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Published online 25 May 2007
Published in J Environ Qual 36:1042-1049 (2007)
DOI: 10.2134/jeq2006.0026
© 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

Phosphorus and Other Soil Components in a Dairy Effluent Sprayfield within the Central Florida Ridge

Kenneth R. Woodarda,*, Lynn E. Sollenbergera, Lewin A. Sweata, Donald A. Graetzb, Vimala D. Nairb, Stuart J. Rympha, Leighton Walkerb and Yongsung Jooc

a Agronomy Dep., Univ. of Florida, Gainesville
b Soil and Water Sci. Dep., Univ. of Florida, Gainesville
c College of Medicine, Univ. of Florida, Gainesville

* Corresponding author (krw{at}ifas.ufl.edu)

Received for publication January 19, 2006.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
There is concern that P from dairy effluent sprayfields will leach into groundwater beneath Suwannee River basins in northern Florida. Our purpose was to describe the effects of dairy effluent irrigation on the movement of soil P and other nutrients within the upper soil profile of a sprayfield over three 12-mo cycles (April 1998–March 2001). Effluent P rates of 70, 110, and 165 kg ha–1 cycle–1 were applied to forages that were grown year-round. The soil is a deep, excessively drained sand (thermic, uncoated Typic Quartzipsamment). Mean P concentration in soil water below the rooting zone (152-cm depth) was ≤0.1 mg L–1 during 11 3-mo periods. Mehlich-1-extractable (M1) P, Al, and Ca in the topsoil increased over time but did not change in subsoil depths of 25 to 51, 51 to 71, 71 to 97, and 97 to 122 cm. Topsoil Ca increased as effluent rate increased. High Ca levels were found in dairy effluent (avg.: 305 mg L–1) and supplemental irrigation water (avg.: 145 mg L–1) which likely played a role in retaining P in the topsoil. An effect of effluent rate on P and Al concentrations in the topsoil was not detected, probably due to large and variable quantities present at project initiation. The P retention capacity (i.e., Al plus Fe) increased in the topsoil because Al increased. Dairy effluent contained Al (avg.: 31 mg L–1). Phosphorus saturation ratio (PSR) increased over time in the topsoil but not in subsoil layers. Regardless of effluent rate, the P retention capacity and PSR of subsoil, which contained 119 to 229 mg kg–1 of Al, should be taken into account when assessing the risk of P moving below the rooting zone of most forage crops.

Abbreviations: DPS, degree of phosphorus saturation • M1, Mehlich-1 extraction • Ox, ammonium oxalate extraction • PSR, phosphorus saturation ratio • UFA, Upper Floridan Aquifer


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
THE HISTORIC Suwannee River originates from the Okefenokee Swamp in southeastern Georgia and flows through northern Florida to the Gulf of Mexico. Basins surrounding Reach 3 (i.e., upper portion of the Lower Suwannee River; Fig. 1 ) have become a major environmental concern for a number of reasons. This area has high potential recharge to the Upper Floridan Aquifer (UFA) because surface soils are deep sands that overlie karst limestone. Also, the UFA is unconfined and relatively close to the surface. These conditions provide a direct route for surface-applied nutrients to leach into the aquifer. Contaminants in the UFA will eventually end up in the river via springs and bottom seeps (Raulston et al., 1998). Elevated nutrient levels in groundwater have been linked to the many dairies and poultry operations located in the Lower Suwannee River basin (Florida United Watershed Assessment, 1999). However, increasing nitrate levels, rather than P concentrations, appear to be the most pressing environmental threat. During a 3-yr period, the average annual load of nitrate originating from Reach 3 basins and reaching the Gulf of Mexico through the Suwannee River system was 965 Mg or 37% of the total load. The annual estimate for P was 41 Mg or 6% of total P reaching the Gulf (SRWMD, 2000, 2002, 2003).


Figure 1
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  		Fig. 1. The Central Ridge region and Suwannee River in Florida. The Central Florida Ridge is dominated by well to excessively drained sandy soils.

