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NUTRIENT MANAGEMENT & SOIL & PLANT ANALYSIS

Laboratory Validation of Soil Phosphorus Storage Capacity Predictions for Use in Risk Assessment

Soil and Water Science Department, Univ. of Florida, Gainesville, FL 32611

^* Corresponding author (vdn{at}ufl.edu).

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Soil P concentrations in agricultural soils have increased over the years, increasing concerns about eutrophication of surface waters. Sandy soils are particularly prone to P leaching due to limited P retention capacity. This study tested the validity of a measure of soil P storage capacity (SPSC) for sandy soils amended with dairy and poultry manure by evaluation of SPSC response to P gain or loss under controlled laboratory conditions. Forty soil samples representing A, E, and Bt horizons were collected from two dairy and two poultry operations within the Suwannee River Basin. Soils were packed into 1.5-cm-diameter columns and 0.05 M KCl solution was passed through the column using unsaturated flow at pore water velocities of approximately 1 cm d^–1. Then the soils were leached with known quantities of P. Several P addition and leaching cycles followed and the whole experiment lasted approximately 30 mo. Phosphorus in the leachate was measured after each P addition. The P saturation ratio (PSR) was calculated from oxalate P, Al, and Fe extracts analyzed before and after the study. The SPSC of the soils was calculated based on a threshold PSR of 0.15 for the oxalate solutions. Changes in SPSC due to repeated P additions corresponded to predicted values calculated from P loading amounts, taking into consideration the P concentration before additional P loading. Results support the validity of SPSC as a means of estimating P loading rates that pose low environmental risk for specific sandy soils.

Abbreviations: DPS, degree of phosphorus saturation • PSR, phosphorus saturation ratio • SPSC, soil phosphorus storage capacity • STP, soil test phosphorus

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Increased soil P contents in agricultural soils from fertilizer and manure applications have raised the potential for P loss to surface waters via hydrologic pathways. The P enrichment of surface water via runoff and leaching can, in turn, lead to eutrophication (Neller et al., 1951; Ozanne et al., 1961; Mansell et al., 1991; Sharpley and Halvorson, 1994; Sims et al., 1998). In sandy well-drained soils, the risk of P loss is higher because of limited P retention capacity. This risk accrues in areas influenced by intensive animal agriculture (Lander et al., 1998; Lanyon, 2000; Eghball, 2003; Nair et al., 2003).

Soil test P (STP) measurements traditionally focused on soil fertility and agricultural productivity, but as concerns about excess P in soils arose, they were used to provide information about the potential risk of soil P to be transported to surface and groundwater (Sharpley et al., 2003; Sotomayor-Ramirez et al., 2004). Limited applicability of this approach, however, has led to the use of saturation indices associated with soluble forms of P for environmental risk interpretation (Gartley and Sims, 1994; Nair et al., 2004).

The degree of P saturation (DPS) concept, introduced in the Netherlands, expresses the P release capability of a soil by relating the amount of P already adsorbed by the soil to its P sorption capacity (van der Zee et al., 1990). Iron and Al control P sorption capacity in low-C, acidic soils (Pautler and Sims, 2000), such as those used in the current study, by contributing to a large extent to P retention. This occurs mainly through the formation of chemical bonds between orthophosphates and Fe and Al at the surface of the soil, a ligand exchange reaction producing an orthophosphate–surface complex (Pierzynski et al., 2005).

A plot of water-soluble P against DPS gives a "change point" above which P release from the soil increases rapidly (Maguire and Sims, 2002; Nair et al., 2004). The DPS has been shown to be closely correlated to P concentrations in leachate waters (Maguire and Sims, 2002). Some researchers have shown a low potential for P losses via leaching when DPS values are <25% and sharp increases in P leaching above these DPS values (Maguire et al., 1998, 2001; McDowell and Sharpley, 2001). The DPS has been suggested as a suitable indicator to predict environmental limits for soil P. Usually, DPS, expressed as a percentage, is calculated as the molar ratio of acid ammonium oxalate-extractable P to oxalate-extractable Al + Fe (van der Zee and van Riemsdijk, 1988). Oxalate extracts most of the reactive Al and Fe present in the soil and seems to well represent its P sorption capacity (Kleinman et al., 2003).

