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Published online 16 October 2007
Published in J Environ Qual 36:1715-1724 (2007)
DOI: 10.2134/jeq2007.0160
© 2007 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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TECHNICAL REPORTS

Waste Management

Controlled Application Rate of Water Treatment Residual for Agronomic and Environmental Benefits

Olawale O. Oladejia,*, George A. O'Connorb, Jerry B. Sartainb and Vimala D. Nairb

a Crop and Soil Science Dep., 512 Plant and Soil Science Bldg, Michigan State Univ., East Lansing MI 48824
b Soil and Water Science Dep., P.O. Box 110510, Univ. of Florida, Gainesville, FL 32611-0510

* Corresponding author (ooladeji{at}msu.edu).

Received for publication March 30, 2007.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and Methods
 Results and Discussion
 Summary and Conclusions
 REFERENCES
 
Water treatment residuals (WTR) are useful soil amendments to control excessive soluble phosphorus (P) in soils, but indiscriminate additions can result in inadequate control or excessive immobilization of soluble P, leading to crop deficiencies. We evaluated the influence of application rates of an Al-WTR and various P-sources on plant yields, tissue P concentrations, and P uptake and attempted to identify a basis for determining WTR application rates. Bahiagrass (paspalum notatum Fluggae) was grown in a P-deficient soil amended with four P-sources at two application levels (N- and P-based rates) and three WTR rates (0, 10, and 25 g kg–1 oven dry basis) in a glasshouse pot experiment. The glasshouse results were compared with data from a 2-yr field experiment with similar treatments that were surface applied to an established bahiagrass. Soil P storage capacity (SPSC) values increased with application rate of WTR, and the increase varied with sources of P applied. Soil soluble P concentrations increased as SPSC was reduced, and a change point was identified at 0 mg kg–1 SPSC in the glasshouse and the field studies. A change point was identified in the bahiagrass yields at a tissue P concentration of 2.0 g kg–1, corresponding to zero SPSC. Zero SPSC was shown to be an agronomic threshold above which yields and P concentrations of plants declined and below which there is little or no yield response to increased plant P concentrations. Applying P-sources at N-based rates, along with WTR sufficient to give SPSC value of 0 mg kg–1 SPSC, enhanced the environmental benefits (reduced P loss potential) without negative agronomic impacts.

Abbreviations: Alox, oxalate-extractable aluminum • Al-WTR, aluminum water treatment residual • APSC, amendment phosphorus storage capacity • DM, dry matter • DPS, degree of phosphorus saturation • Feox, oxalate-extractable iron • PAN, plant-available nitrogen • Pox, oxalate-extractable phosphorus • PSR, phosphorus saturation ratio • SPSC, soil phosphorus storage capacity • STP, soil test phosphorus • TSP, triple superphosphate • WEP, water-extractable phosphorus • WTR, water treatment residuals


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and Methods
 Results and Discussion
 Summary and Conclusions
 REFERENCES
 
LAND application of aluminum-based water treatment residuals (Al-WTR) can serve as a best management practice to reduce the environmental hazards associated with excessive soil soluble P loads. The Al-WTR can increase soil P retention and thereby decrease offsite P loss to water bodies (O'Connor et al., 2002; Dayton et al., 2003; Novak and Watts, 2004). A recent study indicated irreversible immobilization of P by the residuals (Agyin-Birikorang et al., 2007). Knowing the correct amount of WTR to land apply is critical because overapplication of the residuals can lead to excessive immobilization of soil P and induce plant P deficiencies and because insufficient WTR may not be effective at sorbing the excess soluble P.

Applications of WTR are often based on an arbitrary dry weight amendments/soil ratio, with little account taken of the chemical composition of the materials used to determine the WTR rates (Peters and Basta, 1996; Basta and Storm, 1997; Gallimore et al., 1999; Ippolito et al., 1999; Brown and Sartain, 2000; Haustein et al., 2000; Codling et al., 2002; Dayton et al., 2003; Novak and Watts, 2004). Application rates of WTR solely based on dry weight percentages (fixed soil/amendment ratio) can result in excessive or inadequate immobilization of soil soluble P depending on the amount and reactivity of Al and or Fe added in the WTRs. Determining the appropriate application rate of WTR is complicated due to the variations in chemical properties of the residuals as influenced by the source of water, treatment chemicals, and processing chemicals used by treatment plants (O'Connor et al., 2004). Properties that affect the P sorption capacity of WTR include Al, Fe, and P concentrations and metal oxide forms (amorphous and crystalline). Makris (2004) showed that WTRs had Al concentrations that ranged between 37 and 103 g kg–1 for Al-WTRs and between 1.5 and 9.8 g kg–1 for Fe-WTRs, along with varying P and Fe concentrations. Twenty-one Al-WTRs used in a study by Dayton et al. (2003) varied widely in total Al (14.7–177 g kg–1), Fe (5.02–49.9 g kg–1), and P (0.20–4.04 g kg–1) concentrations.