 
Basins surrounding the Lower Suwannee River are part of the Central Florida Ridge region (Fig. 1). Typical soils in these basins are Entisols, dominated by well to excessively drained, deep sands (USDA, 1982). Aluminum and Fe have been associated with P retention in ridge soils, but Al is generally the predominant soil component responsible for P adsorption (Fiskell and Rowland, 1960; Yuan, 1965; Zhang et al., 1997).

Research relating to P movement in animal effluent sprayfields has been limited, especially on rapidly permeable sandy soils. In a swine (Sus scrofa domesticus) effluent sprayfield, King et al. (1990) reported that after a total of 6100 kg P ha–1 was applied over an 11-yr period, the downward movement of P in the soil was limited to the upper 75 cm, but there was a potential risk of P transport offsite via surface runoff and subsurface drainage (Westerman et al., 1985). Several studies involving effluent loading of P have been conducted in municipal wastewater sprayfields. Flaig et al. (1987) applied municipal wastewater over an 8-yr period to ridge soils (Astatula and Chandler fine sands) in Central Florida. Most of the applied P was retained in the upper 120 cm of soil but a substantial amount was retained below the 2-m depth. Overman (1979) applied municipal wastewater on a deep Lakeland fine sand in North Florida. He reported that the removal of P from solution was 99% complete in the upper 120 cm of soil. In Pennsylvania, Kardos and Hook (1976) applied secondary sewage effluent over a 9-yr period to a hardwood forest with a Morrison (Ultic Hapludalf) sandy loam soil. Less than 1.2% of the added total P (1806 kg ha–1) leached below a depth of 120 cm. During the study, soil P fractions associated with Al and Fe increased.

In the current study, forages were grown year-round in a dairy sprayfield with three effluent loading rates over three consecutive 12-mo cycles. The primary objective was to assess the effect of dairy effluent loading on (i) the P concentration in soil water below the rooting zone and (ii) M1-extractable P, Al, Fe, and other elements, and M1-derived P retention capacity and saturation ratio within the upper soil profile of a deep, excessively drained sand.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The study was conducted from 1998 to 2001 in a dairy effluent sprayfield owned by North Florida Holsteins, Inc., near Bell, Florida (29°44' N; 82°51' W). The soil at the site is classified in the Kershaw series (Weatherspoon et al., 1992). It is sandy to a depth of more than 2 m and has a high infiltration rate even when thoroughly wetted (Hydrologic group A). The water table is 15 m below the surface. Precipitation data were collected at the site (Fig. 2 ) with a LI-1200S Minimum Data Set Recording System (LI-COR Inc., Lincoln, NE).


Figure 2
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  		Fig. 2. Weekly total rainfall distribution at a dairy effluent sprayfield near Bell, FL during three 12-mo cycles: (a) 1998–1999; (b) 1999–2000; (c) 2000–2001.

 
A detailed description of the pivot design and site layout has been reported by Woodard et al. (2007). In their study, forages of five year-round systems were grown in a dairy effluent sprayfield and irrigated with three effluent loading rates (low, medium, and high) over five consecutive 12-mo cycles (1996–2001). The current study involves soil and soil water data collected over the last three 12-mo cycles (1998–2001). Our first 12-mo cycle began in April 1998, corresponding to the start of the warm growing season, and ended in March 1999. During each cycle, main plots (15.2 x 76 m) received an average effluent P rate of 70, 110, or 165 kg ha–1 (Table 1). Accompanying effluent N rates were 505, 720, and 950 kg ha–1 cycle–1, respectively. Main plots receiving the high effluent loading rate were irrigated during all effluent applications which averaged 26 per 12-mo cycle. For the medium effluent rate, designated main plots received 75% of the total number of effluent applications. For the low effluent rate, main plots received 50% of effluent applications. When main plots did not receive effluent, an equal volume of fresh water was applied within 3 d. Overall fresh water irrigations were applied during dry times. The average quantity of supplemental fresh water applied overall was 370 mm per cycle.


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  		Table 1. Loading rates of P from dairy effluent and fresh water irrigation during three 12-mo cycles at North Florida Holsteins Dairy.

