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DIVISION S-8 - NUTRIENT MANAGEMENT & SOIL & PLANT ANALYSIS

Taxonomic and Geographic Distribution of Total Phosphorus in Florida Surface Soils

^a Institute of Geography and Natural Resources, Chinese Academy of Sciences, Beijing 100101, China
^b Soil and Water Science Dept., University of Florida, Gainesville, FL 32611-0290

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

	ABSTRACT

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

 
Taxonomic and geographic distributions of background P concentrations are important in assessing whether a soil P level is influenced by anthropogenic activities. This study was conducted to establish an upper baseline concentration (UBC) of soil P, which is defined as 97.5% of the background concentration, using 448 geographically and pedogenically representative Florida surface soils (genetic horizon A, A1, Ap, O, O1 or Op) using total P as determined by the USEPA Method 3052 (HCl–HNO₃–HF digestion). A significant difference existed in total P concentrations between disturbed (126 mg kg^-1, n = 180) and undisturbed (60 mg kg^-1, n = 268) soils. Geometric mean (GM) concentration of total P in the undisturbed soils decreased in the order of Histosols (350 mg kg^-1) > Mollisols (171 mg kg^-1), Inceptisols (140 mg kg^-1) > Ultisols (88 mg kg^-1) > Alfisols (54 mg kg^-1), Entisols (53 mg kg^-1) > Spodosols (24 mg kg^-1). Aquic suborders tended to have greater P contents than the dry suborders, e.g., Aquents (92 mg kg^-1) > Psamments (47 mg kg^-1) and Aquods (27 mg kg^-1) > Orthods (14 mg kg^-1). Total P estimation based on digitized taxonomic soil maps suggested that native soil properties were primary factors in controlling total P in soils. The wide occurrence of P bearing parent materials resulted in many soils having high P concentrations. Twenty-four P-elevated samples from the disturbed soils were identified using the UBC of P for the undisturbed soils at suborder level as reference criterion. Anthropogenic P inputs were related to commercial PO₄–fertilizer application and population growth as nonpoint sources.

Abbreviations: GM, geometric mean • GSD, geometric standard deviation • muid, map unit identifier • STATSGO, state soil geographic database • UBC, upper baseline concentration

	INTRODUCTION

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

 
PHOSPHORUS IS A MAJOR NUTRIENT essential for plant growth. Fertilizer application of P and other nutrients has contributed more than any other measure to increasing crop productivity in agriculture. However, overabundance of P in the nation's waters and soils has become a major environmental concern (Muller et al., 1995; Daniel et al., 1998). Transportation of soluble and soil-bound P influences soil fertility strategies and can accelerate eutrophication in lakes and streams surrounding agricultural areas. Knowing the P status of soils is important for developing an effective P-management strategy (Chen et al., 1999a; Higgs et al., 2000). Numerous studies on P assessment focused on plant-available soil P tests (Sharpley et al., 1996; Daniel et al., 1998; Higgs et al., 2000). Assessment criteria based on total soil P are, however, not emphasized in the literature.

Taxonomic and geographic distributions of background P concentration in soils are important when deciding whether a site is influenced by anthropogenic input of P. Knowledge of background concentration and its variation in soils helps address issues such as the effects of past land-use practices on P levels in soils and establishing proper limits when conducting risk assessments for P eutrophication in water systems. Upper baseline concentration, which is defined as the upper 97.5% and calculated as the GM multiplied by the square of geometric standard deviations (GSD) of the expected range of background concentration, has been used in several studies to assess changes in elemental compositions (Gough et al., 1994; Dudka, 1995; Chen et al., 1999b). Chen et al. (1999a) reported that soil UBC of total P equals 1374 mg kg^-1 and could be used as a reference level for assessing potential nonpoint sources of P enrichment in Florida soils. However, no detailed information is available on how this information can be applied to different land uses and soil category.