In a batch equilibration study that examined components of WTR that contribute to P sorption properties, oxalate extractable Al (Alox) correlated with the linearized Langmuir Pmax values (Dayton et al., 2003). Similarly, sorption capacities of various WTRs were shown by O'Connor et al. (2002) to depend on the oxalate-extractable Al, Fe, and P concentrations of the WTRs. Haustein et al. (2000) compared two Al-rich materials and their potentials to reduce runoff P from excessively P-affected fields. Material with a greater Al concentration (46.7 g kg–1), applied at 9 and 18 Mg ha–1, decreased runoff P amounts to below those of control plots throughout the 4-mo experimental period. However, at the same WTR application rates, the material with the lowest Al concentration (15.9 g kg–1) decreased the runoff P for only 1 mo. Paulter and Sims (2000) reported a relationship (r = 0.61; p = 0.01) between P sorption and amorphous Al and Fe concentrations of soils. Elliott et al. (2002) suggested that the P saturation index (which is the same as P saturation ratio [PSR]) of WTR, determined from 0.2 M oxalate extractable P, Al, and Fe concentrations (Pox, Alox, and Feox, respectively), was useful for determining WTR application rates. However, soils and P-sources that can be co-applied with WTR can also vary in Pox, Alox, and Feox. Thus, the compositional variability of soils, P-sources (if co-applied with WTR), and WTRs needs to be accounted for in determining the amount of WTR to be applied to a soil.

Potential indices of environmental P losses are the degree of phosphorus saturation (DPS) and PSR. Both DPS and PSR are calculated as ratios of Pox to the sum of Alox and Feox of the soil or amendment, but with an {alpha}-value (which depends on soil characteristics) included in the denominator for DPS calculation (van der Zee et al., 1987; Breeuwsma and Silva, 1992; Nair et al., 2004).

A recent study by Nair and Harris (2004) recommended determining the soil P storage capacity (SPSC) values rather than DPS as an index to predict the amount of additional P a soil can sorb before exceeding a threshold soil equilibrium concentration. The SPSC values can indicate the risk arising from P loadings and the inherent P sorption capacity of the soil. The SPSC values range from negative values (for highly P-affected soils) to positive values (for less P-affected soils). Zero SPSC values represent the value at which the soil PSR is at the 0.15 threshold value and above which the P concentration in soil solution increases rapidly (Nair and Harris, 2004).

A growing consensus among researchers is that if soils can be managed to maintain the soil test P (STP) at levels that optimize crop yields, the risk of offsite P transport will be minimized (Higgs et al., 2000; O'Connor and Elliott, 2006). However, this agronomic optimal needs to be determined, and its suitability as a basis for WTR application rate needs to be evaluated. Application of WTR, if based on the agronomic SPSC threshold, targets only the excess P that poses environmental threats and is not expected to negatively affect the P pools needed to meet plant P requirement.

Our objective was to identify the agro-environmental SPSC threshold value, which could serve as basis for applying WTR.


   Materials and Methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results and Discussion
 Summary and Conclusions
 REFERENCES
 
Soil and Amendments Sampling and Characterization
The optimal application rate of WTR was evaluated using glasshouse and field studies. Immokalee fine sand (sandy, siliceous, hyperthermic Arenic Alaquods) was used for field and glasshouse studies. Surface soil samples were collected from A-horizon (0–15 cm) of the field study site and used for the glasshouse pot experiments. Four P-sources of different solubility were evaluated: (i) two biosolids, one from Pompano Beach, FL, and one from Boca Raton, FL; (ii) poultry manure from Indiantown, FL; and (iii) triple superphosphate (TSP), an inorganic fertilizer. The biosolids were selected based on differences in soluble P content. The Boca Raton biosolids have a high water-extractable P (WEP) content (~5.5 g kg–1), whereas the Pompano biosolids have a moderate (~1.2 g kg–1) WEP content. One WTR (aluminum-based, obtained from Manatee Co. Water Treatment Plant, Bradenton, FL) was used in the study. The soil and the amendments (P-sources and WTR) were analyzed as reported in Oladeji et al. (2007).

