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Published online 4 January 2008
Published in J Environ Qual 37:259-265 (2008)
DOI: 10.2134/jeq2006.0302
© 2008 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

Runoff and Leachate Losses of Phosphorus in a Sandy Spodosol Amended with Biosolids

Luis R.F. Alleonia,*, Scott R. Brintonb and George A. O'Connorb

a Soil Science Dep., P.O. Box 09, Univ. of Sao Paulo, Piracicaba, SP, Brazil 13418-900
b Soil and Water Science Dep., P.O. Box 110510, Univ. of Florida, Gainesville, FL 32611-0510

* Corresponding author (alleoni{at}esalq.usp.br).

Received for publication July 31, 2006.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Material and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 
Florida Spodosols are sandy, inherently low in Fe- and Al-based minerals, and sorb phosphorus (P) poorly. We evaluated runoff and leachate P losses from a typical Florida Spodosol amended with biosolids and triple superphosphate (TSP). Phosphorus losses were evaluated with traditional indoor rainfall simulations but used a double-deck box arrangement that allowed leaching and runoff to be determined simultaneously. Biosolids (Lakeland, OCUD, Milorganite, and Disney) represented contrasting values of total P, percent water-extractable P (PWEP), and percentage of solids. All P sources were surface applied at 224 kg P ha–1, representing a soil P rate typical of N-based biosolids application. All biosolids-P sources lost less P than TSP, and leachate-P losses generally dominated. For Lakeland-amended soil, bioavailable P (BAP) was mainly lost by runoff (81% of total BAP losses). This behavior was due to surface sealing and drying after application of the slurry (31 g kg–1 solids) material. For all other P sources, BAP losses in leachate were much greater than in runoff, representing 94% of total BAP losses for TSP, 80% for Milorganite, 72% for Disney, and 69% for OCUD treatments. Phosphorus leaching can be extreme and represents a great concern in many coarse-textured Florida Spodosols and other coastal plain soils with low P-sorption capacities. The PWEP values of P sources were significantly correlated with total P and BAP losses in runoff and leachate. The PWEP of a source can serve as a good indicator of potential P loss when amended to sandy soils with low P-retention capacities.

Abbreviations: BAP, bioavailable phosphorus • DOP, dissolved organic phosphorus • ISP, iron-impregnated strips • PP, particulate phosphorus • PSI, phosphorus saturation index • PWEP percentage of water-extractable phosphorus • SRP, soluble reactive phosphorus • TDP, total dissolved phosphorus • TP, total phosphorus • TSP, triple superphosphate • WEP, water-extractable phosphorus


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Material and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 
EUTROPHICATION is a major threat to water quality in the USA (O'Connor and Elliott, 2006) and is becoming a concern in tropical regions (Shigaki et al., 2006). Phosphorus (P) and nitrogen (N) contribute to eutrophication, but P is the primary agent in freshwater eutrophication because many algae are able to obtain N from the atmosphere (Schindler, 1977). One potential source of water contamination with P is the land application of biosolids. When applied to meet crop N requirements, biosolids can add up to 10 times the amount of P removed in harvests, increasing the risk of P loss to water (Stehouwer et al., 2000).

Many sandy soils in Florida are often P deficient and inherently low in Fe- and Al-based minerals that sorb P (Nair et al., 2004). Because of the low P-retention capacity and hydraulic conditions, P contamination of Florida soils and waters is a concern (He et al., 1999). The flat, coarse-textured soils can attenuate surface runoff but can allow significant P leaching to ground waters (O'Connor et al., 2005).

The National Pollution Discharge Elimination System permit limits effluent P to control eutrophication in receiving waters for many municipalities. The most common method for removing dissolved P from wastewater is precipitation of metal phosphates through the addition of Fe and Al salts. The salts are also used for odor control and as conditioning agents in dewatering. Wastewater treatment plants differ in operational processes, and biosolids from different plants have variable concentrations of Fe and Al depending on treatment objectives. Iron and Al are important factors explaining the P solubility difference between biosolids and manures (Brandt et al., 2004).