At the state level, Florida showed the most striking pattern by having low soil background concentrations of most elements in the U.S. A few soil samples with high P were associated with PO₄ deposits in Florida (Shacklette and Boerngen, 1984). In southern Florida, however, P has been considered the element that most threatens the overall health of lakes and the critical level of dissolved P allowable in drainage water entering the Everglades is currently being debated (McPherson and Halley, 1996). A large, rapidly growing population in the coastal peninsula region and in central Florida since the 1970s continues to impact the Florida environment via elevated N and P levels in soils (McPherson and Halley, 1996). Intensive cattle ranching and farming is another nonpoint source of nutrients that can elevate soil P concentration because of the lack of economically viable alternatives for manure disposal (Harris et al, 1994; Maluk et al., 1998; Sharpley et al., 1996). Field studies have shown that dissolved P concentrations in runoff were related to total P in surface soils (Sharpley et al., 1996). Little is known about taxonomic and geographic distributions of P background concentrations in Florida surface soils.

The objectives of this research were (i) to establish natural and anthropogenic background concentrations of total P in Florida surface soils; (ii) to determine taxonomic distribution characteristics of total P in soils; and (iii) to develop soil P maps in Florida using the state soil databases and geographic information systems. It is hoped that such information will help to evaluate the significance of anthropogenic and natural sources of P inputs in soils by establishing normal total P ranges in different soils. In addition, understanding how P levels vary across the land can lead to information useful in developing site-specific P management strategies.

	MATERIALS AND METHODS

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

 
Sample Selection, Characterization, and Preparation
Soils used in this study had been sampled and characterized as a part of the Florida Cooperative Soil Survey Program conducted jointly by the University of Florida Soil and Water Science Department and the USDA–NRCS. Four hundred forty eight air dried surface soil samples (genetic horizon A, A1, Ap, O, O1, or Op) were selected from a pool of about 8000 archived samples to ensure taxonomic and geographic representation (Fig. 1) . The overall taxonomic representation was achieved by weighing the number of samples for each soil order by their estimated areal extent in Florida. Geographical representation of the samples was achieved via an adequate scattering of samples throughout the state and a representation of the most extensively occurring soil series mapped within each county. The selected soil samples were from rural areas and were originally selected based on how well they represented the soil series being mapped. Most of the selected sites were under native vegetation (undisturbed soils, n = 268) at the time of sampling and 40% (disturbed soils, n = 180) of the samples were described as having surface horizons that had been disturbed either by plowing or clearing (p-subordinate horizon designation). Sampling depth varies from the top 1 to 76 cm, with a mean of 16 cm. These soils were from 51 of 67 counties, and their map units covered 80% of the total land area of Florida (Chen et al., 1999b).

View larger version (75K): [in this window] [in a new window]   Fig. 1. Distributions of Florida soil orders based on the state soil geographic database at a scale of 1:250000 and selected Florida cooperative soil survey program soil sampling sites. White areas are water bodies. Numerous neighboring sampling sites appear coincident at this map scale. White areas are water bodies.

Soil samples were passed through a 0.25-mm screen and digested using the EPA Method 3052 (USEPA, 1995). Approximately 1.000 g of a soil sample was weighed into a 120-mL Teflon pressure digestion vessel, and 9 mL of concentrated HNO₃, 4 mL of concentrated HF, and 1 mL of concentrated HCl were then added. Samples and reagents were well mixed, sealed, and digested in a CEM-2000 digestion microwave oven (CEM Corp., Matthews, NC) for 20 min at 0.83 x 10⁶ Pa (120 psi). After cooling, 2 g of boric acid were added to the digested solution to neutralize excess HF. The samples were then filtered through Whatman #42 filters (Whatman International Ltd., Maidstone, UK) and diluted to 100 mL with deionized distilled water. Total concentrations of Al, Fe, P, and trace metals were analyzed on an inductively coupled plasma spectrophotometer (Thermo Jarrell Ash ICAP 61-E, Norwalk, CT). Method detection limit for the P determination is 10 mg P kg^-1soil. Contents of clay, organic C, and bulk density of the soils were previously determined (Sodek et al., 1990). Concentrations of trace metals were reported separately (Chen et al., 1999b).

