Soil phosphorus dynamics: agronomic and environmental impacts

ECOLOGICAL ENGINEERING ELSEVIER

Ecological Engineering 5 (1995) 261-279

Soil phosphorus dynamics: agronomic and environmental impacts Andrew N. Sharpley USDA-ARS, Pasture Systems and Watershed Management Research Laboratory, Curtin Road, Uni~,ersity Park, PennsylL,ania, PA 16802-3702, USA

Abstract

Phosphorus (P) related eutrophication of surface water is of increasing concern in many areas of the U.S. This concern has arisen where inputs of P in fertilizer and manure to agricultural systems have exceeded output in harvested crops for several years. As the loss of P in surface (r 2 0.85 to 0.96) and subsurface drainage (r z 0.86) have been shown to be related to soil P content, measures to minimize nonpoint source losses of agricultural P are most effective when implemented at the source. To achieve this, identification of soils and management systems vulnerable to P loss and an understanding of the main factors controlling soil P bioavailability is essential. Soil P bioavailability is determined by reaction with hydrous oxides, amorphous and crystalline complexes of AI, Fe and Ca, and organic matter. The rate and extent of these reactions is influenced by soil management and drainage. In wetlands for example, Fe redox dominates P solubility, while organic P mineralization can contribute up to 25 kg P ha-~ y r - l in temperate mineral soils and up to 160 kg P ha - l yr - l in organic soils. However, it is critical that we are able to reliably determine threshold soil P values at which agronomic issues become environmental concerns. Agronomic soil P tests may target problem soils on which environmental soil tests will assess P bioavailability and P sorption capacity. Clearly, soil P dynamics are of agronomic and environmental importance, influencing both crop productivity and eutrophication and thus, must be considered in developing effective management plans.

° Paper presented at the workshop on Phosphorus Behavior in the Okeechobee Basin, sponsored by the South Florida Water Management District and the University of Florida-lnstitute of Food and Agricultural Sciences. 0925-8574/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0925-8574(95)00027-5

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.4.A] Sharply' / Ecological Engineering 5 (1995) .61-:/9

1. Introduction

An increased global awareness of nonpoint source pollution of waters via chemical transport in agricultural drainage and runoff (Sharpley and Menzel, 1987; SCOPE, 1988), has stimulated an urgency in obtaining information on the impact of agricultural management on surface water quality. Advanced eutrophication of surface water leads to problems with its use for fisheries, recreation, industry, or drinking, due to the increased growth of undesirable algae and aquatic weeds and oxygen shortages caused by their senescence and decomposition. Also, many drinking water supplies throughout the world experience periodic massive surface blooms of cyanobacteria (Kotak et al., 1993). These blooms contribute to a wide range of water-related problems including summer fish kills, unpalatability of drinking water, and formation of trihalomethane during water chlorination (Palmstrom et al., 1988). Consumption of cyanbacterial blooms or water-soluble neuro- and hepatoxins released when these blooms die, can kill livestock and may pose a serious health hazard to humans (Lawton and Codd, 1991). Although nitrogen (N), carbon (C), and phosphorus (P) are associated with accelerated eutrophication, most attention has focused on P, due to the difficulty in controlling the exchange of N and C between the atmosphere and a water body, and fixation of atmospheric N by some blue-green algae. Thus, control of P inputs is of prime importance in reducing the accelerated eutrophication of surface water. Phosphorus inputs from point sources are easier to identify and control than more diffuse nonpoint sources. As a result, less attention has been given to management strategies minimizing nonpoint sources of agricultural P. Now, nonpoint sources account for a larger share of all inputs than a decade ago. In Europe, the input of P to many lakes from nonpoint sources, such as agricultural runoff, is sufficient to sustain further eutrophication, irrespective of whether point source inputs of P are controlled or not (Wuhrmann, 1984; Hillbricht-Ilkowska, 1988). Thus, soils and management practices that are vulnerable to P loss, must be identified to implement effective and economically viable management systems that minimize P transport. The negative impacts of P must be balanced with the benefits of P use. Profitable crop production depends on a sound P-management program. For example, fixation and immobilization of soil P in inorganic and organic forms unavailable for crop uptake, necessitates P amendments as fertilizer, animal manure, or crop residue material to achieve desired crop yield goals. Thus, P application has become an integral and essential part of crop production systems in order to provide adequate food and fiber for U.S. consumption and export demands. Also, judicious fertilizer P use can reduce erosion and runoff losses of P via increased crop uptake and vegetative cover. On the other hand, in areas of intensive crop and livestock production, continual P applications as mineral fertilizer and manure have been made at levels exceeding crop uptake (Sharpley et al., 1994). As a result, surface soil accumulations of P have occurred to such an extent that the loss of P in surface runoff has become a priority management concern. In an increasing number of cases, the

