn State University, University Park, PA, pp. 19-45. runoff.J. Environ. Qual. 28:170-175. Breeuwsma, A., and S. Silva. 1992. Phosphorus fertilisaRoss, G.J., and C. Wang. 1993. Extractable Al, Fe, Mn tion and environmental effects in the Netherlands and and Si. In Soil Sampling and Methods of Analysis. the Po region (Italy). Report 57. DLO The Winand M.R. Carter (ed.). Lewis Publishers, Boca Raton, Staring Centre, Wagenmgen, The Netherlands. FL, pp. 239-246: Heckrath, G., P. C. Brookes, P. R. Poulton, and K. W. Ryden, J. C., J . R. McLaughlin, and J . K. Syers. 1977. I. 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