Data Analysis and Mapping
Phosphorus concentrations are presented on a dry matter basis. Undetected data were censored using a half of the method detection limit, i.e., 5 mg P kg^-1, for calculation. Analysis of variance was used to assess significant differences between soil categories or land uses. The confidence level for the Student t-Test was calculated at equals 0.05. Cumulative probability distributions of total P in different soil category (Fig. 2) and land uses (Fig. 3) were plotted using methods provided by Myers (1997). For a given P concentration in soils on the x-axis, the percentage of soil samples below that the concentration can be read off the graph. The shape of the curve represents the distribution characteristic of total P for a given soil category or land use, with steeper slopes indicating a narrower spread of concentrations and horizontal offsets indicating greater differences in total P. Because of the log-normal distribution of P concentrations in Florida surface soils (Chen et al., 1999a), GM and GSD were used to describe the central tendency and variation of the data. Upper baseline concentration was defined as the 97.5% of the expected range of background concentration and calculated as GM multiplied by GSD² (Dudka, 1995; Gough et al., 1994; Chen et al., 1999b). Soil samples were screened using UBC P values at different taxonomic levels to determine possible anthropogenic P enrichment. For example, sites with P concentrations equal to or larger than the UBC P values for individual soil suborder were used to indicate possible P enrichment at the suborder level.

View larger version (30K): [in this window] [in a new window]   Fig. 2. Cumulative probability curves for total P in 448 Florida surface soils based on soil orders. Dashed lines represent upper baseline concentrations (UBC). Numbers in parenthesis are sample number for each soil order.

View larger version (18K): [in this window] [in a new window]   Fig. 3. Cumulative probability curves for total P in Florida surface soils: disturbed vs. undisturbed soils. Numbers in parenthesis are sample number for each soil category.

(1)

where C_muid represents the area weighted P concentration for a map unit; C_i represents GM P concentration for suborder i of a map unit; and p_i represents component percentage of suborders in a map unit.

Geographic distribution of soils with high P was evaluated by plotting soils with total P > UBC values at suborder level (Fig. 4) . County P fertilizer application rates were calculated by multiplying the 1992 Census of Agriculture annual state use (USDA-ERS, 1994) by the ratio of county fertilizer expenditures to state PO₄ fertilizer expenditures (Bureau of the Census, 1992), and divided by county farm acreage (USDA-ERS, 1994).

View larger version (73K): [in this window] [in a new window]   Fig. 4. Map of estimates of total soil P for Florida soil suborders based on the state soil geographic database at a scale of 1:250000 and geographic distribution of potential P elevated sites for disturbed and undisturbed soils. White areas are water bodies.

	RESULTS AND DISCUSSION

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

It is necessary to separate the database into disturbed and undisturbed soils to define UBC P levels separately, since influences of human activities on P concentrations in soils are expected. Total P in the undisturbed soils ranged from 5 to 1680 mg kg^-1, with a GM of 60 mg kg^-1, whereas total P in the disturbed soils ranged from 5 to 2870 mg kg^-1, with a GM level of 126 mg kg^-1 (Table 1). Cumulative probability distributions indicate total P in the disturbed soils was greater than that in the undisturbed soils (Fig. 3). Statistical analysis showed significant difference (P = 0.05) existed between total P in the disturbed soils and in the undisturbed soils, indicating anthropogenic contribution of P in soils. Upper baseline concentration of P in the disturbed soils was greater than that in the undisturbed soils: 1983 vs. 917 mg P kg^-1 (Table 1). The UBC of P in the undisturbed soils was, therefore, a better criterion than the UBC of P in the disturbed soils, for assessing anthropogenic P inputs in the disturbed soils.

Undisturbed Ultisols had the third greatest GM P concentration (88 mg kg^-1) among the seven soil orders, and the greatest among the three dominant (areal base) soil orders (Spodosols, Entisols, and Ultisols) in Florida (Table 1). This agrees with Yuan (1992), who stated that P retention in Ultisols was considerably higher than that in Entisols and Spodosols, which could be attributed to the relatively higher contents of total Al and Fe in Ultisols (Table 2). Attention should be drawn to the fact that around 60% of the randomly selected Ultisols samples were from disturbed soils, the highest ratio among all soil orders studied. This is especially true in the case of Udults, which are very common in the northern part of the panhandle and the central highlands (Fig. 1). Statistical analysis showed significant difference in GM P between the disturbed (n = 46) and undisturbed (n = 24) Udults (188 vs. 75 mg kg^-1). No significant difference, however, was found between GM P between the disturbed (n = 8) and undisturbed (n = 8) Aquults (81 vs. 104 mg kg^-1).