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A.N. Sharpley/ Ecological Engineering 5 (1995) 261-270

capacity of the soil to sorb further P additions has become limited. As a result, increasing losses of P in ground water have been observed by Breeuwsma and Silva (1992) in the Netherlands and by Federico et al. (1981) in Florida. Before we can develop agronomically and environmentally sound agricultural systems for P, we need to understand what forms of P occur in soils, its plant availability, and the processes controlling soil P removal by and transport in runoff. Using this information, we can assess how to manage agricultural P to maximize soil productivity, while minimizing environmental impacts. This paper presents information on the processes controlling the forms, amounts, and bioavailabilities of P in soil and its release to and transport in agricultural runoff and drainage water. Considering these processes, methods to assess the environmental as well as agronomic availability of soil P will be discussed. Particular reference will be made to soil and water processes occurring in Florida, as a conceptual synthesis of research published in this issue of Ecological Engineering, resulting from a workshop held by the South Florida Water Management District on P dynamics in Lake Okeechobee watershed. 2. Soil phosphorus dynamics 2.1. Forms and distribution

Soil P exists in inorganic and organic forms (Fig. 1). These forms are characterized by chemical extractions and relative lability assigned as to the chemical SLOW INORGANIC

RAPID CYCLING ORGANIC & INORGANIC

SLOW ORGANIC

ANIMAL

P.~M~'( P

"

MINERALS

m"

Ii

..'"'

i

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ta~)t'X,I

~\

OCCLUDED

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II

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LABILE AND LABILE AND Iv MODERATELY LABILE MODERATELYLABILE / INORGANIC P ORGANIC P (amorphous sesquloxides (phospholipid$, i and some ¢ry=talline AL and Fe)

,

inositols fulvic aci , cO

/ J

Fig. 1. The soil P cycle: its components and measurable fractions (from Stewart and Sharpley, 1987).

264

A.~: Sharpley/ Ecological Engtneering 5 (1995) 26l-279

species extracted (Fig. 1). Such fractionation of soil P is based on the premise that extractants of increasing acidity and alkalinity, sequentially remove P of decreasing lability or bioavailability (Hedley et al., 1982). However, the forms generalized in Fig. 1 are not discrete entities, as intergrades and dynamic transformations between forms occur continuously to maintain equilibrium conditions. In most soils, the P content of surface horizons is greater than subsoil due to the sorption of added P and greater biological activity and accumulation of organic material in surface layers. However, soil P content varies with parent material, texture, and management factors, such as rate and type of P applied and soil cultivation. These factors also influence the relative amounts of inorganic and organic P. In most soils 50 to 75% of the P is inorganic, although this fraction can vary from 10 to 90%. Inorganic P forms are dominated by hydrous sesquioxides, amorphous, and crystalline AI and Fe compounds in acidic, noncalcareous soils and by Ca compounds in alkaline, calcareous soils (Fig. 1). Organic P forms include relatively labile phospholipids, inositols and fulvic acids, while more resistant forms are comprised of humic acids (Fig. 1). The role of microbial biomass P as a dynamic intermediary between inorganic and organic forms is evident from Fig. 1. With the development of fumigation-extraction techniques to measure soil microbial biomass P (Brookes et al., 1982; Hedley and Stewart, 1982), its importance in P cycling has been quantified (McLaughlin et al., 1988; Stewart and Tiessen, 1987). In a study of P cycling through soil microbial biomass in England, Brookes et al. (1984) measured annual P fluxes of 5 and 23 kg P h a - t yr-1 in soils under continuous wheat and permanent grass, respectively. Although biomass P flux under continuous wheat was less than P uptake by the crop (20 kg P h a - t yr-~), annual P flux in the grassland soils was much greater than P uptake by the grass (12 kg P ha -~ yr-~). Clearly, microbial P plays an important intermediary role in the s h o r t - term dynamics of organic P transformations and thereby, management of soil P availability. With the application of P, available soil P content increases as a function of certain physical and chemical soil properties (Larsen et al., 1965; Lopez-Hernandez and Burnham, 1974; Barber, 1979; McCollum, 1991). The portion of P remaining as plant available P (resin P) 6 months after application, decreased as clay, organic C, Fe, Al, and CaCO 3 content increased for over 200 widely differing soils (Table 1: from Sharpley et al., 1984a,1989; Sharpley, 1991). With an increase in degree of soil weathering, represented by soil taxonomic and other related properties, a general decrease in availability of applied P was evident. Where no P is added, a net loss of P from the system via removal in the harvested crop is often accounted for by a decrease in soil organic P, while inorganic P generally remains constant. For example, the growth of cotton on a Mississippi Delta soil, Dundee silt loam for 60 years (1913-i973), with no reported fertilizer P applied, had little affect on inorganic P content (Sharpley and Smith, 1983). However, a decrease in the organic P content of the cultivated (93 mg P kg -~) compared to virgin analogue (223 mg P kg -1) surface soil (0-15 cm) was

