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a Univ. of Florida, IFAS, RCREC, 3401 Experiment Station, Ona, FL 33865
b Univ. of Florida, IFAS, Dep. of Agron., Newell Hall, Gainesville, FL 32611
c Univ. of Florida, IFAS, GCREC, 5007 East 60th Street, Bradenton, FL 34203
* Corresponding author (hari{at}ifas.ufl.edu)
Received for publication December 17, 2003.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONPHOSPHORUS ACCUMULATION IN SOILS...FORAGE PRODUCTION AND PHOSPHORUS...CONCLUSIONSREFERENCES |
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Abbreviations: TP, total phosphorus
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONPHOSPHORUS ACCUMULATION IN SOILS...FORAGE PRODUCTION AND PHOSPHORUS...CONCLUSIONSREFERENCES |
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In general, livestock producers face problems associated with the disposal of manures from their animal operations due to limited available land and strict environmental regulations. Moreover, in concentrated animal operations, feeds are transported to farms while lack of manure transportation from the farms has resulted in net accumulation of the nutrients. Such scenarios have created pockets of highly impacted sites within many ranches/farms. A case in point is the Lake Okeechobee Basin. Phosphorus has been identified as a major cause of eutrophication of Lake Okeechobee (Federico et al., 1981). Thus, to control algal blooms in the lake, South Florida Water Management District (SFWMD) has been directing its resources to reduce P loadings from both the external and internal sources (SFWMD, 1997). It has been recognized that the major sources of P loading to the Lake Okeechobee are surface runoffs from highly fertilized pastures and dairy animal wastes of the Lake Okeechobee Basin (Gunsalus et al., 1992). Hence, the tasks for water quality improvement in the lake have primarily been aimed at fertilizer and animal waste management in the basin.
The amounts of manure generated by concentrated animal operations often exceed the capacity of nearby croplands to use and retain the nutrients; thus, limited areas receive excessive amounts of manure (Carpenter et al., 1998). Such practices have led to a buildup of P along with other nutrients and have created potential hazards to water quality. Animal manures, however, can be an effective source of nutrients for forages, and their applications to pastures could help substantial amounts of nutrients, such as N and P, be recycled through herbage production. Linking forage production with manure utilization is a sound approach for addressing both the problems of manure disposal and its negative impacts on water quality. High quantity and quality of herbage can be produced from impacted sites although optimal management of forage production and manure depend on specific local site characteristics (Newton et al., 2003).
Animal manures have been used in agricultural (especially crop) production systems for centuries (Simpson, 1991). Unlike commercial inorganic fertilizers, manures have the disadvantage of not having the right nutrient forms and/or ratios for specific crop/forage requirements. Thus, their use may lead to accumulations of excess nutrients, including N and P in soils, and cause potential hazards to water quality. As eco-consciousness increases, developments of environmentally and economically sound agricultural production systems are receiving a high priority around the world. Perennial pastures are desirable recipients of animal manures (Edwards et al., 1996) because of the low application cost and broadcast application ease (i.e., no need of land preparation and easier to maneuver hauling and spreading equipment). To reduce potential threats from nutrient runoffs and leaching effects on water quality, recycling nutrients through forage production system may provide attractive alternatives to farmers/ranchers to comply with environmental rules and regulations, especially in the region where ecologically sensitive water bodies exist, including the Lake Okeechobee Basin.
Animal manures, in different physical forms (in solid, slurry, or liquid), have been used in crop production systems, resulting in various yield responses (Powers et al., 1975). Grasses are known to remove nutrients including P from manures and soils in varying degrees. Yield response to P at relatively high N supply has also been reported for coastal bermudagrass (Cynodon dactylon cv. Coastal) (Welch et al., 1963) and midland bermudagrass (cv. Midland) (Taliaferro et al., 1975). Similarly, various grasses have been used for phytoremediation of P from wastewater and animal lagoons. This review is intended to provide consolidated information on the limitations and potentials of various forages on P uptake, particularly from manure-applied soils. This review also discusses problems associated with water quality impairment due to P buildup in manure-impacted soils.