Alfisols (54 mg kg^-1) and Entisols (53 mg kg^-1) had the fourth and fifth greatest GM P concentration, whereas Spodosols (24 mg kg^-1) had the lowest total P in the undisturbed soils. This is also true for disturbed soils (Table 1), which is consistent with results of metal concentrations published by Ma et al. (1997), who found that total trace metals generally decreased in the order, Ultisols > Entisols > Spodosols, in Florida surface soils. Spodosols had the lowest contents of clay and total Fe and Al of the seven soil orders in both undisturbed and disturbed surface soils (Table 2), which would contribute to the low P content of the soils. It has been reported that Spodosols in Florida have little retention capacity for ions in the surface horizon (Yuan, 1992). As a result, most elements, if there were much to begin with, were possibly leached from the surface.

It is interesting to note that the Aquic (wet) suborders of Entisols and Spodosols (Aquents and Aquods) had significantly greater total P (with a GM of 92 and 27 mg kg^-1, respectively) than the dry suborders (Psamments and Orthods) (with a GM of 47 and 14 mg kg^-1, respectively). Aquents are typically found in wetlands and are dominated by silt- and clay-sized CaCO₃ (marl soils), which are common in southern Florida, whereas Pamments are typically found on the central Florida ridge and are excessively to moderately well drained. Anthropogenic inputs of P to those wet soils were confirmed by the fact that GM P concentration in the disturbed Aquents was nearly three times greater than that of the undisturbed Aquents (268 vs. 92 mg kg^-1).

Because of the significant differences in total P concentrations among different soil category and land uses, UBC of P values based on soil orders and suborders should be more accurate than UBC of P value for all soils in assessing potential P enrichment. A surface soil with total P > 917 mg kg^-1 could occur naturally for Aquents, Aquepts, Aquolls, Hemists, and Saprists, but may indicate anthropogenic P input in other soil suborders (Aqualfs, Aquods, Aquults Orthods, Psamments, and Udults). Thus, UBC of P values for the undisturbed soils at soil order and suborder levels should be more appropriate in assessing potential anthropogenic P inputs.

Geographic Distribution of Total Phosphorus Concentrations in Soils
Estimates of total P concentrations for soil suborders indicate that generally total soil P in the southwest through the central to the northeastern Florida (5–60 mg kg^-1) was lower than average. Soils on the Gulf and the Atlantic coastal plains had generally higher than average total P (60–120 mg kg^-1). Soils with higher total P (120–240 mg kg^-1) extended into the northwestern panhandle and the central highlands of Florida, and in the southern Everglades. Soils with the greatest total P levels (240–480 mg kg^-1) were generally located in the Everglades areas, and in part of the central, and western Florida (Fig. 4). These patterns were consistent with differences in soil categories and soil properties. High total P concentrations occur in soils with high organic C (Histisols) and high limestone (Aquents) in the Everglades; low total P concentrations occur in Spodosols on the coastal plains, and Ultisols in the northwestern panhandle and the central highlands of Florida (Fig. 1). Soils in the Everglades had the highest concentration of total Fe and Al, whereas soils in the coastal plains had the lowest (Fig. 5a, b) . This confirms the importance of soil properties in controlling total P in Florida surface soils (Chen et al., 1999a).

View larger version (55K): [in this window] [in a new window]   Fig. 5. Maps of Florida showing factors influencing total P contents in surface soils. (a) Total Fe. (b) Total Al. (c) Phosphate deposits. (d) Commercial phosphate fertilizer application rates.

Impact of Anthropogenic Input on Total Phosphorus Concentrations in Soils
The principal factors affecting P fixation in natural acid soils are the clay mineralogy, clay content, x-ray amorphous colloid content, exchangeable Al, and soil organic C. The primary controlling factors for P concentrations in Florida surface soils are soil category and properties, such as contents of clay, organic C, and total Fe and Al (Chen et al., 1999a). At least 50% of the concentration variations for the elements As, Cr, Cu, Ni, Pb, Sn, and Zn in sediments can be accounted for by covariation with Fe and Al along the coastal USA (Daskalakis and O'Connor, 1995). Sediment metal concentrations are correlated with Al over large, presumably uncontaminated areas of the coasts of Florida (Windom et al., 1989; Schropp et al., 1990). In our study, significant correlation coefficients exist between total P concentration and clay (r = 0.97), organic C (r = 0.93), and total Fe (r = 0.87) in undisturbed soils (Table 2), suggesting that natural clay minerals might be the dominant P-bearing phases. However, no significant correlation exists between total P concentration and these factors in disturbed soils (Table 2), confirming human influence on the disturbed soils.