The P concentration in runoff from biosolids-amended soils depends on the types of wastewater and solids processing methods used to generate the biosolids. Because of additions of Al and/or Fe during wastewater treatment, or additional solids processing like heat drying, some biosolids produce runoff P losses less than those from unamended soil (Brandt and Elliott, 2003; O'Connor and Elliott, 2006).

An indication of the degree to which biosolids P is potentially bound with Fe and Al is the phosphorus saturation index (PSI = moles of oxalate-extractable P/moles of oxalate-extractable Fe + Al) (Pote et al., 1996). According to Elliott et al. (2002), PSI values <1 suggest excess Fe and Al for binding of P (little available P), whereas PSI values >1 suggest available P beyond that associated with Fe and Al precipitates. The P-binding soil components affect P availability once biosolids are land applied, but the PSI value of biosolids is useful as an a priori index of biosolids-P lability, especially in very sandy soils.

The solubility of P in an organic amendment exerts a major influence on the potential for off-site migration of P at land application sites in sandy soils. Phosphorus loss from agricultural soils amended with organic sources of P is proportional to the water-extractable P (WEP) concentration of the source material (Brandt and Elliott, 2003; Brandt et al., 2004; Kleinman and Sharpley, 2003; Kleinman et al., 2002; Sharpley and Moyer, 2000)

In addition to dissolved P, particulate P (PP) can be highly important, especially in runoff for some organic P sources, and consideration of the potential lability of PP seems appropriate. Sharpley (1993a,b) showed that P extracted from sediment-laden runoff by iron-impregnated strips (ISP) was strongly correlated with the growth of P-starved Selenastum capicornutum and suggested that ISP could be used as a good estimate of biologically available P (BAP). The BAP should include dissolved P forms and the labile portion of PP. Summing the BAP losses for runoff and leachate should give a good indication of the total pool of P lost that is of environmental significance.

Rainfall simulation has been used with success throughout the last 80 yr to conduct research on infiltration, surface water runoff, and soil erosion. Natural rainfall is desirable, but spatial and temporal distribution of rainfall intensity cannot be controlled (Moore et al., 1983). On the other hand, simulator conditions can be closely matched with natural rainfall, and simulators offer control of variables such as rainfall intensity and uniformity (Humphry et al., 2002).

The vast majority of box rainfall simulations have been designed to evaluate only runoff. Few rainfall simulation experiments have addressed BAP losses or have been conducted with sandy soils. The objective of this study was to determine runoff and leachate P losses (including BAP losses) from a sandy soil amended with triple superphosphate (TSP) or one of four biosolids with different P solubilities. We hypothesized that P leaching losses would be greater than P runoff losses and that P losses (as BAP) would be correlated to P-source solubility.


   Material and Methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Material and Methods
Results and Discussion
 Conclusions
 REFERENCES
 
Soil and Phosphorus Sources
A typical Florida Spodosol (Immokalee fine sand, sandy siliceous hyperthermic Arenic Alaquods), four biosolids (Lakeland, OCUD, Disney, and Milorganite), and one mineral fertilizer (TSP) were used in this study. Native Immokalee sand, not contaminated by manure depositions and having "very low" soil test P, was collected from the University of Florida Research and Education Center in Immokalee, FL. Multiple random bulk samples were collected from the A horizon (0–15 cm) and thoroughly mixed to yield a composite sample. Soil characteristics are: pH, 5.5; organic matter, 15 g kg–1; total Fe + Al, 0.2 g kg–1; total P, 0.02 g kg–1; Mehlich-1 P, 5 mg kg–1.

Characteristics of the four biosolids and TSP are given in Table 1 . The Lakeland biosolids is generated by autothermal thermophilic aerobic digestion. The OCUD biosolids is an anaerobically digested material, and the Disney material results from composting a mixture of dewatered primary and secondary wastewater solids with a bulking agent (wood chips) via the aerated static pile method. Milorganite represents anaerobically digested material that is then heat-dried and pelletized.


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  		Table 1. Selected properties of source materials.