Glasshouse Pot Experiments
The 5-mo glasshouse study was a three-factorial experiment arranged in a randomized complete block design with three replicates per treatment. An extra pot per block was left untreated as the control. Two application rates of the P-sources were used to attain P-based and N-based nutrient management. The application rates of P-sources equivalent to 44 kg total P ha–1 (P-based) and 179 kg plant available nitrogen (PAN) ha–1 (N-based) (Kidder et al., 2002) were calculated from the total P and N contents of the P-sources. Mineralization rates of 40% of total N in biosolids and 60% of total N in manure were assumed in the calculation, based on previous experience in similar studies (O'Connor et al., 2004). The P applied at the N-based rate varied (Table 1 ) because of differences in the P/N ratios of the P-sources. The equivalent TSP fertilizer N-based rate used was 88 kg P ha–1, which was smaller than the rate of P applied when biosolids or manure are applied at an N-based rate but twice the applied P at a P-based rate. Ammonium nitrate was applied to the P-based treatments of the biosolids and manure treatments to equalize the N supplied by the amendments that differ in total N levels (Table 1). No ammonium nitrate was applied to the N-based treatments except for the TSP. Three rates of WTR application (0, 10, and 25 g kg–1 oven-dry basis) were used for the study, based on previous work (O'Connor et al., 2002). Potassium-magnesium sulfate (22% S, 18% K, and 11% Mg), equivalent to 444 kg ha–1, was added to each treatment to provide adequate and uniform S, K, and Mg. Each treatment combination was thoroughly mixed with soil (8.5 kg), wetted to the maximum water-holding capacity of the soil, and allowed to equilibrate for 1 wk before planting. The P supplied at the P-based rates was fixed, and the only variables affecting the SPSC of the P-based treatments were the Al and Fe loads of the P-source applied and the WTR rate. The P, Al, and Fe supplied at the N-based rates varied with P-sources. Hence, SPSC values of the N-based rates were affected by the P, Al, and Fe loads of the P-source applied and by the WTR rate. The wide range of soil P, Al, and Fe loads at P-based and N-based rates, with and without WTR, ensured the variability in SPSC values needed for the regression studies. All 25 treatments (including control) received the same amount of N (179 kg PAN ha–1).


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  		Table 1. Summary of P and N supplied and supplemental N applied at the two rates (P- and N-based) of the different P-sources during the glasshouse study.

 
Each amended soil sample was transferred to a growing pot after the equilibration period, packed to a bulk density of 1.3 Mg m–3, and planted with bahiagrass (paspalum notatum Fluggae) at a seeding rate of 7 g per pot. The soil surface of each pot was covered with moist filter paper, which was wetted daily to reduce evaporation. The filter paper was removed after seed germination, and the soil was wetted daily and to initial weight weekly. The aboveground portion of the grass was harvested monthly to determine dry matter (DM) yield and P uptake. Harvesting was at a height of 5 cm above soil surface with scissors and electric clippers. Cuttings were placed in a pre-weighed labeled paper bag for drying to constant weight at 65°C, and DM weight was determined as the difference between the dried paper bags with cuttings and the pre-weighed empty bag. After each harvest, supplemental N (0.6 g pot–1 NH4NO3) was split-applied for the first 3 mo. Water was added to each pot as necessary to attain initially determined pot weights. After the fourth (final) bahiagrass harvest (December 2004), soil samples (~50 g) were taken with an auger of 5 cm diameter from each pot for analysis.