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PHOSPHORUS ACCUMULATION IN SOILS AND ITS IMPACTS ON WATER QUALITY |
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TOPABSTRACTINTRODUCTIONPHOSPHORUS ACCUMULATION IN SOILS...FORAGE PRODUCTION AND PHOSPHORUS...CONCLUSIONSREFERENCES |
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In recent years, the accumulation of nutrients, especially P in soils, has received a major attention in ecologically sensitive agricultural catchments because of the eutrophication it causes to surrounding water bodies. Similarly, concentrated activities, such as dairy operations in farmlands, have raised concerns about increasing P inputs to watersheds. Although animal manures are usually targeted at crop/forage production, the manures often move in substantial quantities from the targeted agricultural parcels to aquatic systems through runoff and leaching. Such scenarios may cause undesirable changes, directly or indirectly both to agricultural parcels and the nearby water bodies. In general, cropping systems, climate, topography, and fertilization practices affect the concentrations of nutrients, including N and P, in runoff waters. Land management of highly impacted sites, such as that of the Lake Okeechobee Basin, may have dramatic effects on P export to streams and lakes in the region.
Muir (2001) reported that kenaf grass (Hibiscus cannabinus) removed 10.4 and 6.8% of the equivalent P in composted dairy manure when applied, respectively, at 10 and 20 Mg ha–1 yr–1 (on dry weight basis), suggesting inability of the grasses to prevent P buildup in soils when manure was applied annually. Kingery et al. (1993) and Sharpley et al. (1998) reported that poultry litter, applied based on N requirements, provides more P than required by forages; hence, P accumulations in soils occur with potential threats to water quality. Liu et al. (1997) reported that soil P moved down to 40-cm soil depth, suggesting that when P exceeded requirements of forage uptake and the sorption capacity of soils, it leached down to the lower soil layers. It is known that grasses mine P from soils so that the supply of P usually has little or no effect on dry matter yield and P removal (Liu et al., 1997). On the basis of chemical fractionation data, Graetz and Nair (1995) suggested that about 80% of total P (TP) in the A horizon soils of Spodosols of Lake Okeechobee Basin could be considered leachable/mobile. They reported poor P retention capacities of soils from A and E horizons, but the retention was relatively higher and variable in Bh horizon depending on soil types (Myakka 
Immokalee > Pomello). Some soils may have high capacities to buffer P removals from soil solutions with reserve P; consequently, grasses grown in such soils do not show a response to applied P fertilizers (Ziadi et al., 2000).
Pastures are significant contributors of nutrients including P (86.2 x 103 t yr–1) to water bodies in the USA (Havens and Steinman, 1995). In general, nonpoint sources such as agricultural runoffs are considered major sources of P to surface waters of the USA. If current practices are continued, the negative impacts of nonpoint sources on water quality would perhaps increase in the future. A number of measures have been taken to reduce eutrophic impacts from dairy operations on water quality. Forage productions, such as exporting hay from the impacted sites, would help to reduce P loss potential from soils to aquatic systems and subsequently minimize water quality impairment. Thus, studies investigating optimal P removals by forages and reductions in P pools that are susceptible to runoff and leaching are beginning to receive more attention.
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FORAGE PRODUCTION AND PHOSPHORUS PHYTOREMEDIATION |
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TOPABSTRACTINTRODUCTIONPHOSPHORUS ACCUMULATION IN SOILS...FORAGE PRODUCTION AND PHOSPHORUS...CONCLUSIONSREFERENCES |
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Fox et al. (1974) showed that desmodium [Desmodium aparines (Link) DC.] (a pasture legume) required 0.2 mg P L–1 in soil solution during establishment, but the requirement dropped to 0.01 mg P L–1 after the second cutting. Carvalho et al. (1994) reported 5.4 and 9.45 mg P kg–1 as critical soil P level (TP), respectively, for gamba grass (Andropogon gayanus Kunth) and golden timothy [Setaria sphacelata (Schumach.) Stapf & C.E. Hubb.] in a red-yellow clayed latosol in the first year of their establishment. Vicente-Chandler et al. (1974) reported that tropical grasses, e.g., stargrass (Cynodon nlemfuensis Vanderyst), guineagrass (Panicum maximum Jacq.), and napiergrass (Pennisetum purpureum Schum.) require at least 73 kg P ha–1 annually when grown in some Puerto Rican soils and harvested at 40- to 60-d intervals. They also suggested that these grasses appeared to have little tendency for luxury P consumption. Kikuyugrass (Pennisetum clandestinum Hochst. ex Chiov.) and pangolagrass (Digitaria eriantha Steud.) in Hawaii, however, had dramatic positive dry matter yield responses to P supply (Tamimi, 1972).