 
Biosolids and the mineral fertilizer (TSP) were analyzed for percent total solids (TS), water extractable P (WEP), and total P (TP) concentrations. The percent of total P that is water extractable (PWEP) was then calculated. Following digestion according to the USEPA Method 3050A (USEPA, 1986), total recoverable P was determined using ICP–AES (PerkinElmer Plasma 3200; PerkinElmer, Wellesley, MA). Total C and N concentrations were determined in the biosolids by combustion at 1010°C using a Carlo Erba analyzer (NA-1500 CNS; Carlo Erba, Milan, Italy). Oxalate (200 mM)-extractable P, Fe, and Al of biosolids were determined by ICP–AES after extraction at a 1:60 solid/solution ratio, following the procedures of McKeague et al. (1971). Percent solids were determined by drying materials at 105°C (Sparks, 1996), and pH measurements were performed on the materials (1:2 solid/water ratio). Quality assurance and quality control protocols included 5% repeats, spikes, certified samples, and blanks for each procedure (duplicates). Analyses that did not satisfy this quality assurance/quality control protocol were rerun.

The P saturation index (PSI = [Pox]/[Alox + Feox]) was calculated from the Pox, Feox, and Alox molar concentrations in the biosolids. Because only Feox and Alox are considered, the index is not useful for Ca-dominated amendments, such as TSP. For water-extractable phosphorus (WEP) analysis, distilled/deionized water was added to a 0.5 g (dry weight equivalent) of P-source (as received) to give a 1:200 solid/solution ratio. High solid/water ratios reportedly relate better to runoff potential than lower solid/water ratios (1:200, Sharpley and Moyer, 2000; 1:250, Brandt et al., 2004; O'Connor and Elliott, 2006). Samples were shaken on an end-over-end shaker at ~15 rpm for 1 h at room temperature. The suspensions were centrifuged (15 min at ~1250 rpm) and vacuum-filtered through pre-wetted 0.45-µm filter paper. Phosphorus concentrations in the extracts were determined by the Murphy and Riley (1962) method. Samples were prepared for ICP–AES analysis by adding 0.5 mL HCl (1:1) to 24.5 mL of filtrate. All extractions were performed in duplicate.

Rainfall Simulation Experiment
Phosphorus losses in runoff and leaching were evaluated using a modification of the U.S. National Phosphorus Research Project indoor runoff box protocol (National Phosphorus Research Project, 2005). The normal 100-cm-long, 20-cm-wide, and 7.5-cm-deep wooden runoff boxes were modified to quantify P leaching in addition to P runoff by adding a second empty (water proof) box under the first in a double-deck design. This design allows runoff and leachate collection simultaneously. The top boxes were each packed with 5 cm (14 kg) of soil to a bulk density of 1.4 g cm–3, and the entire double-deck box assemblage was sloped to 3%.

All P-sources were surface applied uniformly to the soil at an equivalent total P application rate of 224 kg P ha–1, corresponding to a typical biosolids application rate based on crop N requirements. Although these rates can minimize N leaching losses to groundwater, the resulting P application rate greatly exceeds crop requirements (Stehouwer et al., 2000; Sarkar and O'Connor, 2004). The Lakeland material was applied to the soil as slurry (31 g kg–1 solids). The TSP, Milorganite, and the Disney compost were dry materials (660–950 g kg–1 solids), and OCUD contained 110 g kg–1 solids. Amended soils were pre-wetted to near saturation to control for antecedent moisture and to promote runoff in the subsequent rainfall simulation. Rainfall simulations were conducted three times at 1-d intervals between rainfall events. Rainfall was delivered at 7.1 cm h–1 from a height of 3 m above the boxes, mimicking a 25-yr, 1-h rainfall in central Florida. Rainfall continued until 30 min of runoff was generated for each box, and the runoff volume was recorded. Simultaneously, leachate (which appeared before runoff occurred) generated during the entire rainfall was collected, and the volumes were recorded. Subsamples of runoff were immediately filtered (0.45 µm) for soluble reactive P and total dissolved P (TDP) analysis. Representative, well mixed samples of the unfiltered runoff and of the leachate (~250 mL each) were taken from each replicate for additional analysis.