A.N. Sharpley / Ecological Engineering 5 (1995) 261-279

265

Table 1 Percent fertilizer P available (as resin P) 6 months afterapplication (data adapted from Sharpley, 1991 and Sharpley et al., 1984a, 1989) Related properties

Number of soils

Availability Mean (%)

Range (%)

Calcareous CaCO 3

56

45

11-72

80

47

7-74

27

32

6-51

40

27

14-54

Slightly weathered Base saturation Available P oH

Moderately weathered Clay Available P Organic C Highly weathered Clay Extractable AI Extractable Fe

measured. Apparently, organic P mineralization replenished the inorganic pool via the microbial P and provided adequate amounts of plant available P (Fig. 1). 2.2. Organic P mineralization Even though inorganic P has generally been considered the major source of plant available P in soils, the incorporation of fertilizer P into soil organic P (McLaughlin et al., 1988) and lack of crop response to fertilizer P due to organic P mineralization (Doerge and Gardner, 1978), emphasize the importance of organic P in soil P cycling. Organic P mineralization in several unfertilized and P fertilized soils in the Southern Plains was quantified by Sharpley (1985) as the decrease in soil organic P content during the period of maximum crop growth (spring and early summer). Amounts mineralized were related to total organic P content for both unfertilized and fertilized soil (Fig. 2, from Sharpley, 1985). For a given organic P content, mineralization was greater for Woodward than Houston Black and Kirkland soils. As a function of moderately labile organic P, however, no difference between locations was observed (Fig. 2). Apparently, organic P mineralization dynamics were a function of moderately labile organic P, the level of which is determined by climatic and soil factors. Of this, moderately labile organic P contributed 83 to

266

A.?~: Sharpley / Ecological Engineering 5 (1995) 261-279

240 i

'-= oi

200

MODERATELY LABILE

160 120

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80

uJ

40

,.I

o

N_

nuJ z 0.

200

. ~

Z

I

~

=

I

~

I

d slit loam " Houston Black clay • Woodward loam t

I

TOTAL

16o 12o

z

<

O

8o

4o o

200

ORGANIC

'

,oo

P CONTENT (mg

'

600

kg "1)

Fig. 2. Annual net mineralization of organic P as a function of toal and moderately labile organic P for three soils.

93% of that mineralized. As labile and resistant organic P pools remained fairly constant (Sharpley, 1985), mineralization of moderately labile organic P replenished the available P pool, when it fell below a critical but as yet undefined level. Tate et al. (1991) also found labile organic P mineralization was an important source of P to pasture in both low- and high-P fertility soils in New Zealand. Both studies (Sharpley, 1985; Tate et al., 1991), suggest that management practices maximizing the build-up of organic matter during autumn and winter, may reduce external P requirements for plant growth during the following spring and early summer. Amounts of organic P mineralized in the three Southern Plains soils studied by Sharpley (1985) are similar to other temperate soils (Table 2). Further, organic P mineralization (15 to 33 kg P ha - l y r - t ) was not completely inhibited by fertilizer P application (20 to 28 kg P ha-~ yr-t), with similar amounts of P contributed by both sources (Table 2). However, losses from temperate soils are generally lower than for soils from the tropics (67 to 157 kg P ha-~ yr-~), where distinct wet and dry seasons and higher soil temperatures can increase the amounts of organic P mineralized (Table 2). Similarly, Reddy (1983) estimated that organic P mineralization amounted to about 38 to 185 kg P ha -1 yr -~ for organic soils in central Florida and 16 to 23 kg P h a - l yr-~ in south Florida. As a result, flooding these organic soils increased P release into drainage water by about 4 to 8 times that from drained soils.