In general, the P requirement is lower in fine-textured soils due to more restricted P movement than in sandy soils (Woodruff and Kamprath, 1965). Similarly, the P requirement in soil solution for optimal availability to plants is lower for soils with higher P sorption capacity (Smax) compared with soils with lower sorption capacity. Fox (1969) reported an optimal growth of kikuyugrass in Hawaiian soils (Smax = 200 mg P L–1) at 0.4 mg P L–1 in soil solution while the requirement was greater than 1 mg P L–1 when the grass was grown in soils that had low P sorption capacity (Smax = 70 mg P L–1). Although the P requirement of plants largely depends on forage species, and soil types, it is indicative that P equilibrium status of soils is critical for the estimation of supply of P requirements for forage productions in a given region.
Soil Fertility and Phosphorus Pools
Nutrient requirements of forages depend on their abilities to mine essential elements from soils. Forage plants include diverse groups of grasses, legumes, and various nonlegumes (annuals, biennials, and perennials) adapted to different climatic zones and varying soil fertility. Unlike legumes, nonlegumes require sufficient supply of N from soils. Similarly, lime often limits forage production on acidic soils of tropics and subtropics by affecting the utilization/availability of nutrients including P. Soils can rapidly tie up a large amount of P in relatively less bioavailable forms. Phosphorus availability in soils is greatly influenced by pH, e.g., application of P in acidic soils without liming is virtually useless (Woodhouse and Griffith, 1982), just as the P availability drastically decreases in alkaline soils (Stevenson, 1986). The levels of soil nutrients may be reduced slowly or remain relatively constant in continually manured soils (Kingery et al., 1993). However, intensive forage production can deplete P levels in highly manure-impacted soils, and such forage production may represent a crucial component for nutrient management in pastures.
Applications of fertilizers/manures are necessary, especially to sustain herbage production as the soil fertility is depleted due to crop removals, leaching, runoff, etc. Forages are grown in various types of soils. Fertilizer applications need to be geared toward meeting requirements for optimum production of given forages from a given pasture. The abilities of different forage species to take up different elements from soils vary, depending on their anatomical and physiological characteristics as well as the levels and forms of the elements in soils and soil characteristics. Differential P uptake by various forage species are expected, depending on the forms of P present in soils (Cihacek, 1993) and the capacities of the plants to mine the relatively stable P. Turtola and Yli-Halla (1999) indicated that surface applications of slurry and mineral fertilizers in soils with low levels of P may significantly increase P level in the soil surface. Hence, P loading to the surface runoff could increase sharply. However, various practices such as tandem disk operation in pastures before manure application may help to reduce runoff P losses (Osei et al., 2003).
Impacted soils such as in high-cattle-density areas often have accumulations of high amounts of mobile P. Graetz et al. (1999) showed that an average of 3.4% of TP (TP ranged from 750 to 2500 mg kg–1) was water-soluble P in some of the Lake Okeechobee Basin soils. Stanley and Rhoads (2000) reported that bahiagrass (Paspalum notatum Flüggé) did not respond to P fertilization if the soil test P (Mehlich-extractable P) was >16 mg P kg–1 soil, and 39 mg P kg–1 soil was sufficient for 2 yr; however, N application increased P uptake by the bahiagrass. Bailey et al. (2000) suggested that Olsen soil P test could provide erroneous assessment of forage/plant available P in iron-rich soils. Although there is no foolproof method for the determination of P availability in soils, relatively suitable methods should be used to estimate P availability in given soils. Proper estimations of P requirements can only be possible if existing P availability in soils is determined, and it is critical to reduce P losses in runoff due to overfertilization.
Phosphorus Uptake Potential of Forages
Phosphorus plays an important role in plant growth and energy transfer at the cellular level. It is probably one of the most universally applied nutrients to forage crops. Abe and Ozaki (1998) reported that the annual ryegrass (Lolium multiflorum) had the highest P and N removal rates among 11 spring-grown species in plant beds used to filter wastewater. Similarly, Lucero et al. (1995) and Vervoort et al. (1998) reported that plant N and P uptake increased with the rates of poultry litter application on bluegrass (Poa pratensis L.)–tall fescue (Festuca arundinacea Schreb.) and bermudagrass–tall fescue pastures. Robinson (1996) reported that yield and nutrient uptake had a typical positive correlation in hybrid bermudagrass pasture.