 
To determine application volume, six 20-L containers were placed in each main plot. During a typical irrigation, 16 mm of effluent or fresh water was applied. For nutrient analysis, effluent and fresh water were collected by placing lab-cleaned, 20-L containers ahead of the pivot. Effluent rates of P were calculated using application volumes and P concentrations in dairy effluent and fresh water.

Subplots (15.2 x15.2 m) contained year-round forage systems consisting of bermudagrass (Cynodon spp.)-rye (Secale cereale L.), perennial peanut (Arachis glabrata Benth.)-rye, corn (Zea mays L.)-forage sorghum [Sorghum bicolor (L.) Moench]-rye, corn-bermudagrass-rye, and corn-perennial peanut-rye. Treatment combinations of the 3 by 5 split-plot factorial were replicated three times in a randomized complete block design.

Suction Lysimeters
Suction lysimeters were constructed by attaching a round-bottom, porous ceramic cup (5.1-cm o.d. x 6-cm length, one bar high flow; Soil Moisture Equipment Corporation, Santa Barbara, CA) to the end of 4.9-cm-o.d. polyvinyl chloride tubing. Three suction lysimeters were installed in each subplot and spaced a minimum of 2 m apart. One lysimeter was installed so that its ceramic cup was 91 cm below the soil surface which was considered to be below the primary rooting zone. The other two were installed so that cups were 152 cm below the soil surface. This depth was considered to be below the rooting zone. In the sprayfield, there was a total of 45 suction lysimeters installed at the 91-cm soil depth and 90 lysimeters at the 152-cm depth. Soil water sampling began in April 1998 and continued through March 2001 at 14-d intervals. Two days before sampling, any water contained in the lysimeter cups was evacuated before a suction (40 to 45 kPa) was placed on it. At sampling, suctions were released. Samples were acidified (2 pH) and placed in a cooler within 15 min of collection. Lysimeters were also installed at the 91- and 152- cm soil depths nearby in an unimpacted area characterized as an open woodland. This area was about 400 m away from the edge of the dairy sprayfield.

Soil Sampling
The soil was sampled in April 1998, January 1999, March 2000, and June 2001. The soil profile was divided into (i) topsoil (A horizon) which averaged 25 cm in depth and (ii) subsoil from beneath the base of the topsoil to 51 cm, and (iii) subsoil depths of 51 to 71, 71 to 97, and 97 to 122 cm. The subsoil depths were within C1 and C2 horizons. During a sampling date, one soil profile probe was made in each subplot. The soil profile of a nearby unimpacted area was also sampled at the same soil depths.

Analytical Procedures
Phosphorus concentration in soil solutions from suction lysimeters was determined by inductively coupled argon plasmo spectroscopy (USEPA Method 200.7; USEPA, 1994) utilizing a Model 61-E analyzer (Thermo Jarrell Ash Corporation, Franklin, MA). Soil samples taken throughout the project were extracted with the Mehlich-1 (M1) procedure (Mehlich, 1953). For comparison, soil collected during the last two sampling dates, including those of the unimpacted area, were also extracted with a 0.1 M oxalic acid/0.175 M ammonium oxalate solution (Ox) in darkness (McKeague and Day, 1966). Concentrations for P, Al, and Fe in M1 and Ox solutions were determined by USEPA Method 200.7. Soil concentrations of Ca, Mg, K, Cu, and Zn were measured with the same EPA method, but in M1 solutions only. Values for the P saturation ratio (PSR) were calculated with molar concentrations of P, Al, and Fe from both M1 and Ox extractants using the following equation: PSR = [P/(Al + Fe)]. Soil pH was determined by USEPA Method 150.1. Organic matter concentration in initial soil samples from the sprayfield was determined by the Walkley–Black method (Nelson and Sommers, 1982). The average organic matter concentration was 15 g kg–1 for the topsoil and 4 g kg–1 for subsoil from beneath the base of the topsoil to a soil depth of 51 cm. The concentration in subsoil from depths of 51 to 71, 71 to 97, and 97 to 122 cm, was 3, 2, and 1 g kg–1, respectively.