[1]

Included in the DPS calculation is an empirical factor in the denominator to account for the fraction of Al and Fe responsible for P sorption for soils of a given region. The corrective factor may be omitted and a simple ratio of molar P to molar Fe + Al, commonly referred to as the P saturation ratio (PSR), used for soils with similar properties (Maguire and Sims, 2002; Nair and Harris, 2004).

Although STP, DPS, and PSR are often used to make environmental risk assessments, they do not give information about how much more P can be added to the soil without creating an environmental problem. In effect, they identify present-day problems but fail to capture risks of future additions. Therefore, there is a need to better predict the ability of sandy soils to safely retain additional P. A new concept, the soil P storage capacity (SPSC) has been recently proposed by Nair and Harris (2004). The SPSC refers to the amount of P that can be added to a certain volume or mass of soil before the soil becomes an environmental concern. It is applicable to both surface-runoff and leaching risks. In the latter case, SPSC accounts for the volume of soil that is susceptible to P leaching. Knowing the safe P storage capacity of a soil profile is critical in nutrient management judgment because it indicates how long P could be safely applied to soils under specific loading conditions. This information is particularly needed in manure application scenarios, where nutrient loading is based on crop N requirements, resulting in an excess of P in the soil due to the high P/N ratio of manure (Robinson and Sharpley, 1996).

The overall objective of this study was to test the validity of the SPSC for sandy soils amended with dairy and poultry manure by evaluating the SPSC response to P loading and loss in soil columns under controlled laboratory conditions. The specific objectives were to: (i) determine the SPSC for soils amended with dairy and poultry manure (minimally and heavily P impacted) before and after additions of known amounts of P to soil columns; and (ii) compare observed changes in SPSC with changes predicted from measured P gains or losses from soil columns.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Soil Sampling
Four study sites were selected within the Suwannee River Basin, southeastern USA: two dairy spray fields, disposal sites for P-rich lagoon effluent, and two poultry manure application sites (Fig. 1 ). Two sites (Dairy 1, Lafayette County, FL, and Poultry 1, Lanier County, GA) were minimally impacted by P (Mehlich 1 P concentrations <40 mg kg^–1) for the surface horizons (Tables 1 and 2). The other two sites (Dairy 2, Dixie County, FL, and Poultry 2, Suwannee County, FL) were highly impacted by P (Mehlich 1 P concentrations >40 mg kg^–1) for the surface horizons (Tables 1 and 2). Soils of Dairy 1 and Poultry 2 are mainly Grossarenic and Arenic Paleudults and Typic Quartzipsamments. Soils of Dairy 2 are mainly Grossarenic Paleudalfs and Typic Quartzipsamments, and soils of Poultry 1 are mainly Arenic Paleudults. Selected soil horizons encompassed sandy (A and E) to loamy or clayey (Bt) textures and provided a range in P sorption capacities.

View larger version (58K): [in this window] [in a new window]   Fig. 1. Map of the Suwannee River Basin showing the location of the study sites (source: USGS, gulfsci.usgs.gov/suwannee/; verified 20 May 2007).

Soil Characterization
Particle size analyses (sand, silt, and clay fractions) were determined for the 300 samples using the pipette method (Day, 1965). The pH of all the soils was obtained using a 1:2 soil/water ratio. Total C in the air-dried soil samples was determined using an automated combustion procedure with a Carlo Erba CNS Analyzer (Milan, Italy). Mehlich 1 P was determined colorimetrically (Murphy and Riley, 1962) after extraction with a double acid solution (0.05 M HCl + 0.0125 M H₂SO₄) at a 1:4 soil/solution ratio (Mehlich, 1953).

Phosphorus Loading of Soil Columns
Fifteen grams of each of the 40 samples studied were packed into laboratory polyethylene columns, approximately 1.5 cm diameter. The columns were leached continuously with a background electrolyte (0.05 M KCl) under unsaturated flow at pore water velocities of approximately 1 cm d^–1 (corresponding to less than half a pore volume per day). After steady-state conditions were achieved, P was measured until a constant P concentration was reached in the leachate (Rhue et al., 2006). Then, concentrations between 5 and 40 mg of P L^–1 of solution were added to the soils at the same rate (1 cm d^–1) using the same background solution (Tables 1 and 2). These concentrations are representative of measured concentrations in spray field effluent of the dairy sites. Phosphorus concentrations were arbitrarily selected for each column to mimic P application variability under field conditions. We did not distinguish between A and E or Bt horizons in this selection since Bt horizons could, on occasion, receive heavy P concentrations due to vertical movement of P in these sandy soils.