Leachate and runoff (filtered and unfiltered samples) pH and EC values were determined using standard procedures (Sparks, 1996). Soluble reactive P (SRP) was determined on the filtered runoff and the leachate samples colorimetrically (Murphy and Riley, 1962). Total dissolved P was measured on the filtered runoff and the leachate samples after digesting 10 mL of the samples with 0.5 mL 11 N H2SO4 and 0.15 g of potassium persulfate in an autoclave for 1 h (Pote and Daniel, 2000a,b). Total P in the unfiltered runoff samples was determined by digesting 5 mL of the samples with 1 mL of 11 N H2SO4 and 0.3 g of potassium persulfate on a digestion block and then diluting to 10 mL with distilled deionized water (Pote and Daniel, 2000b). All digested samples were analyzed for P colorimetrically (Murphy and Riley, 1962). The iron-oxide impregnated paper strip method (Myers and Pierzynski, 2000) was used to estimate bioavailable P (BAP) in unfiltered runoff waters. Particulate phosphorus (PP) was calculated by subtracting TDP from the TP of each runoff sample. Dissolved organic P (DOP) was assumed to be the difference between SRP and TDP.

Flow-weighted P concentrations (SRP, TDP, or TP) were determined for the runoff and the leachate samples by summing the product of the P concentrations and volumes for the three runs to yield cumulative P mass and dividing the P mass by the total volume of the runs. The masses of runoff and of leachate P losses (mg) were calculated as the product of flow-weighted concentrations (mg L–1) and the runoff and leachate volumes (L), respectively. Leachate samples, with the exception of the samples from Lakeland biosolids treatments, were clear with no observable particulate matter. Therefore, TDP in leachate was taken to represent BAP (Sharpley, 1993c) on the assumption that any DOP in the leachate would eventually mineralize to become bioavailable. Oladeji (2006) has confirmed that when no turbidity in leachates is observed, BAP equates to TDP and TP. Turbidity in leachates from the Lakeland biosolids-amended treatments prompted actual determination of the BAP concentrations. Total BAP losses for each treatment were determined by summing the masses of runoff BAP and the leachate BAP loss.

Statistical Analyses
All runoff and leachate data were logarithmically transformed to conform to ANOVA assumptions of normality and equal error variances. Treatment effects were evaluated using the general linear model (PROC GLM) of the SAS software version 9 (SAS Institute, 2002) along with Tukey's mean separation. Treatment differences were tested at a significance ({alpha}) level of 0.05.


   Results and Discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Material and Methods
 Results and Discussion
Conclusions
 REFERENCES
 
The wide ranges of values found for TP, WEP, solids contents, and PSI were desirable for the study. Total P concentrations ranged from 12.5 to 194 mg kg–1 TP, WEP concentrations ranged from 0.1 to 165 mg kg–1, and solids content ranged from 31 to 950 g kg–1 (Table 1). Milorganite was the only biosolid with a PSI <1, reflecting the relatively high concentration of oxalate-extractable Al + Fe in the material (Table 1). The low PSI value suggests that Milorganite has little available (labile) P (Elliott et al., 2002) and little P environmental loss is expected. The Lakeland, OCUD, and Disney materials had PSI values >1, suggesting more P available for loss.

Runoff and Leachate Volumes
Total volumes of runoff + leachate were not statistically different among the treatments, but some differences were found in the separate runoff and leachate volumes (Table 2 ). Total average runoff volumes ranged from 9.8 to 21.9 L (Table 2), the greatest value being found in the soil amended with Lakeland. Runoff volumes were not significantly different for all the other treatments. The Lakeland slurry had only 31 g kg–1 solids (Table 1) and nearly sealed the soil surface after application. In the first rainfall event, the volume of runoff was 8.4 L and appeared within 2 to 3 min of rainfall in the Lakeland treatment, whereas runoff values ranged from 2.8 to 5.0 L and appeared 10 to 15 min after rainfall commenced when the soil was amended with the other P sources. The average leachate volume for the Lakeland-amended samples was very low (3.5 L). In the first rainfall event, no leachate was collected in two of the three boxes, and only 170 mL were collected in the third box.