For total P concentrations in dairy effluent and fresh water, USEPA Method 351.2 was used (USEPA, 1983). The method involves a standard Kjeldahl digestion, followed by semi-automated colorimetry using a Technicon Auto Analyzer (Technicon Instruments Corp., Tarrytown, NY). For K concentration in dairy effluent, samples were filtered and determined by inductively coupled argon plasmo spectroscopy (USEPA Method 200.7). After the termination of treatments in the sprayfield, dairy effluent and fresh irrigation water were sampled over a 3-mo period and analyzed for total Al, Ca, and Mg concentrations. For Al, additional effluent samples were taken on two later dates. Samples were digested with a nitric acid procedure (method 3030 E; American Public Health Association, 1989). Concentrations were determined with a Model 220FS atomic absorption spectrophotometer (Varian Analytical Instruments, Walnut Creek, CA). The pH of dairy effluent and fresh water was determined by USEPA Method 150.1.

Statistical Procedures
Originally, an objective of this study was to measure the effect of forage system on soil P over the 3-yr period. Results were inconclusive and will not be addressed. We think that the relatively small differences in soil P removed among systems (Woodard et al., 2007) were obscured by the large, variable amount of P present in the topsoil at the start of the project. Therefore, the effects of effluent rate, soil depth, and period (time) were analyzed across forage systems.

Phosphorus concentrations in soil water (collected from suction lysimeters at 14-d intervals) were averaged over 3-mo periods for each lysimeter depth and analyzed. Within a 12-mo cropping cycle, 3-mo periods were April through June, July through September, October through December, and January through March. Due to the high degree of soil variation within and across plots, soil data from the first two sampling dates (April 1998 and January 1999) were merged and designated as Period 1. Data from the March 2000 and June 2001 soil samplings were merged and designated as Period 2. Soil data for P, Al, Fe, Ca, Mg, K, Cu, and Zn were then converted using natural logarithmic transformation to improve the normalcy of data distribution. Soil pH data were analyzed directly. All data were analyzed by fitting mixed linear models (Littell et al., 1996) using the PROC MIXED procedure of SAS (SAS Institute, 2000). In the analysis of P in soil water, period was considered a repeated measure. For soil components, period and soil depth were analyzed as repeated measures. Main effects and interaction terms were considered significant if P was ≤ 0.10. Analysis showed that for the majority of soil components measured, the effect of soil depth commonly interacted with period and/or effluent rate. Therefore, results are presented for each soil depth. Pairwise comparisons were made using the lsmeans/pdiff procedure (P ≤ 0.05). Least squares means from transformed data were converted to original units and are presented.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Soil Water Phosphorus below the Primary Rooting Zone
For P concentration in soil water at 91 cm below the surface, the main effect of period was significant (P < 0.01). The effect of effluent rate was not detected. Mean P concentration was ≤0.4 mg L–1 for 11 3-mo periods during the 3-yr study (Fig. 3A ). An exception occurred during the second period (July-September) of the 2000–2001 cycle when the mean P concentration was 0.8 mg L–1. The main effect of period was also significant for P concentration in soil water at the 152-cm depth (P < 0.01). Mean P concentration was <0.1 mg L–1 during 11 3-mo periods (Fig. 3B). The mean concentration during the second period of the last cycle reached 0.6 mg P L–1. It is unknown whether heavy rainfall in some weeks during that period caused the increase (Fig. 2c). In the unimpacted area, soil water P concentration from the two profile depths remained ≤0.1 mg L–1.


Figure 3
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  		Fig. 3. Average P concentration in soil water at (A) 91 cm and (B) 152 cm below the surface in a dairy effluent sprayfield for 3-mo periods from April 1998 to March 2001. An arrow following a letter indicates consecutive means with the same letter. Means with the same letter are not different (P > 0.05). Standard deviations for means (in consecutive order over the 3-yr period) were (A) 0.9, 0.5, 0.6, 0.4, 0.3, 0.3, 0.3, 0.2, 1.2, 1.9, 0.3, and 0.2; and (B) 0.2, 0.1, 0.1, 0.1, 0.1, 0.1, 0.1, < 0.1, 0.3, 2.3, 0.2, and 0.2.