Anthropogenic inputs of P in Florida surface soils might come from various point and nonpoint sources. Point sources of P inputs are suspected because Florida has the largest deposits of PO₄ rock in the world, supplying 80% of the domestic PO₄ fertilizer and accounting for 25% of the total world production (Armstrong, 1988; Lane, 1994). In the current investigation, the soil with the highest total P (2870 mg kg^-1) was located in Polk County, location of the central land-pebble PO₄–deposits district. The soil with the second highest P (2610 mg kg^-1) was in Marion County, location of the northern land-pebble PO₄–deposits district (Fig. 5c). It was reported that PO₄ had been mined only in these two districts within the whole state (Cathcart, 1989; Lane, 1994). By design, however, the Florida Cooperative Soil Survey Program sampling sites were intentionally selected away from known anthropogenic P contaminated area to provide a representative coverage of Florida native soils. Thus the high P concentrations in soils in this study should not relate to isolated P contamination because of P mining or processing.

Commercial fertilizer was reportedly a primary nonpoint source of P elevation in waters and coastal drainages of North and South Carolina (Maluk et al., 1998). Historical fertilizer consumption in Florida showed the heaviest use was in the citrus-producing area of central Florida and widely separated areas of concentrated use were in the truck-crop areas of southern Florida (Bureau of the Census, 1992). Drainage and development irreversibly altered Florida's natural watershed and subsequently made south Florida an area with the highest application rates of N and P (McPherson and Halley, 1996). Comparison of sites with potential elevated P in southern Florida with P fertilizer application rates confirms this hypothesis (Fig. 5d). The soil with the third highest P level (2430 mg kg^-1) was found in southeastern Florida, with annual P fertilizer application rates as high as 1260 kg ha^-1. The soil with the fourth highest P (1680 mg kg^-1) was in an area with P fertilizer application between 300 and 600 kg ha^-1. Population density was another possible nonpoint source for elemental sediment contamination (Daskalakis and O'Connor, 1995). In our investigation, however, when commercial P fertilizer application rates and county population density were used as surrogates for nonpoint sources, total P in surface soils had strong association (P < 0.001) with commercial P fertilizer application rates (r² = 0.67), and slightly weaker association (P < 0.01) with population density (r² = 0.40). Although these statistical-based associations do not provide conclusive evidence of relations, they do support our hypothesis that P enrichment in disturbed Florida surface soils is likely related to human activity as anthropogenic nonpoint sources.

	CONCLUSIONS

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

 
Taxonomic and geographic distributions of total P in Florida soils were investigated and soil properties (contents of clay, organic C, and total Al and Fe) were found to be the primary controlling factors for total P concentrations in undisturbed Florida surface soils. Taxonomic distribution by GM P concentrations in the undisturbed soils followed the order, Histosols > Inceptisols, Mollisols > Ultisols > Entisols, Alfisols > Spodosols, as well as Aquents > Psamments, and Aquods > Orthods. Upper Baseline Concentration of P values for the undisturbed soils at the order and suborder levels were better criteria in screening potential P-elevated sites than using a single criterion. Anthropogenic P inputs in disturbed Florida surface soils were confirmed by the fact that total P in the disturbed soils was significantly greater than that in the undisturbed soils. Further research on mechanism and process for nonpoint anthropogenic P inputs in different soil categories and land uses is needed.

	ACKNOWLEDGMENTS

 
This research was sponsored in part by the Florida Center for Solid and Hazardous Waste Management (Contract No. 96011017). The authors thank those who participated in the Florida Cooperative Soil Survey. Their collection and characterization of a large number of Florida soil samples made this study possible. The helpful suggestions made to this manuscript by Drs. W.G. Harris, A. Mallarino, and three anonymous reviewers are gratefully acknowledged.

	NOTES

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

 
Approved for publication as Florida Agricultural Experiment Station Journal Series No. R-06744.

Received for publication September 28, 2000.

	REFERENCES

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

 

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