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  		Table 2. Runoff, leachate, and total volumes collected from the three rainfall events.

 
Amendment Effects on Runoff and Leachate Phosphorus
There were differences in runoff P concentrations among biosolids types after surface application to soil boxes exposed to simulated rainfall (Fig. 1 ). Experimental conditions mimicked the spreading of biosolids without incorporation. Studies have shown that biosolids applied at the same total P rate can exhibit significantly different runoff P losses (Penn and Sims, 2002; Brandt and Elliott, 2003; Sims et al., 2003; O'Connor and Elliott, 2006).


Figure 1
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  		Fig. 1. Runoff total phosphorus (TP) and total dissolved phosphorus (TDP) concentrations for five P sources at the first, second, and third rainfall events (note scale differences). Bars indicate 1 SD.

 
Trends in TP and TDP concentrations in runoff with P source treatment were similar for all rainfall events (Fig. 1). As expected, total P concentrations exceeded TDP concentrations, especially for the Lakeland treatment, where PP contributed greatly to total runoff P. Total dissolved P concentrations in runoff tended to decrease dramatically with subsequent rainfall events for the relatively high-solubility P sources (TSP and Lakeland). In the first rainfall event, runoff losses of TDP for Lakeland, TSP, and OCUD treatments were 72, 89, and 34% of the total for all events, respectively. The same general trend was observed by Rostagno and Sosebee (2001) and Penn and Sims (2002), who documented that TP and dissolved P in runoff decreased with successive rainfall events after surface application of biosolids. Kleinman and Sharpley (2003), Pierson et al. (2001), Sims et al. (2003), and Withers and Bailey (2003) also observed that the majority of TDP loss from biosolids and manures generally occurs during the first runoff event. In the third event, TDP losses represented less than 10% of cumulative TDP losses for Lakeland and TSP treatments but 37% of TDP losses for OCUD. The OCUD material released P at a relatively constant rate during the three events and seemed to function as slow-release P-source of moderate P solubility. The greater P solubilities in TSP and Lakeland may have allowed greater P loss in the first rainfall, leaving less P available for loss in subsequent events. The moderate P solubility in the OCUD material may have reduced P loss in the first event but was able to re-establish moderate soluble P concentrations before the next rainfall event. Runoff TP and TDP concentrations from the lower soluble P materials (Disney and Milorganite) were low in all three events.

For the first rainfall event, BAP concentration in the leachate was very low (1.3 mg L–1) for the Lakeland treatment when compared with the other treatments (Fig. 2 ). Bioavailable P concentration was 139 mg L–1 for the TSP treatment and <7 mg L–1 for the other treatments. In the second and in the third events, BAP concentrations were greater for TSP (41 and 25 mg L–1, respectively) and Lakeland (33 and 28 mg L–1, respectively) than for OCUD (9 and 11 mg L–1, respectively), Disney (5 and 3 mg L–1, respectively), and Milorganite (3 and 2 mg L–1, respectively), without significant differences among OCUD, Disney, and Milorganite (Fig. 2). Bioavailable P leachate concentration in Lakeland treatment increased in the second and third rainfall events because the crust formed in the first event had fractured and let more P leach through. On the other hand, leachate BAP for TSP treatments decreased from 139 mg L–1 in the first rainfall event to 40 mg L–1 in the second event and to 24 mg L–1 in the third event. The data reinforce the importance of evaluating flow-weighted concentration data instead of focusing on a single rainfall event. Similar to runoff results, leachate P loss from the OCUD biosolids remained constant rate during the three rainfall events (Fig. 2).


Figure 2
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  		Fig. 2. Leachate bioavailable phosphorus (BAP) concentrations for five P sources at the first, second, and third rainfall events. Bars indicate 1 SD.