 
Soil Components
Mehlich-1 Phosphorus, Aluminum, and Iron
Soil P decreased sharply from the topsoil (avg.: 231 mg kg–1) to the underlying subsoil layer (avg.: 50 mg kg–1). Subsoil P gradually declined with depth (Table 2). Phosphorus concentration increased over time in the topsoil (P < 0.01) but not in subsoil layers. In the topsoil, the effect of effluent rate was not detected but there was a rate x period interaction (P = 0.07). In Period 2, the rate effect was not significant but means were 219 mg P kg–1 for the low rate, 251 mg P kg–1 for the medium rate, and 264 mg P kg–1 for the high. The absence of a rate effect may have been attributable to the large quantities of P present in the topsoil at the start of the project.


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  		Table 2. Mehlich-1 extractable P, Al, Ca, and Zn in the upper soil profile of a dairy effluent sprayfield during a 3-yr study in northern Florida.

 
In the unimpacted area, soil P levels were much lower in magnitude (Table 3). It would appear that the greatest impact of dairy effluent irrigation in previous years occurred in the topsoil.


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  		Table 3. The pH, element concentrations, and P saturation ratio (PSR) of an unimpacted soil in a sparse woodland near the project sprayfield site. Soil P, Al, and Fe were extracted with Mehlich-1 (M1) and ammonium oxalate (Ox) methods and the PSR for each method was computed{dagger}.

 
In the sprayfield, there was a more gradual decline in soil Al with soil depth compared to P (Table 2). The most notable finding was that Al concentration in the topsoil increased over time (P < 0.01). Dairy effluent applied during the project was not analyzed for Al concentration. However, when an increase of soil Al was found, effluent and fresh water were collected post-study from the same dairy sprayfield on five sampling dates and analyzed for total Al. Mean Al concentration was 30.5 mg L–1 in effluent and 0.7 mg L–1 in fresh water. Based on these mean concentrations and volumes of effluent and fresh water applied during the study, total quantities of Al applied during the 3-yr period were 175 kg ha–1 for the low rate, 260 kg ha–1 for the medium rate, and 365 kg ha–1 for the high rate. An effluent rate effect on soil Al was not detected, although the mean concentration of topsoil Al in Period 2 was 321 mg Al kg–1 for the low rate, 327 mg kg–1 for the medium rate, and 335 mg kg–1 for the high. Also, the Al level in the topsoil of the sprayfield in Period 1 was substantially greater in magnitude than that measured in the unimpacted topsoil, while subsoil Al values for the four depths were somewhat similar. This would suggest that topsoil Al in the sprayfield had increased during previous years of effluent irrigation. In Central Florida, Flaig et al. (1987) reported an increase in Al concentration throughout the soil profile following secondary municipal wastewater applications on deep sands over an 8-yr period. They suggested that Al in wastewater partially regenerated the soil's P sorption capacity.

The mechanism by which Al becomes an effluent component is unknown. One possibility is direct ingestion of soil by dairy cows (Fries et al., 1982). Another possibility is that soil-contaminated forages were consumed by dairy cows. Robinson et al. (1984) found that Al concentration in ungrazed forages normally remained below 100 mg kg–1 of dry matter. Soil contamination of grazed forages during cold, wet periods of low forage production caused a substantial increase of Al concentration in forage and in the rumen of dairy cows.

Soil Fe gradually decreased with depth (P < 0.01), but at a given depth did not increase during the study. In the topsoil, the average Fe concentration was 19 mg kg–1. In the subsoil layers, concentration ranged from 12 to 16 mg kg–1.