Daylength and temperature requirement affect optimal growth of bermudagrass in summer (Ball et al., 1991). Although in early spring, favorable moisture conditions would help growth of forages like bermudagrass, Sharpley et al. (1994) indicated that nutrients including P could be lost in runoff from a pasture receiving broiler litter if the litter was applied during early spring. Similarly, Pant and Warman (2000) indicated that applying manure to timothy (Phleum pratense L.) pasture in a cool climatic zone (Nova Scotia, Canada) in summer would be better than early spring to reduce P loss to runoff as well as for the utilization of native soil P. Brinson et al. (1994) and Daniel et al. (1998) suggested that litter applied in summer may be utilized by optimum growth of grasses (because of favorable temperature for growth). However, N could be lost because of NH3 volatilization. Brink et al. (2002), however, reported that P uptake was not affected by timing of broiler litter application in bermudagrass pasture, possibly due to higher-than-required levels of P in soils. It is apparent that depending on the bioavailability of P in soils, the timing of manure applications could be crucial for the utilization of nutrients including P by forages.
Sanderson et al. (2001) reported that ‘Alamo’ switchgrass (Panicum virgatum L.) reduced the concentration of soluble reactive P in surface runoff by an average of 47% from the filter strip receiving dairy manure on 150 kg N ha–1 basis compared with without the filter strip. Sanderson and Jones (1997) found that when large amounts of dairy manure were applied to bermudagrass–wheat (Triticum aestivum L.) pasture, up to 20% of the equivalent manure P was removed in the herbage. The P utilization/uptake by grasses depends on the levels of P in soils. Once the P demands of grasses in pastures/grasslands are met, the efficiencies of grasses in removing/utilizing P usually decrease drastically. Banszki (1997) indicated that at 25 kg P ha–1 yr–1 application rate, grasses removed 77 to 81% of applied P, and as the application rate increased to 100 kg P ha–1 yr–1, 29 to 32% was removed.
Different grasses have variable capacities to remove nutrients. Belanger et al. (2002) reported variations among timothy genotypes on tissue P concentrations and uptake. Newton et al. (2003) indicated that grasses tended to outperform broadleaf forages in dry matter yields and nutrient uptakes when dairy manure was applied. Bermudagrass is known to have high yield and tissue N and P concentrations in response to applied N (Brink et al., 2003; Newton et al., 2003). Belonging to the same genera as bermudagrass, stargrass may also accumulate substantial amounts of N and P; thus, it can be used for P phytoremediation of impacted soils. Similarly, Griffin et al. (2002) reported that nutrient removal by forage swards accounted for all applied N and almost all applied P. Although data on nutrient removals by different forages from differentially manured soils exists, it is critical to evaluate the forage species that are best suited for particular sites and conditions.
Importance of Liming and Nitrogen Fertilization
Nitrogen application is usually essential to produce high herbage yield, but the application may alter composition of forages and soil properties in the long term. Nitrogen often increases forage yield and N uptake by the plants (Ziadi et al., 2000; Malhi et al., 2002a). Singh (1999a) indicated that application of N increases root length and root density in grasses. Similarly, Loeppky et al. (1999) reported that N application increases seed productions from grasses. Changes in pH due to N fertilization or lime application can greatly influence the concentration of potentially mobile P because of their effects on Al solubility (McDowell et al., 2002).
The amount of P accumulation in grasses often depends on increase in yields (Adeli and Varco, 2001). Pederson et al. (2002) suggested that improvement in N fertility would improve P concentration (i.e., increase in mg P kg–1 dry matter yield) in forages due to enhanced uptake of P by the plants. Apart from soil characteristics and climate, response of plants to changing photoperiod could also limit grass response to N (Pitman and Read, 1998). It is thus apparent that soil N availability is crucial for forage production even for legumes in some instances (Raun et al., 1999). However, maximizing N utilization efficiency is required for sustaining profitability and reducing ecological risk associated with excess residual N.