 
Flow-Weighted Runoff and Leachate Phosphorus Concentrations
Flow-weighted runoff P concentrations were highly correlated with percentage water-extractable phosphorus (PWEP) values of the biosolids (Fig. 3 ) but not for TSP because most of the fertilizer-P loss was in leachate. For the range of materials used in this study, PWEP and runoff TDP were linearly related (P < 0.01) (Fig. 3a), whereas an exponential relationship (P < 0.01) best related PWEP and runoff TP in runoff (Fig. 3b). Withers et al. (2001) reported that differences P loss in runoff from soil amended with TSP fertilizer, cattle slurry manure, and two biosolids were more related to P-source WEP than total P application rates, even though the latter ranged from 150 to 329 kg P ha–1. In our experiment, WEP was greater in the Lakeland material than in the other biosolids, and the Lakeland material resulted in the greatest P concentration in runoff, which supports the hypothesis that WEP can accurately predict TDP in runoff (Kleinman et al., 2002).


Figure 3
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  		Fig. 3. Flow-weighted runoff total dissolved phosphorus (TPD) (a) and total P (TP) (b) concentrations in the first rainfall event as a function of the percent water-extractable P (PWEP) for biosolids P sources.

 
Flow-weighted concentrations of the various P forms in leachate were dominated by SRP (Table 3 ). Soluble reactive P in leachate was greater than in runoff samples for all treatments, and more so for fertilizer-P (Table 3). The SRP data emphasize the dominant role of leaching as a P loss mechanism in sandy soils with low P-sorbing capacities. Such soils are common in the Coastal Plain region of the USA. Site assessment tools for these soils must include consideration of leaching, as well as runoff, to realistically assess P loss risk (O'Connor and Elliott, 2006). For all the treatments, there was minimal DOP (calculated as the difference between TP and SRP) in leachates (4.4 mg L–1 for Lakeland, 2.0 mg L–1 for Disney, 1.3 mg L–1 for Milorganite, 1.1 mg L–1 for TSP, and 0.7 mg L–1 for OCUD treatment). The concentration of PP in Lakeland leachate was small (0.5 mg L–1).


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  		Table 3. Soluble reactive phosphorus (SRP) and bioavailable phosphorus (BAP) concentrations in the runoff and the leachate (flow-weighted over three rainfall events).

 
Flow-weighted leachate BAP concentrations (assumed to equal TP for all P sources except for Lakeland, where BAP was determined by the Fe strip method) ranged from 2.3 to 68.6 mg L–1 (Table 3) and were positively correlated (P < 0.01) with PWEP of P sources (Fig. 4a ). The PWEP values can be a good indicator of P-source leachability in sandy soils with low P-sorption capacities. Flow-weighted BAP concentrations were greater in leachate than in runoff, especially for TSP (Table 3).


Figure 4
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  		Fig. 4. Leachate bioavailable phosphorus (BAP) flow-weighted concentrations (a) and cumulative mass of BAP (b) in the leachate as a function of the percent water-extractable P (PWEP) for all P sources.

 
Phosphorus Losses
To compare total runoff and leachate P losses for each source, flow-weighted concentrations were converted to masses by multiplying by the corresponding runoff and leachate volumes. In soil samples amended with Lakeland, TSP, and Disney, the initial rainfall event accounted for more than 60% of total runoff P loss over the three rainfall events (Table 4 ). Similar results have been reported by Sims et al. (2003), who found that the initial rainfall event accounted for 52 to 73% of the total P lost in runoff from biosolids-amended plots over four rainfall events. On the other hand, total P loss from soils amended with OCUD and Milorganite was uniformly distributed among the three events.


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  		Table 4. Masses of total phosphorus (TP) in runoff and in leachate for each rainfall event, and cumulative mass losses of TP.