Phosphorus concentrations in the plant samples were determined colorimetrically (Murphy and Riley, 1962) after ashing, and digestion with 6 M HCl was performed as detailed in Jones and Case (1990). Soil samples taken were analyzed for 0.2 M oxalate-extractable P (Pox), Al (Alox), and Fe (Feox) by inductively coupled plasma atomic emission spectroscopy (Plasma 3200; PerkinElmer, Wellesley, MA) after extraction at a 1:60 soil/solution ratio (Schoumans, 2000). The SPSC values were calculated from the soil oxalate extractable P, Fe, and Al concentrations as:

Formula 1[1]
where the PSR = ([Pox]/[Alox + Feox]); all elements are expressed in mmoles (Nair and Harris, 2004). The 0.15 value is the threshold PSR value suggested by Nair and Harris (2004) for Florida soils, and the value 31 in the equation is the atomic mass of P. Values of SPSC > 0 suggest that a soil acts as a P sink, whereas SPSC < 0 values suggest that a soil acts as a P-source. The SPSC = 0 is equivalent to a PSR value of 0.15, which is the environmental threshold (Breeuwsma and Silva, 1992; Nair and Harris, 2004).

Water-extractable phosphorus was determined in the soil samples by shaking the samples with deionized water at a 1:10 soil/solution ratio for 1 h (Kuo, 1996). The P concentration in the filtered (<0.45 µm) extract was determined colorimetrically using the Murphy and Riley (1962) method.

Field Experiment
The field experiment had similar treatments as the glasshouse experiment, and the data obtained were used to validate the glasshouse experiments. The field site consisted of 51 plots (20.7 x 95 m) arranged in three blocks of 17 plots. The experimental setup, P-sources and application rates, and WTR type were identical to those used in the glasshouse study. However, unlike the glasshouse experiment where three rates of WTR applications were used, the field experiment used two application rates (0 and 10 g kg–1). The P-sources and WTR were surface applied in May 2003. Ammonium nitrate was applied to the plots that received P-based rates of the organic sources and TSP plots to equal the N supplied in treatments at N-based rates of the organic sources. The N fertilizer (22.7 kg per plot) was split applied twice (at the start of the experiment and after the first harvest 2 mo later). The amendments were applied once during the study (May 2003), but the study extended through December 2004. Soil samples from the A (0–5 cm), E, and Bh horizons of each plot were taken in June 2003 (1 mo), January 2004 (8 mo), and December 2004 (19 mo) after treatments were surface applied. The samples were analyzed for WEP, Pox, Alox, and Feox, and the SPSC was calculated for each soil sample as described for the glasshouse study. The test plant (established bahiagrass) was harvested twice (July and October) in 2003 and four times (July, August, October, and November) in 2004 from each plot, and the DM yields and tissue P concentrations were determined for each harvest. Grass cuttings were obtained by laying out a 1 by 1 m frame on each plot and cutting all the grass within the frame with hand shears to a height of 5 cm above the ground surface. Bahiagrass cuttings were placed in pre-weighed bags and dried for several days to constant weight at 65°C. Phosphorus concentrations were measured in samples of each plant harvest as described in the glasshouse study, and P uptake was calculated as the product of the P concentration and plants yield. Weighted P concentrations was determined for 2003 and 2004 harvests as total P uptake divided by total DM yield for each year.

Statistical Analysis
Analysis of variance (ANOVA) was performed on SPSC values calculated for the two sets of soil samples collected during the glasshouse experiment and three samples from the field experiment. Differences among treatments were analyzed as a three-way factorial experiment with a randomized complete block design, using the PROC GLM procedure of the SAS software (SAS Institute, 1999). Means of the various treatments were separated using contrast or Tukey test at a significance ({alpha}) level of 0.05. Regression between soil WEP and SPSC values was used to determine the environmental thresholds and between plant P concentrations and SPSC to determine the agronomic threshold. The critical plant P concentrations and SPSC values (change point) were identified using the Cate-Nelson method (Cate and Nelson, 1971).


   Results and Discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
Summary and Conclusions
 REFERENCES
 
The low extractable P values (Mehlich 1P and WEP) of the Immokalee A-horizon soil (Table 2 ) make the soil suitable for the P-response experiments. A soil Mehlich 1P value of <10 mg kg–1 is considered very low for agronomic crops, including bahiagrass (Kidder et al., 2002). The low soil P is expected to enhance identification of plant response to the treatments. Soil pH 5.5 coincides with the "target" pH for bahiagrass (Hanlon et al., 1990). Although the soil is low in extractable P, the slightly negative SPSC value suggests that the storage capacity of the soil is saturated. This is expected for sandy, low P-sorbing soils with low Alox and Feox values (Nair and Harris, 2004). The SPSC was close to zero, which was expected to enhance treatment effects.