After the P concentration in the leachate exceeded half the value of the concentration in the input solution, the soils were treated with another flushing cycle, with 0.05 M KCl solution as described above. Then, another P addition cycle was conducted followed by another flushing cycle (Rhue et al., 2006). The experiment was conducted for approximately 30 mo. Phosphorus concentrations were measured in the filtered leachate after each P addition to determine the amount of P that had been sorbed. After completion of the experiment, the soils were air dried and stored for P, Al, and Fe analyses.

Oxalate-extractable P, Fe, and Al were determined for the soils before and after the P loading using a 0.1 M oxalic acid + 0.175 M ammonium oxalate solution, equilibrated at a pH of 3.0 (McKeague and Day, 1966). Phosphorus, Fe, and Al in the oxalate solution were determined using inductively coupled Ar plasma spectroscopy (Thermo Jarrel Ash ICAP 61E, Franklin, MA). Total P was determined for all the soil samples before and after the loading using the ignition method (Anderson, 1976). The solutions were analyzed for P using the Murphy–Riley procedure (Murphy and Riley, 1962).

This experiment was designed to "create" negative SPSC, as achieved by P loading until there was a breakthrough of P concentrations in the effluent that exceeded half the concentration in the input solution.

Calculations
The PSR was calculated for each sample as the molar ratio of oxalate-extractable P(Ox-P) to oxalate-extractable Fe + Al [Ox(Fe + Al)]:

[2]

The soil P storage capacity was calculated based on a threshold PSR of 0.15 (Nair et al., 2004). Nair et al. (2004) used their professional judgment to identify the 0.15 PSR threshold for Florida soils as the best approximation to maintain the 0.10 mg L^–1 critical P solution concentration proposed by Breeuwsma and Silva (1992).

[3]

where the PSR is for the specific soil for which SPSC is being calculated. Phosphorus, Fe, and Al in Eq. [2] and [3] are expressed in moles. The SPSC can be expressed on a mass basis (e.g., mmol kg^–1, mg kg^–1, or kg ha^–1) or on a volume basis (e.g., mg cm^–3, or mg m^–3). The SPSC calculated before the column study was conducted using initial values for oxalate-extractable P, Al, and Fe is referred to as the initial SPSC:

[4]

The final PSR (PSR_final) was calculated with oxalate-extractable P, Al, and Fe measurements determined in the soils after completion of the column study. The value of PSR_final was used to calculate observed SPSC:

[5]

A new oxalate-extractable P (predicted value) was calculated as the sum of the initial oxalate-P values and the net P gained or lost by the soil column at the end of the experiment. This value was calculated from solution P concentrations added to the original oxalate P:

[6]

The predicted SPSC (SPSC_predicted) was calculated as follows:

[7]

An accurate prediction using this approach requires that the oxalate solution extracts all added P.

Statistical Analysis
The observed SPSC of the soils was compared with the predicted SPSC for all the samples. Additionally, observed and predicted SPSC values were compared separately in samples collected from minimally impacted sites because soils at these sites carried some remaining capacity, as opposed to the heavily manure-impacted soils. A paired t-test was used to compare observed and predicted values for SPSC at the end of column treatments. Analyses were performed using Microsoft Excel 2002 and SAS (SAS Institute, 2001).

	RESULTS AND DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Soil Characteristics
The physical and chemical characteristics of the soils from the two dairy sites (Table 1) and the two poultry sites (Table 2) reflect the coarse-textured, acidic, low-C conditions typical of southeastern USA Coastal Plain soils. Mehlich 1 P, the soil P test used in Florida, was <40 mg kg^–1 for the minimally impacted soils (Dairy 1 and Poultry 1), whereas the heavily impacted soils (Dairy 2 and Poultry 2) had STP values up to 270 mg kg^–1 (Tables 1 and 2).