 
Field simulation results suggest that the nature of the P source (compost, slurry, granular fertilizer) influences the erosion potential and, consequently, the TP lost during a rainfall event. McDowell and Sharpley (2003) found that more TDP was transported from dairy manure–ammended than poultry manure–amended soil, but more TP was lost from the latter treatment due to a greater contribution of PP. In our study, soil amended with Lakeland lost more TDP in runoff than the other sources (165.0 mg for Lakeland, 38.9 mg for TSP, 42.7 mg for OCUD, 21.4 mg for Disney, and 6.9 mg for Milorganite), but the differences in magnitude was much larger for PP values (1046.9 mg for Lakeland, 26.5 mg for TSP, 40.7 mg for OCUD, 13.8 mg for Disney, and 7.1 mg for Milorganite).

Soil amended with TSP lost 69% of total leachate P mass collected in the first rainfall event (Table 4). For Disney- and Milorganite-amended soil, larger P losses were also measured in the first event than in subsequent events, but the percentages of cumulative P loss were not as high as in the TSP treatment. On the other hand, TP loss from OCUD-amended soils gradually increased over the three rainfall events, whereas total P loss from Lakeland-treated soil was negligible during the first event.

Total BAP losses were smaller from the organic amendments than from TSP (Table 5 ). Differences in cumulative BAP losses among the various organic sources were statistically significant and suggested greater losses for sources with greater WEP values (Table 1). Leaching was the predominant mechanism for BAP loss from TSP-amended soils (Table 5 and Fig. 4b), accounting for 94% of the total P loss. Similar large percentage losses were noted for OCUD (69%), Disney (72%), and Milorganite (80%), whereas BAP mass losses from Lakeland were much greater in the runoff than in the leachate (81 vs. 19%). When comparing the amounts of P lost and the total mass of P applied to the boxes, it is important to state that although Milorganite leached 80% of the total BAP lost and TSP leached 94% of the total BAP lost, the percentages of the applied P are drastically different for the two P sources. Due to the high WEP and PWEP contents of TSP, total BAP loss from TSP-amended soils was at least 3.6 times greater than that from biosolids-amended soils, despite equal P application rates in all treatments.


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  		Table 5. Cumulative masses of bioavailable phosphorus (BAP) in runoff, TP in leachate, and their sum over the three rainfall events.

 

   Conclusions
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Material and Methods
 Results and Discussion
 Conclusions
REFERENCES
 
The vast majority of box rainfall simulations have quantified only runoff P losses. Few researchers have attempted to relate various measures of P loss to BAP, and few studies have been conducted with sandy soils where leaching losses of P can be important. The objective of this study was to determine runoff and leachate P losses (including BAP losses) from a sandy soil with low P-sorption capacity amended with fertilizer- and biosolids-P sources, all applied at the same P rate. We hypothesized that P leaching losses would be greater than runoff P losses and that P losses would be correlated with P-source solubility.

All biosolids-P sources lost less P than fertilizer-P (TSP): total (runoff + leachate) BAP mass loss was ~1000 mg for TSP and varied from 30 to 284 mg for the various biosolids. Leachate P losses dominated (69–94%) runoff P losses for all materials, except a slurry biosolids (31 g kg–1 solids) that sealed the soil surface and resulted in substantial runoff particulate P. Total P and BAP losses in runoff plus leachate were significantly correlated with source P solubility, as expressed by PWEP. This study showed that assessments of P loss risk potential can account for P leaching in sandy soils with low P-sorption capacities and can distinguish between various P sources on the basis of source-P solubility.

The double-deck rainfall simulation boxes used here were useful to account for possible leachate P losses and to contrast with possible runoff P losses in sandy soils, but field validation of the results are critical. Vertical leaching of P to shallow water tables, which are intercepted by drainage systems (called subsurface runoff), can complicate runoff P management and risk assessment.


   ACKNOWLEDGMENTS
 
The authors thank Dr. Sampson Agyin-Birikorang for providing detailed information regarding the laboratorial methods used in this study.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 Material and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 
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   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Material and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 




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S. Agyin-Birikorang, G. A. O'Connor, and S. R. Brinton
Evaluating Phosphorus Loss from a Florida Spodosol as Affected by Phosphorus-Source Application Methods
J. Environ. Qual., May 1, 2008; 37(3): 1180 - 1189.
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