A.N. Sha rpley / Ecological Engineering 5 (1995) 26 l - 279

267

Table 2 Average net amount of organic P mineralized annually for several climatic regions. Data adapted from Sharpley (1985) and Stewart and Sharlaley (I.987)

Region

Fertilizer P applied

Southern Plains

0 25 0 34 0 40

Temperate

Tropics

Organic P mineralized kg P ha - t yr -I 23 17 11 6 157 67

Percent mineralized (% yr- t ) a 1l 8 2 1 15 18

a Percent of total soil organic P which is mineralized annually.

2.3. Wetland soils The rate and extent of inorganic and organic P transformations in wetlands are modified by intermittent aerobic and anaerobic conditions compared to dryland soils described above. Under aerobic conditions the solubility of P associated with amorphous and sesquioxide AI and Fe compounds can be increased but some P associated with crystalline Al and Fe oxides is desorbed only under extended waterlogged condition (Fig. 3). Although Al-P complexes are not affected by

water

aerobic

anaerobic

leaching Fig. 3. Generalized scheme of P transformation in a wetland system.

268

A.N. Sharpley / Ecological Engmeering 5 (1995) 261-279

oxidation-reduction reactions brought about by aerobic-anaerobic conditions, pH and organic matter influence the solubility of these P forms. Thus, Fe speciation dominates the dynamics of P solubility in many wetland soils. Ferric oxyhydroxides in the surface oxidized zone of a waterlogged soil can act as sink for P in both the overlying water column and underlying anaerobic soil, as represented in Fig. 3. Thus, the thickness of this surface layer, which is dependent on oxygen demanding species at the interface, will determine to a large extent, the mobility of P associated with Fe complexes in wetland soils (Reddy et al., 1995). Calcium-bound P is generally biologically unavailable and in many wetlands, particularly in the Okeechobee Basin, is present in small amounts because of low pH conditions (Reddy et al., 1995). However, in alkaline Aquods of the Lake Okeechobee, Wang et al. (1995) found P was primarily retained in Ca and Mg forms. In addition, organic P mineralization may be enhanced by alternate soil wetting and drying cycles, changes in soil pH, and an increased in microbial activity. As a result, the bioavailability and mobility of P in wetland soils under aerobic conditions is generally greater than for aerobic or dryland soils (Reddy et al., 1995). This enhances the potential for P movement in runoff and drainage water from wetland soils. Wetland soils can function as sinks and sources of P depending on water residence time, sediment physiochemical properties and vegetative assimilation (Richardson, 1985; Reddy et al., 1995). When the dissolved P concentration of water flowing into wetlands is greater than that present in the pore water of wetland soils, P is retained by AI, Fe, organic matter and to a lesser extent Ca complexes. However, with low P loadings, wetland soils can act as a P source, releasing P to the water column (Khalid et al., 1977; Bostrom et al., 1988; Reddy et al., 1995).

2.4. Analytical limitations Sequential extraction procedures may fractionate soil inorganic and organic P according to its chemical stability and these fractions then related to bioavailability (Bowman and Cole, 1978; Hedley et al., 1982; Potter et al., 1991). Thus, the fractionations provide indirect evidence of P bioavailability and chemical form of P extracted. However, comparing the results of Boers et al. (1984), Bostrom (1984), and Williams et al. (1980), indicate that both the fractionational composition of soil P and species within each chemically determined fraction, varies between different edaphic and ecosystem regions. This variability complicates the assessment of soil P bioavailability from relationships between bioassay-extractable P and chemically determined P fractions. Thus, interpretations should be limited to edaphically similar soils. Soil organic P has not been accurately quantified or identified. Total organic P is determined indirectly by difference (total-inorganic P) following either extraction or ignition methods (Olsen and Sommers, 1982). However, solubilization or complexation of mineral P during ignition and hydrolysis a n d / o r incomplete removal of adsorbed or occluded organic P during extraction, may introduce errors