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  		Table 2. Selected chemical properties of Immokalee fine sand (Oladeji et al., 2007).

 
The three organic P-sources had pH values of ~7.6 (Table 3 ), whereas the WTR had a pH of 5.6. The WTR total Al concentration was 157 g kg–1, >90% (145 g kg–1) of which was amorphous (0.2 M ammonium oxalate extractable) (McKeague et al., 1971). Total P was greatest for Boca Raton biosolids (47.3 g kg–1), representing the upper limit of P concentrations typically found in biosolids produced nationally (USEPA, 1995). The poultry manure had the least total P but had a moderately high WEP and a greater percentage of WEP than in either biosolid. The WEP values of the organic sources were greatest in the Boca Raton biosolids (5.52 g kg–1), followed by poultry manure (4.57 g kg–1) and Pompano biosolids (1.16 g kg–1). A greater portion of total P in manure is water soluble, as indicated by greatest percentage of WEP in manure (18%), followed by Boca Raton biosolids (11%) and Pompano biosolids (4%). Total and oxalate Al and Fe concentrations were smaller in manure than in the two biosolids, but manure total Ca concentration was greater than in the biosolids. The greater Ca in manure likely results from Ca-rich additives in the animal feeds, the greater part of which is excreted in the manure (Nair et al., 1998). The greater Pox and smaller Feox and Alox values of Boca Raton biosolids resulted in a greater PSR (1.44) value than for Pompano biosolids (0.7) (Table 3). A PSR >1.25 was identified as a critical value for biosolids P loss in a leaching study by Elliott et al. (2002) and is consistent with the greater water-soluble P measured in Boca Raton biosolids. The greater PSR of the Boca Raton biosolids suggests greater P lability than in Pompano biosolids. Phosphorus sources with larger PSR values are expected to have lower SPSC (or negative values) when applied at equal P rates than those with smaller PSR values. The amendment P storage capacity (APSC; equivalent to SPSC in soil) of the Al-WTR is positive due to a large Al concentration (Table 3). Total P and N concentrations, the basis for the application rates, were greatest in Boca Raton biosolids and least in poultry manure. These differences affected the P, Fe, and Al loads and resulted in different SPSC values of the soils amended with different P-sources at P- or N-based rates. The SPSC values of the surface soils (0–5 cm) from the field and the samples taken during the glasshouse study were affected by the WTR, P-source, and P-source rate at p = 0.05 (ANOVA; data not shown).


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  		Table 3. Selected chemical properties of P-sources and aluminum water treatment residuals (Al-WTR) used in the studies (Oladeji et al., 2007)

 
Soil Phosphorus Storage Capacity
Soil samples taken at time zero of the bahiagrass cropping were expected to indicate how SPSC values were affected by the treatments. The treatment effects on the SPSC values of time-final soils can be confounded by nutrient uptake by the crop. However, trends of time-zero and time-final SPSC values with treatments were generally similar (Fig. 1 ). The control treatment had similar SPSC values to P-based rate treatments (without WTR) at time zero and at time final. For all P-sources, SPSC values increased with increasing WTR rates and declined with P rate (Fig. 1). The increase in SPSC values with WTR rates results from the added Al and Fe, which increases the P storage capacity of the soils. In the absence of WTR, the SPSC values of treatments with higher P loads (N-based rate) were negative for the four P-sources, which suggests that added P exceeded soil P storage capacity The smaller SPSC values (more negative values) at the N-based rates than at the P-based rates resulted from greater added P, which saturated the P sorption sites.


Figure 1
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  		Fig. 1. Soil phosphorus storage capacity (SPSC, mg kg–1) values for the different treatments in (a) time zero and (b) time final samples taken during the glasshouse study. *Significant contrast at p = 0.005. ***Significant contrast at p = 0.0001. NS, nonsignificant contrast at p = 0.005; TSP, triple superphosphate; WTR, water treatment residuals.

 
The variation of the SPSC values at P-based rates (where equal P loads were applied) in the absence of WTR reflects the differences in the P-source chemical compositions (especially Al, Fe, and P). Differences in the SPSC values suggest that soils amended with P-sources of different PSR values require different amounts of Al and or Fe added as WTR to achieve equal soil SPSC values.