Mehlich-1 Calcium and Magnesium and pH
The topsoil had the greatest Ca concentration (Table 2). There was a sharp decline in Ca in the underlying subsoil layer. In subsoil layers, Ca concentration gradually declined with depth. Calcium concentration in the topsoil increased over the two periods while subsoil Ca remained unchanged. In the topsoil, the effect of effluent rate was significant (P = 0.1). For the latter period, topsoil Ca was 920 mg kg–1 for the low rate, 1020 mg kg–1 for the medium rate, and 1075 for the high. The increase in topsoil Ca with time and rate occurred because of high Ca levels in both dairy effluent (post-study avg.: 305 mg L–1) and fresh irrigation water (avg.: 145 mg L–1). Based on the average concentrations, the loading rates of Ca per 12-mo cycle from dairy effluent plus fresh irrigation water were 1330 kg ha–1 for the low effluent rate, 1525 kg ha–1 for the medium rate, and 1725 kg ha–1 for the high.

Magnesium concentration declined with soil depth (P < 0.01) but did not increase during the study. Highest Mg concentration was in the topsoil (avg.: 105 mg kg–1). In subsoil layers, Mg gradually declined with depth (range: 15–30 mg kg–1). In Period 2, soil Mg increased with effluent rate (P = 0.02) in the topsoil but not in subsoil layers. Means for topsoil were 90 mg kg–1 for the low rate, 110 mg kg–1 for the medium rate, and 120 mg kg–1 for the high. The Mg concentration averaged 67 mg L–1 in effluent and 14 mg L–1 in fresh water.

The pH gradually declined with depth (P < 0.01) but did not change during the project. The pH was 6.7 in the topsoil and 6.1 in the lowermost subsoil layer. Calcium and other basic cations in the dairy effluent and fresh water were likely responsible for maintaining soil pH during the project. The study site is located in a region with underlying highly karst limestone, known as the Chiefland Limestone Plain (Weatherspoon et al., 1992). Apparently, basic cations in dairy effluent (pH: 7.7) and fresh water (pH: 8.0) were of adequate quantities to neutralize acidity arising from the nitrification of ammoniacal N contained in effluent. Ammoniacal N averaged 68% of the total effluent N with the remainder being organic N (Woodard et al., 2003a). With swine effluent applied to bermudagrass over an 11-yr period, King et al. (1990) reported that soil concentrations of M1 Ca and Mg increased during the latter 6 yr of the study, which coincided with an increase in pH.

Mehlich-1 Potassium
Highest K concentrations were found in the topsoil but there was little variation among the lower concentrations in subsoil layers (Table 4). Soil K levels did not change over time. The presence of large amounts of divalent cations in the sprayfield soil (Ca in particular) would compete with K for adsorption sites (Khomvilai and Blue, 1977), therefore K retention was not favored. Although K did not accumulate per se in the soil, higher concentrations were maintained throughout the upper profile as effluent rate increased. This was likely related to the large differences in the continuous loading of K at the three effluent rates. During the study, plots received an average of 475 kg K ha–1 per 12-mo cycle for the low rate, 730 kg K ha–1 for the medium rate, and 1030 kg K ha–1 for the high.


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  		Table 4. Mehlich-1 extractable K in the upper soil profile as affected by low, medium, and high dairy effluent rates over three consecutive 12-mo cycles.

 
Mehlich-1 Copper and Zinc
Soil Cu concentrations only differed among soil depths (P < 0.01). The topsoil had the highest concentration (avg.: 1.8 mg kg–1). Soil Cu was substantially lower in subsoil layers and gradually declined with depth (range: 0.12–0.39 mg kg–1). Mehlich-1 Cu concentration in the topsoil was mostly greater than critical soil levels reported by others (Makarim and Cox, 1983; Fageria, 2001). The topsoil also had the highest Zn concentrations (Table 2). Levels were substantially lower in subsoil layers and gradually declined with depth. Mehlich-1 Zn concentration in the topsoil far exceeded critical levels reported by others (Alley et al., 1972; Lins and Cox, 1988). In all soil depths, Zn increased during the study. The most likely source of Zn was the dairy ration. Elevated Zn levels are commonly fed to dairy cows to prevent hoof problems (Harris et al., 1994).