Determination of the Soil Phosphorus Storage Capacity for the Soils in the Columns
The oxalate solution was chosen as the extractant to calculate the SPSC of the soils because oxalate extracts most of the reactive Al and Fe present in the soil and represents its P sorption capacity (Schoumans and Groenendijk, 2000; D'Angelo et al., 2003). In fact, oxalate P represents >90% of the total P of the soils (all 300 samples; data not shown), which is consistent with research results by van der Zee and van Riemsdijk (1988). Mehlich 1 P, on the other hand, accounts for only about 35% of total P for the soils. Also, mean oxalate-extractable Fe and Al concentrations for these soils were 268 and 327 mg kg^–1, respectively, whereas mean Mehlich 1 Fe and Al concentrations were 13 and 129 mg kg^–1, respectively.

The differences between extractants in the calculation of SPSC suggest that some calibration needs to be made to obtain the true capacity of the soils to retain additional P that is independent of the extractant used, because a given soil can have only one remaining capacity. Preliminary data for sandy soils suggest that SPSC calculated with oxalate data is approximately twice SPSC calculated with Mehlich 1 data:

[8]

Further research is underway to confirm that this correction factor is applicable to a wide range of surface and subsurface soils under various conditions.

The initial SPSC calculations showed negative SPSC for almost all the surface A horizon soils regardless of initial P status, suggesting that these soils were a P source even before P additions. The other horizons had a wide range of SPSC values that reflected loading conditions at the sites (Tables 1 and 2). The predicted and observed remaining capacity of the soils determined after column loading showed negative capacity in almost all cases because all the soils had been heavily loaded with P for the column study. Adding large amounts of P to the soils increased the PSR above the threshold value of 0.15, and this higher saturation generated negative SPSC. This is consistent with research conducted by Kleinman et al. (2003), which determined that manure additions to soils and consequently sorption of added P increased the P saturation of soils.

The relationship between initial SPSC and the mean of the initial leachate P concentrations from the column before any P loading showed positive SPSC values when P concentration in the leachate was low (Fig. 2 ). On the other hand, the P concentration in the leachate increased linearly with increasingly negative SPSC values (R² = 0.88), suggesting that SPSC has the potential to predict P that will be released from soils at excessive P application rates.

View larger version (13K): [in this window] [in a new window]   Fig. 2. Comparison between the initial soil P storage capacity (SPSC) and the mean of the initial leachate P concentration from the column study. The soil samples represent A, E, and Bt horizons of minimally and heavily P-impacted dairy and poultry sites.

View larger version (12K): [in this window] [in a new window]   Fig. 3. Soil P storage capacity (SPSC) predicted vs. SPSC observed for all 40 soils in the column study. The soil samples represent A, E, and Bt horizons of minimally and heavily P-impacted dairy and poultry sites. The SPSC predictions are based on oxalate-extractable P values, i.e., the sum of initial values and net P gained or lost by the soils during the column experiment.

View larger version (12K): [in this window] [in a new window]   Fig. 4. Soil P storage capacity (SPSC) predicted vs. SPSC observed for minimally P-impacted dairy and poultry sites. The soil samples represent A, E, and Bt horizons. The SPSC predictions are based on oxalate-extractable P values, i.e., the sum of initial values and net P gained or lost by the soils during the column experiment.

	CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

Predictions of SPSC following P loading of soil columns closely corresponded to observed values, especially for low-impacted soils, indicating that net additions or losses of P from soils under these controlled conditions produced a reasonably proportionate change in SPSC. Our results suggest that SPSC has the potential for closely estimating the additional P that can be loaded to sandy soils in the field before bringing about environmental risks. It also has the potential to predict how much P would ultimately be released from a previously loaded soil before excessive P release would abate. Therefore, SPSC would be a more useful indicator of P loss risk than STP or PSR in P indexing schemes because it conveys the remaining safe loading capacity as well as current risks arising from previous loading.

Important field applications of the SPSC concept would include the following: (i) predicting the reduction in the P storage capacity of a soil with time if the P loading to a soil is known, such as in dairy spray fields: (ii) evaluating how much P can be safely applied to a soil before the soil becomes an environmental risk if manure application is based on N requirements of a crop (currently often a practice in Florida and in other parts of the USA) instead of P requirements: (iii) using SPSC as a parameter in the Florida P Index as a replacement for soil test P; (iv) using SPSC to estimate how long a P-loaded site would continue to release P at environmentally elevated levels; (v) identifing suitable areas for location of animal-based agriculture by selecting soils that have a greater capacity to retain P; and (vi) verify the suitability of potential locations for the construction of stormwater treatment areas.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.

Received for publication February 27, 2006.

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