The addition of WTR increased P storage capacity for all P-source treatments applied at P- or N-based rates. However, SPSC values were greater at the P-based than at the N-based rates when an equal amount of WTR was applied with each P-source. Varying amounts of WTR are needed depending on the source PSR to achieve equal SPSC values for soils treated with various P-sources. The variations in SPSC values could be adjusted by applying WTR based on the desired SPSC value.

Values of SPSC at time zero increased with WTR addition in all samples taken during the study (Fig. 1) due to reduction in soil P:Al+Fe ratio (PSR) of the soil. The SPSC values at the P-based rate (without WTR) were close to 0 mg P kg–1, confirming the rate (P-based) as environmentally friendly. Negative SPSC values (an indication of P in excess of soil P sorption capacity) were observed at the N-based rates (without WTR) and at 10 g kg–1 WTR time final samples, except for the TSP treatment. Negative SPSC values at the N-based rates of P-sources indicated the soils received excess P and is consistent with other studies reporting that N-based rates of residuals load soils with excess P that could cause negative environmental impact (Reddy et al., 1980; Pierzynski, 1994; Peterson et al., 1994; Maguire et al., 2000). The SPSC values were increased by the addition of 25 g WTR kg–1 to the N-based rate of all P-sources and by 10 g WTR kg–1 in some treatments (Fig. 1).

The SPSC values of field samples taken in June 2003, January 2004, and December 2004 from the soil A-horizons during the field study are shown in Fig. 2 . The soil chemistry of subsurface horizons was unaffected by the surface-applied treatments, and E and Bh horizons had similar SPSC values in all treatments (data not shown). The SPSC values of the E-horizon soil samples were similar, negative, and approximately zero and indicate saturation with P and an inability to hold added P. The SPSC values of the Bh horizon soils were positive and similar (~147 mg kg–1) for all treatments. Positive SPSC values were expected for the Al-rich Bh horizon. Other researchers have noted high P-retention capacity in the spodic Bh horizon in Spodosols compared with surface A and E horizons (Mansell et al., 1991; Nair et al., 1998; Nair et al., 2004).


Figure 2
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  		Fig. 2. Soil phosphorus storage capacity (SPSC, mg kg–1) values of A horizon (0–5 cm) samples from the field in (a) June 2003 as affected by the different treatments and (b) January and December 2004 as affected by P rates and WTR. Treatments in (a) ending in P and N are P- and N-based rates of the sources, respectively. Treatments in (a) or within the same sampling period in (b) capped with the same letters are not different at p = 0.05 by Tukey test. TSP, triple superphosphate; WTR, water treatment residuals.

 
The impacts of the surface-applied treatments were obvious in the SPSC values of samples from A horizons (0–5 cm). Soil samples from plots amended with P-sources, but not with WTR, had negative SPSC values, and SPSC values for the N-based rates were more negative than for P-based rates (Fig. 2). Treatments receiving WTR had greater SPSC values than equivalent treatments without WTR. Thus, the field results are consistent with the glasshouse results that SPSC values increased with addition of WTR and decreased with increasing amounts of P added to the soil.

Soil P Storage Capacity, Soluble P, and Plant Growth
Plant yields and tissue P concentrations varied with P-sources, P-source application rates, and the amounts of WTR added (data not shown). For most of the P-sources and at either application rate, plant yields and tissue P concentrations values were greatest in the absence of WTR and least with the 25 g WTR kg–1 soil treatment.

Soil soluble P, as indicated by the WEP values, increased as soil SPSC values decreased in the glasshouse study (Fig. 3a ). However, the rate of change in the WEP values with SPSC (slope) was greater for soils with negative SPSC values than for soils with positive SPSC values and suggests a change point at zero SPSC value (Fig. 3). Similar trends were obtained with the field data (Fig. 3b). Thus, applications of the P-sources and/or WTR to achieve zero or positive SPSC values were accompanied by minimal soil soluble P and could be considered environmentally friendly.


Figure 3
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  		Fig. 3. Soil P storage capacity (SPSC, mg kg–1) and water-extractable P (WEP, mg kg–1) values of soil samples obtained from (a) glasshouse and (b) A horizon (0–5 cm) in the field study. Dashed lines locate zero SPSC values. WTR, water treatment residuals.