A.N. Sharpley /Ecological Engineering 5 (1995) 261-279

269

in organic P estimation between and within soil types. Recent modifications of extraction and ignition methods (Soltanpour et al., 1987; Bowman, 1989) may simplify and increase the accuracy of organic P quantification. Although complete characterization of soil organic P has not been accomplished, application of solid state neutron magnetic resonance (NMR) spectroscopy may improve its identification.

3. Soil phosphorus content

The continual long-term application of fertilizer and manures at levels exceeding crop requirements can raise soil test P above levels required for optimum crop yields. Once soil test P levels become excessive, the potential for P loss in runoff and drainage water, is greater than any agronomic benefits of further P applications. After high levels of soil test P have been attained, considerable time is required for significant depletion. For example, McCollum (1991) estimates that without further P addition, 16 to 18 years of cropping corn (Zea mays L.) or soybean (Glycine max (L.) Merr.) would be needed to deplete the soil test P content (Mehlich III) of a Portsmouth soil from 100 mg P kg -~ to the threshold agronomic level of 20 mg P kg - t In recent years, the number of soils with soil test P exceeding levels required for optimum crop yields, has increased in areas of intensive agricultural and livestock production. In 1989, several state soil test laboratories in the eastern U.S. reported that the majority of soils analyzed had soil test P levels in the high or excessive categories (Fig. 4). Although these categories vary between states, soil test P limits ranging from > 10 to > 75 mg P kg -t for high and from > 25 to > 150 mg P kg - t for excessive. Of particular concern, is the efficient utilization on locally limited land areas, of manure produced in confined animal operations. For example, Graetz and Nair (1995) found soil test P levels (double acid) in several Spodosol A horizons of 400 mg P kg-t in intensive and 453 mg P kg-~ in holding areas of several dairy farms for up to 32 years. Nonimpacted areas had soil test P levels of 3 mg P kg-~. In Florida, double acid P levels of 66 mg P kg -t are considered high and no P additions are recommended for soils above these levels. Further, Graetz and Nair (1995) clearly demonstrated the environmental impact of P contained in these treated soils by calculating that about 4000 and 1800 kg P ha-~ would be available for transport from intensive and holding areas, respectively. Also, soil test P levels (Bray-I) of up to 200 mg P kg -~ in soils receiving long-term applications of dairy manure were observed in Wisconsin (Motschall and Daniel, 1982) and up to 279 mg P kg- ~ in soils receiving poultry litter in Oklahoma (Sharpley et al., 1991b). Long-term manure applications can also increase soil organic P content. For example, the organic P content of the A horizon of several Spodosols treated with dairy manure for up to 32 years was 219 mg P kg-t and for untreated soils was 9 mg P kg -~ (Graetz and Nair, 1995). Of this, organic P