Phosphorus Retention and Saturation Ratio Computed from Mehlich-1 Soil Components
In Florida, the M1 extraction method has been used extensively to estimate P input requirements of a soil for optimum crop production (Mylavarapu and Kennelley, 2002). Nair et al. (2004) concluded that molar concentrations of M1 P, Al, and Fe, and computed P saturation indices could be used to assess environmental risk of P loss. In the current study, the molar concentration of M1 Al plus Fe (i.e., estimate of P retention capacity) was highly correlated with Al concentration (r = 0.998). This occurred because Fe contributed only 3% in topsoil and an average of 5% in subsoil layers to the sum of Al and Fe. Aluminum plus Fe concentration gradually declined with soil depth (P < 0.01) and increased in the topsoil (P < 0.01; Table 5).


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  		Table 5. Mehlich-1 molar concentration of Al plus Fe and phosphorus saturation ratio (PSR) for the upper soil profile of a dairy effluent sprayfield during a 3-yr study in northern Florida.

 
Phosphorus saturation ratio decreased dramatically from the topsoil to the underlying subsoil layer, followed with a gradual decline with depth (Table 5). In the topsoil, PSR was greater in Period 2 than in Period 1. It remained unchanged in the subsoil layers. The PSR [P/(Al + Fe)] was almost identical to P/Al (r = 0.999). Therefore, PSR could be computed for the dairy sprayfield soil without including soil Fe.

Mehlich-1 vs. Oxalate Extraction Methods and Phosphorus Saturation Ratio
The ammonium oxalate extraction method (Ox) was designed to remove P associated with amorphous oxides of Al and Fe in non-calcareous soils (Hooda et al., 2000). It has been recognized as a standard extraction method for environmental risk assessments involving P saturation indices for which other methods have been compared (Khiari et al., 2000; Nair et al., 2004; Sims et al., 2002). In the present study, M1 and Ox extractions of P, Al, and Fe were conducted on soil samples of Period 2 and results were compared. All soil components derived from Ox decreased with depth (Table 6). Throughout the upper soil profile, concentrations of POx were greater than PM1 while AlOx was much greater than AlM1. A notable finding was the high FeOx levels. The molar concentration of FeOx contributed from 23 to 29% to the sum of AlOx and FeOx. As a consequence of higher AlOx and FeOx concentrations, Ox estimates for the P retention capacity of the soil layers were much greater than those of M1.


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  		Table 6. Average P, Al, Fe, and Al plus Fe concentrations, and phosphorus saturation ratio (PSR) derived from the ammonium oxalate extraction method for soil samples taken in March 2000 and June 2001 (Period 2).

 
Molar concentrations of M1 P, Al, and Fe and associated Al + Fe and PSR variables were expressed as a percentage of Ox respective counterparts. With the exception of Fe, relative values decreased with soil depth (Table 7). Though PSR derived from the two extraction methods were highly correlated (r = 0.975; P < 0.01), the percentage for topsoil showed that PSRM1 values were nearly twofold greater than those for PSROx. However, in the two lowermost subsoil layers PSRM1 values were only slightly higher. It is possible that in the topsoil, and to a lesser extent in the first subsoil layer, M1 extraction (0.0125 M H2SO4 + 0.05 M HCl) was efficient at recovering P associated with a number of soil fractions including organic matter, Ca, and amorphous Al, thereby inflating the term: PM1/AlM1. Flaig et al. (1987) reported that 30% more P was extracted from the topsoil of a municipal wastewater sprayfield with 0.1 M HCl compared to acid ammonium oxalate. In the present study, amorphous forms of P and Al in the three lowermost subsoil layers were probably predominant and the recovered portions of the two components by M1 were directly related to (not equal to) those of Ox, resulting in comparable PSR values for the two extraction methods. Similarly, differences in PSR between extraction methods were small throughout the upper soil profile of the unimpacted area (Table 3). Values for PSROx were actually higher than PSRM1 in the topsoil and subsoil layers of the unimpacted area. In the unimpacted topsoil, CaM1 concentrations were low while AlM1 was high, therefore most of the PM1 was probably associated with Al.