 
Most WTR-amended soils in the glasshouse and the field studies had positive SPSC values. However, some soils amended with WTR still had negative SPSC values, indicating insufficient added WTR. This was expected because of the variations in the chemical compositions (Al, Fe, and P concentrations) and application rates of the P-sources. Data from the glasshouse and the field studies showed that the amount of WTR needed to achieve equal SPSC value depends on the composition and application rates of the applied P-sources.

Zero SPSC values have been suggested (Nair and Harris, 2004) as a conservative environmental threshold and could be recommended because the agro-environmental threshold based on the rationale that agronomic threshold is below the environmental threshold. No negative agronomic impact is expected at the environmental threshold, which is typically 3 to 4 times greater than agronomic threshold (O'Connor and Elliott, 2006). Thus, STP can be maintained at levels that optimize crop yields while minimizing the risk of offsite P transport (Higgs et al., 2000). This can be achieved through WTR application based on the agronomic threshold.

The range of bahiagrass P concentrations (1.5–6 g kg–1) encompasses the critical P concentration value of ~2.0 g kg–1 identified by a Cate-Nelson type of approximation (Fig. 4 ) (Cate-Nelson, 1971). The critical (agronomic threshold) P concentration can be defined as the concentration above which there is no plant yield response to increased P concentration. Below a P concentration of 2.0 g kg–1, the bahiagrass DM yield was reduced, and little or no response in the plant DM yield to increasing P concentrations was observed above a P concentration of 2.0 g kg–1. Kincheloe et al. (1987) indicated that tissue P concentrations of 2 to 4 g kg–1 are within a range sufficient for grass production, but the 1.6 to 1.7 g kg–1 P concentrations observed in pastures (which include bahiagrass) by Adjei et al. (2000) were considered limiting values.


Figure 4
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  		Fig. 4. Bahiagrass dry matter yields (DM) and soil P storage capacity (SPSC, mg kg–1) values as a function of plant P concentrations in the glasshouse. Also shown are SPSC vs. plant P concentrations in the field study. Dashed lines locate critical P concentrations and SPSC values by Cate-Nelson approximation (Cate-Nelson, 1971). WTR, water treatment residuals.

 
Among the six common statistical models available to relate STP to plant yields (Cate-Nelson, linear plateau, quadratic plateau, quadratic, and exponential Mitscherlich type equations), the Cate-Nelson method was selected as the best for guiding fertilization recommendations (Mallarino and Blackmer, 1992). The soil SPSC values at planting of bahiagrass decreased (greater negative values) with increasing tissue P concentrations, and the identified critical P concentrations (2 g kg–1) were observed at zero soil SPSC value using the Cate-Nelson method (Fig. 4).

The glasshouse data suggest that by applying a P-source, WTR, or both to attain a zero SPSC value, sufficient P concentrations in the plant for growth are ensured without causing negative environmental impacts. Similarly, a zero SPSC value ensured plant P concentrations of 2 g kg–1 in the field study (Fig. 4c). A SPSC value of zero could serve as an agronomic threshold and a basis for determining the rates of WTR to be applied. Thus, P-sources can be applied at any rate without negative environmental impact if sufficient WTR is applied to achieve an amended soil SPSC value of zero. Applying WTR to attain a SPSC soil value of zero maintains soil soluble P values below the environmental change point but above the optimum plant P concentration. Application rates of WTR based on desired soil SPSC values ensure applying the amount needed for optimum plant growth with no risk of excessive P immobilization.

The SPSC and APSC values can be used to determine the amount of WTR needed to be applied to a P-affected soil or to be co-applied with the P-sources. The SPSC-based rate can account for the P, Al, and Fe concentrations in the residuals and the soil and the (environmental) threshold soil P value. Thus, the WTR rate required to attain a desired SPSC value can be calculated to ensure a soil P concentration below the environmental threshold as well as sufficient plant available P. Similar to the SPSC, the APSC of the P-sources (APSCsource) and WTR (APSCWTR) can be calculated as in Eq. [2].

Formula 2[2]
The amount of WTR to be added could then be determined from Eq. [3].