270

A.,'~: Sharpley / Ecological Engineering 5 (1995) 261-279

~~~.~25 56

d':-'Sl I

~"'65 35

\64

14

Fig. 4. Percent of soil samples testing high or above for P in 1989 in the eastern U.S. Highlighted states have 50% or greater of soil samples testing in the high or above range (data adapted from Potash and Phosphate Institute, 1989 and Sims. 1993).

increase, approximately 14% occurred in the moderately labile NaOH extractable fraction. Sharpley et al. (1993) also found that broadcast applications of poultry, litter (35 to 130 kg P ha -~ yr -t) for 12 to 35 years increased the organic P content of 12 Oklahoma soils up to 54% (from 440 to 678 mg P kg- ~). The major portion of this increase in organic P occurred in the labile, NaHCO3-extractable fraction (40%). Application of cattle feedlot waste (90 kg P ha -1 yr-t) for 8 yr also increased the organic P content of a Pullman clay loam (283 mg P kg -l) under irrigated grain sorghum (Sorghum bicolor [L.] Moench) compared to untreated soil 203 mg P kg -1) (Sharpley et al., 1984b). Labile organic P accounted for 43% of the total increase. In many cases, manure applications have been N based, considering only soil N content and crop N requirements. This strategy can lead to an increase in soil P, due to the generally lower ratio of N:P added in manure than taken up by crops. A P driven strategy may mitigate the excessive build up of soil P and at the same time lower the risk for nitrate leaching to ground water. However, basing manure applications on P rather than N management, could present several problems to many landowners. A soil test P-based strategy could eliminate much of the land area with a history of continual manure application, from further manure additions, as many years are required to lower soil test P levels once they become excessive. This would force landowners to identify larger areas of land to utilize the generated manure, further exacerbating the problem of local land area limitations.

A.N. Sharpley / Ecological Engineering 5 (1995) 261-279

6o01

271

Oklahoma

i

C,,p*=nd

40O

200 2.~%....~ j ~ "

y = loo,.

O.

~-t IJJ

4.4, •

r z : 0.90

New Zealand

iii

0 _~

|

0

200

.~

= Undrainecl y

104.4 + 3.34 x

~

~

i

L 100

°o

J

"

y = Do.r~i~e:61 x ,., = o.9:3

~ }

3o

L 5o

,:o

SOIL TEST P (rag kg .1) Fig. 5. Relationship between soil test P content and dissolved P concentration of runoff for several watersheds in Oklahoma and New Zealand.

Clearly, high soil test P levels are a regional problem, with the majority of soils in several states testing medium or low (Fig. 4). For example, most Great Plains soils still require fertilizer P for optimum crop yields. However, Fig. 4 clearly illustrates that in the eastern U.S., problems associated with high soil test P soils are aggravated by the fact that many of these soils are located near sensitive water bodies such as Florida lake's the Great Lakes, and Chesapeake Bay.

4. Relationship between soil and runoff phosphorus The P content of surface soil influences the loss of P in runoff. In fact, a highly significant linear relationship was obtained between the soil test P content (Bray I) of surface soil (1 cm) and the dissolved P concentration of runoff from cropped and grassed watersheds in Oklahoma (Sharpley et al., 1986) and drained and undrained field plots in New Zealand (Sharpley et al., 1977) (Fig. 5). The consistently higher dissolved P concentration in surface runoff from the cropped and undrained areas than grassed and drained areas, is attributed to the desorption of P from the higher sediment loads found in these waters. A similar dependence of the dissolved P concentration of runoff on Bray-1 P was found by

272

A.N. Sharpley / Ecological Engineering 5 (1995) 261-279

Romkens and Nelson (1974) for a Russell silt loam in Illinois (r 2 = 0.81) and on water extractable soil P (r 2= 0.61) of 17 Mississippi watersheds by Schreiber (1988) and 11 Oklahoma watersheds by Olness et al. (1975) (r 2 = 0.88). Vaithiyanathan and Correll (1992) observed that the loss of P in runoff from forested and cropped watersheds in the Atlantic Coastal Plains was closely related to soil P content (r 2 -- 0.96). In fact, the high organic P content of the forest soils (331 mg P kg-t; 70% of total P) contributed to the high organic P loss in runoff from these soils, while the high inorganic P content of the cropped soils (486 mg P kg-t; 75% of total P) resulted in a higher inorganic P loss from these soils (Vaithiyanathan and Correll, 1992). Other studies have also demonstrated the close dependence of P loss in runoff upon surface soil P content (Barisas et al., 1978; Reddy et al., 1978; Uhlen, 1978; Rekolainen, 1989). The sorption of P by P-deficient subsoils generally results in lower concentrations of dissolved P in subsurface than surface runoff. Exceptions may occur in organic or peaty soils, where organic matter may accelerate the downward movement of P together with organic acids and Fe and AI (Fox and Kamprath, 1971; Singh and Jones, 1976; Miller, 1979). Also, P is more susceptible to movement through sandy soils with low P sorption capacities (Ozanne et al., 1961; Adriano et al., 1975; Sawhney, 1977) and in soils that have become anaerobic through waterlogging, where a decrease in