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  		Table 7. Average percentage of soil components and variables derived from the Mehlich-1 extraction method in relation to those derived from the ammonium oxalate method.

 
High PSRM1 estimates relative to PSROx could be a unique feature of topsoil of dairy sprayfields in the Lower Suwannee River basin, where appreciable amounts of Ca are applied to the soil surface via effluent and supplemental fresh water irrigation. Woodard et al. (2003b) reported much higher ‘degree of P saturation’ percentage (DPS) derived from M1 components compared to that from Ox components in the topsoil of several dairy sprayfields in the region. Mean values for DPSM1 were 93% higher than DPSOX estimates. In the topsoil of unimpacted sites, DPS values derived from the two extraction methods were similar in magnitude.

Reported threshold PSR values for environmental assessments are generally near 0.15 (Breeuwsma et al., 1995; Sims et al., 2002; Nair et al., 2004). Throughout the current project, PSR estimates for the topsoil far exceeded 0.15, regardless of extraction method. Yet, PSR for the subsoil layers remained unchanged. In addition, P concentration in soil water at 152 cm below the surface remained <0.1 mg L–1 throughout most of the study. One explanation is that the continuous loading of copious amounts of Ca from dairy effluent and fresh water irrigation onto the soil surface of the sprayfield resulted in Ca-P forms which remained in the topsoil during the project. The Ca-P fraction in dairy manure-impacted soils has been characterized as a labile form and is considered to be subject to sustained leaching if suitable conditions occur (Nair et al., 1995; Josan et al., 2005).

In the current sprayfield where the soil is a deep, excessively drained sand, surface runoff and subsurface drainage are generally considered to be minimal. In the event that P moves out of the topsoil, it should be retained in the subsoil layers, given the quantities of Al contained in them. Subsoil horizons should provide a significant buffer to the downward movement of P. Therefore in regions with similar soils, the P retention capacity of the entire upper soil profile should be taken into account when evaluating the potential risk of P moving out of the rooting zone.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
The findings of this study suggest that when assessing the environmental impact of applying manure effluent to grow forage crops in dairy sprayfields, there may be site-specific features that play a significant role in the retention of P. At the start of the project, the sprayfield topsoil had P levels already considered to be far beyond saturation of its retention capacity, based on Al plus Fe concentration. Yet P concentration continued to increase in the topsoil while remaining unchanged in subsoil layers. Also, low P concentrations were measured in soil water below the rooting zone during the 3-yr study. We think that P continued to be retained because dairy effluent applications continuously supplied substantial amounts of Ca to the sprayfield soil. In addition, the soil was supplied with significant amounts of Al which could have contributed to the increase of P in the topsoil.

The effect of effluent rate on soil P and Al was not detected, probably due to the masking effect of large and variable amounts of these elements in the topsoil at project initiation. But regardless of the effluent rate used, monitoring the PSR of the upper subsoil layers should provide an early warning of P moving below the rooting zone of most forage crops. Evaluating the P retention capacity of the subsoil layers within the rooting zone would require deeper than normal soil sampling. One possible recommendation for nutrient management planners would be to assess soil conditions just below the primary rooting zone, thereby limiting the amount of P reaching a soil depth where it cannot be effectively extracted by commonly grown forage crops (i.e., grasses with fibrous root systems).Though this approach would result in underutilization of effluent P, the environmental threat of it reaching the water table would be low.


   ACKNOWLEDGMENTS
 
This project was funded through the Florida Dep. of Environmental Protection, Tallahassee, FL, by a Section 319 NPS Management Implementation Grant from the USEPA. We are grateful to Mr. Don Bennink, owner of North Florida Holsteins, Inc. and his managers and staff for their active involvement in this project and donation of time, equipment, irrigation system, and land. We recognize contributions made by Mr. Winston Tooke, Natural Resource Conservation Service, USDA, who long before the initiation of this study, identified specific research needs and assisted in grant procurement.


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




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