Formula 3[3]
The SPSC value of the soil and the APSC value of P-source and the WTR can be determined from their chemical compositions. The weight of the P-sources is known from the application rate, and the weight of soil can be determined from the land area to depth of impact (depending on application method; 15 cm depth if incorporated or 5 cm when surface applied) and the soil bulk density. The only unknown in Eq. [3] is the weight of WTR. Thus, Eq. [3] can be used to calculate the amount of WTR needed to achieve a particular soil SPSC value under any given condition. For example, to determine the amount of WTR needed to increase the SPSC value of a highly P-affected soil to zero, the equation is used without P-source parameters (because no P-source is added) and the formula equated to zero.

The SPSC values expected and observed in time zero soil samples of each treatment at the three rates of WTR in the glasshouse study are shown in Table 4 , along with estimated amounts of WTR needed to achieve SPSC value of 0 mg kg–1 at the two rates of the four P-sources. At the P-based rate, applying at least 10 g kg–1 WTR gave measured and observed SPSC values greater than zero for all the P-sources, which indicates that more WTR was applied than necessary. However, N-based rates <25 g kg–1 but >10 g kg–1 WTR were indicated by the measured SPSC values as needed by manure, Pompano, and TSP treatments. The measured SPSC also suggested that >25 g kg–1 WTR is needed by Boca Raton biosolids to achieve a SPSC value of 0 mg kg–1. Estimated SPSC values (expected) differed from the observed values and suggested that <10 g kg–1 is needed in all the treatments except at N-based rate of Boca Raton biosolids. For this treatment, >10 g kg–1 but <25 g kg–1 WTR is expected to give a zero SPSC value. Further study may be needed to explain the differences between the observed and the expected SPSC values. However, among the reasons that could explain the differences is the inability to achieve thorough mixing of amendments with soil despite daily mixing during the 1-wk incubation period (before time zero sampling). For example, pellets of undissolved TSP were observed in the time zero samples during the analysis. Also, reaction of the oxalate extractant with Ca in Ca-rich amendments such as manure and TSP can reduce the measured Pox, Alox, and Feox (Loeppert and Inskeep, 1996) and can contribute to the differences in expected and observed SPSC values. Nevertheless, the SPSC values estimated using Eq. [3] related well with the observed SPSC values at time zero (r2 = 0.8; p < 0.05) and support the use of SPSC as a basis for WTR rate determination.


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  		Table 4. Expected and observed soil phosphorus storage capacity (SPSC) values (mg kg–1) of time zero soils at 0, 10, and 25 g kg–1 water treatment residuals (WTR) and calculated amounts of WTR needed to achieve 0 mg SPSC kg–1 when co-applied with the four P-sources at the two P-source rates (glasshouse study).

 

   Summary and Conclusions
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
 Summary and Conclusions
REFERENCES
 
Applying P-sources at P-based rates to a P-deficient Florida Spodosol results in soil SPSC values close to 0 mg kg–1. However, when the P-sources were applied at N-based rates, the SPSC values were negative, and the magnitude depended on the Pox, Alox, and Feox values of the P-sources. Similarly, co-application of equal amounts of the same WTR with different P-sources resulted in different soil SPSC values, reflecting different chemical compositions (Alox and Feox) of the P-sources.

The application rate of WTRs required to minimize negative agronomic and/or environmental impacts depends on the soil; WTR; and P-sources Al, P, and Fe composition. A WTR with greater oxalate Al and Fe concentrations results in greater SPSC values and, hence, lower soil soluble P concentrations than WTRs with lower Al concentrations. Application rates of WTR based on a desired soil SPSC value ensure applying the amount of amendment needed to control P loss without the risk of excessive plant-available P immobilization. The zero soil SPSC value was identified as the critical point above which the plant P concentrations can be sufficiently reduced to decrease plant yields and below which the soil soluble P (and hence potential P loss) may increase. Amendment P storage capacity, an equivalent term of SPSC for the P-sources, needed for the calculation of WTR rate was also suggested. Glasshouse and field data show that applying P-sources and WTR to attain zero SPSC values optimizes agronomic and environmental benefits of the residuals.


   ACKNOWLEDGMENTS
 
We gratefully acknowledge the Water Environment Research Foundation for funding the glasshouse study and the South Florida Water Management District for funding the field study. We thank Dr. S. Agyin-Birikorang, for critical reading of the manuscript.


   NOTES
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 Materials and Methods
 Results and Discussion
 Summary and Conclusions
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   REFERENCES
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