soluble Fe (II) content and organic P mineralization occurs (Gotoh and Patrick, 1974; Khalid et al., 1977). An interaction of these biogeochemical processes contribute to inefficient P retention by several sandy Haplaquods in areas of Okeechobee Basin, Florida, with a high density of dairy farms (Graetz and Nair, 1995). A low P sorption capacity of the A and highly eluted E horizons and a lack of percolation of dissolved P into deeper Al-rich spodic (Bh) horizons, due to high water tables (Reddy et al., 1995; Yuan, 1965), contribute to high dissolved P concentrations in drainage discharged from these basins (Campbell et al., 1995). Because of the variable path and time of water flow through a soil with subsurface drainage, factors controlling dissolved P in subsurface waters are more complex than for surface runoff. However, soil P content has been shown to influence the loss of P in drainage water as well as surface runoff. For example, Sharpley et al. (1977) found that the amount of P extracted by 0.1 M NaCI from soil at the tile drain depth (40 to 50 cm) was related (r 2 = 0.86) to the loss in tile drainage during storm events (Fig. 6). No relationship was found between extractable soil P at shallower depths or with dissolved P concentration. The fact that dissolved P loadings rather than concentrations were related to extractable soil P suggests that the 0.1 M NaCI extraction removes a pool of readily leachable inorganic P, which is normally exhausted during a flow event, but replenished between events (Sharpley et al., 1977). Replenishment may occur by mineralization of soluble organic matter leached down the profile during the preceding rainfall event and by soil P desorption between events. Nicholls and MacCrimmon (1974) also attributed a seasonal 6-fold increase in dissolved P concentration in subsurface drainage from cultivated mucks in Holland Marsh, Ontario, to organic P mineralization.

273

A.N. Sharpley / Ecological Engineering 5 (1995) 261-279

.

o

d~

o

_1 n rs tkl -I

o

!

2

1

= ~

•

°o0

L

r a = 0.86

,

L

02

I

L

I

o4

o.

EXTI~CTABLE SOIL P ( kg ha .1 ) Fig. 6. Relationship between the extractable soil P content (0.1 M Nacl) of subsoil (40-50 cm) and dissolved P loss in tile drain discharge events from a grassland watershed in New Zealand.

A similar dependence of dissolved P concentration in tile drainage on the P sorption-desorption properties of subsoil material was found for Histosols in Florida (Hortenstine and Forbes, 1972), New York (Duxbury and Peverly, 1978; Cogger and Duxbury, 1984), Ontario (Nicholls and MacCrimmon, 1974), and for Haploquolls in Ontario and Michigan (Culley et al., 1983).

5. Environmental soil testing for phosphorus Soil testing programs must measure P through the use of rapid chemical extraction procedures, if they are to provide recommendations in a timely and cost-effective manner. However, as we move from agronomic to environmental concerns with soils containing high P levels in excess of crop requirements, will such soil test methods developed to assess plant availability of P estimate forms important to eutrophication? If not, are appropriate methods available? For an environmental assessment, the bioavailability of soil (or sediment) P to aquatic organisms is needed. For wastewater irrigation systems, such as liquid manures applications common in the Lake Okeechobee watershed, estimates of the longterm capacity of a soil profile to retain P against leaching will be needed. While it is unrealistic to expect that routine soil tests can provide the information needed for all environmental management programs, recent research has shown that soil test P is well correlated with several parameters needed to assess nonpoint source pollution (Sims, 1993). Additionally, a number of alternative tests for soil P are available that, while not as easily conducted as a routine soil test, can provide supplemental information on P sorption, desorption, and bioavailability. Amounts of soil and sediment P extracted by 0.1 M NaOH (Dorich et al., 1985; Sharpley et al., 1991a) or Fe-oxide impregnated paper strips (Sharpley, 1993), have

274

.4.:~: Sharpley / Ecological Engineering 5 (I995) 261-279

been shown to be closely related to the growth of P-starved algae in bioassays. While these methods were proposed to estimate algal-available P, they present some difficulties for soil testing laboratories because of the extraction time (16 h) and wide NaOH:soil ratio (1000 or 500:1) required, other research has shown that routine soil tests are well correlated with this measure of bioavailable P (Wolf et al., 1985). Consequently, in areas with P-related water quality problems, soil test laboratories could use routine soil tests to provide preliminary rankings of the algal available P content of soil (or sediment) and identify those on which the environmental test should be conducted. The potential for soils to adsorb P is also important in the design of wastewater (manure and sewage sludge) irrigation systems, or in areas where leaching and lateral flow of P in drainage waters may be important (Sims, 1993). The long-term capacity of soils to retain P is commonly estimated by adsorption isotherms that can be used to derive adsorption maxima for soil horizons. However, these isotherms require equilibration of soil with a series of P solutions of increasing P concentration, normally for 24 h, and are not well adapted to routine soil testing laboratories. Bache and Williams (1971), however, suggested that a single-point isotherm could be used to estimate the P adsorption maxima of soils with reasonable accuracy. This was recently confirmed by Mozaffari and Sims (1993) for surface and subsoil horizons of four Atlantic Coastal Plain soils. Interpretation of soil test P results for environmental problems will require continued innovation. It is essential that the long-term, proactive role of soil testing laboratories in the development of sampling procedures, analytical methods, and practical recommendations for efficient P management for crop production be applied to environmental management of soil P. Such soil testing programs can contribute greatly to the development of conceptually sound and technically feasible solutions to the complex problems of nonpoint source pollution by soil P.

6. Conclusions

Data presented in this paper demonstrates both the agronomic and environmental importance of inorganic and organic P dynamics in soils. Clearly, soil biogeochemical properties and management via system inputs and outputs, determine threshold soil P contents, above which agronomic issues become environmental concerns. However, to determine these threshold values for a given soil and management and thereby develop sustainable and economic agricultural systems, we must understand the variables and relative importance in controlling soil P dynamics. The main processes controlling soil P bioavailability are P interactions with AI, Fe, and Ca hydrous oxides, amorphous, and crystalline complexes, along with organic P mineralization. The rate and extent to which these processes occur are influenced by soil management factors including P amounts, tillage, and drainage. In most cases, soil P amendments as fertilizer and manure are an integral part of profitable and sustainable crop production. However, the continual long-term land

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application of P at levels exceeding crop P removal, can result in the accumulation of P in soil above threshold values and increase the potential for P movement in drainage water. Initially we must identify soils and management practices presenting a greater risk to P-sensitive waters. As agronomic soil P tests methods may not reliably assess eutrophication potential, environmental soil tests estimating bioavailable soil P and P sorption capacity, should be used on soils targeted as being "high risk" by standard agronomic methods. Comprehensive P-management plans must then be adopted. Remediation of targeted "high risk" soils, which are the source of P in eutrophic waters, is the most cost-effective way of minimizing P associated problems. The costs of remediation measures generally increase as they are implemented further and further from the problems' source. Papers presented in this issue, address methods to reduce the amounts and bioavailability of P. Obviously, decreasing P inputs is essential in most cases. However, for soils already containing P in excess of crop requirements, management of the biogeochemical processes controlling soil P bioavailability, should be considered. On high pH Spodosols ( > 7.0) receiving long-term dairy manure applications in Okeechobee Basin, Florida, increasing soil Ca through gypsum application was effective in reducing P solubility up to 63% (Anderson and Faber, 1995a). Gypsum also reduced dissolved organic C content, a potential source of dissolved P in these Spodosols, by suppressing bacterial mineralization of manure residues (Anderson and Faber, 1995b). As a result, Anderson et al. (1995) suggested that land application of gypsum, as a waste material from several Florida industries, may reduce P mobility. Thus, sandy textured Spodosols, heavily loaded with animal manure, gypsum application may be a practical and effective best management practice, for the reduction of dissolved P and C levels in wetland drainage waters. If the water flowing into a wetland has a higher dissolved P concentration than the interstitial P concentration of surface sediments, then P will be retained by the wetland (Reddy et al., 1995). Directing water flow through wetlands rather than stream channels, particularly in the Okeechobee Basin, will increase overall P retention. Management of the system to include incoming P concentration and water residence times will be important in determining the efficiency of P retention. Management of soil P bioavailability is one of the main challenges facing agriculture in many regions of the U.S., particularly where high soil P contents already occur in the drainage area of P-sensitive water bodies. It is hoped that the general information presented in this review and subsequent papers will provide some practical measures to meet these challenges. References Adriano, D.C., L.T. Novak, A,E. Erickson, A.R. Woolcoot and E. Ellis, 1975. Effect of long-term disposal by spray irrigation of food processing wastes on some chemical properties of the soil and subsurface water. J. Environ. Qual., 4: 242-248.

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