Soil pH may be lowered by prolonged and/or higher rates of N application (Haby et al., 1999; Singh, 1999b) and ultimately affect the availability of nutrients including P (Singh, 1999b). Bahiagrass can tolerate low soil fertility and acidity, while bermudagrass can tolerate moderate acidity, and is very responsive to N and P fertilization (Haby, 2002). Similarly, ryegrass is highly responsive to lime, N, and P (Haby, 2002). Liming could induce reduction in NO3–N, NH4–N, and P in some shallow soil layers along with forage production (Malhi et al., 2002b) due to increases in pH to a suitable range for the forage growth. Moreover, lime enhances the growth of beneficial microbes and reduces the Al and Mn toxicity, i.e., acts as a regulator of soil conditions (Woodhouse and Griffith, 1982). Due to increasing N application rates, concentration of N increases in bromegrass (Bromus inermis Leyss.) hay while that of P tends to decrease (Malhi et al., 2002b). It is evident from these studies that liming may induce reduction in NO3–N, NH4–N, and P concentrations and increase pH in shallow soil layer as well as increase dry matter yield of bromegrass. However, forages such as elephantgrass (Pennisetum purpureum Schumach.; tropical forage) could efficiently utilize P in acidic soils and grow well (Shen et al., 2001). Moreover, responses to lime and P application to ‘Georgia-5’ tall fescue may be greater than previously thought; Pitman (2000) reported that the grass had a linear response to P application up to 80 mg kg–1 (soil P = 142 mg kg–1) and a quadratic response to liming.
Banszki (1997) reported an increase of 38 to 98% in dry matter yield (i.e., increases in dry matter yield from per-unit area, kg ha–1) from grasslands in chernozem soil with the supply of higher N fertilizer rates (up to 450 kg N ha–1 yr–1) compared with control. Evers (2002) reported that addition of commercial inorganic N fertilizer together with broiler litter increased the removal of P by 23% (compared with no added N) from an annual ryegrass–bermudagrass pasture. Jacobsen and Surber (1995) reported that N and P applications increased alfalfa (Medicago sativa L.) and orchardgrass (Dactylis glomerata L.) production as well as the N and P concentrations in plant tissue. Sufficient supply of N is crucial for optimal forage production in many pastures. Johnson et al. (2001) reported 129% increase in dry matter yields of bermudagrass, bahiagrass, and stargrass by application of 78 kg N ha–1 per cutting compared with no N fertilization. Higher dry matter yields, however, may not always guarantee higher tissue P content (Banszki, 1997). It is apparent that N and lime applications are very important for optimal herbage production as well as P uptake by plants. Nitrogen application rates and sources, however, should be carefully determined to avoid soil acidification.
Forage Quality and Quantity as Affected by Phosphorus Availability
Although luxury uptake of P by forages is not as well established as that of K, increase in tissue P content due to increase in P fertilization rates (from 56 to 112 kg P ha–1 yr–1) have been reported in some (‘Pensacola’) bahiagrass (Burton et al., 1997) and rangeland grasses such as woollybut wanderrie (Eriachne helmsii Domin), a native grass in Western Australia (Islam and Adams, 1999). Reinbott and Blevins (1997) reported that annual P fertilization (28 kg P ha–1) of tall fescue pasture in soils with low P levels (Bray-1 P < 18 kg ha–1) increased both the herbage production and mineral contents in early spring. Rhoads et al. (1997), however, indicated that tissue P content in bahiagrass did not respond to P application > 84 kg P ha–1 yr–1.
Differential responses to nutrient availabilities from various grasses are usually common even from the grasses belonging to the same genera. Nitrogen fertilization can enhance forage yields and nutritive value such as crude protein and in vitro organic matter digestibility in buffalograss (Tripsacum dactyloides L.) (Springer and Taliaferro, 2001). Leyshon (1991) indicated that herbage production from bromegrass increased linearly in response to N fertilization of up to 200 kg N ha–1 yr–1 from flood-irrigated medium- to heavy–textured soils in southern Saskatchewan. Adjei et al. (1999) demonstrated a need for periodic application of P along with K and micronutrients to maintain productivity from grass–legume systems in Florida. Muir (2001) found that application of composted dairy manure at 20 Mg ha–1 yr–1 (on dry weight basis) increased kenaf (Hibiscus cannabinus L.) yield by 25.7% by the second year of the establishment compared with control (pastures receiving no manure). A perennial Mediterranean forage legume, trefoil (Lotus glaber Mill.), thrives in P-deficient soils and responds to small amounts of P fertilization with significant increase in yields; however, P utilization efficiency decreases with an increase in fertilization (Torales et al., 2000). Torales et al. (2000) also reported that seradella [Ornithopus micranthus (Benth.) Arechav.] showed low productivity from the P-deficient soils, and another species, red clover (Trifolium pratense L.), showed increased production with increased P application. Rechcigl et al. (2002) indicated that annual application of P and K may not often be necessary to improve establishment or yield of legumes {joint vetch (Aeschynomene americana L.) and Brazilian stylo [Stylosanthes guianensis (Aubl.) SW]}–bahiagrass-grazed pastures on Spodosols that have a history of P and K fertilization and that are not intensively managed. They, however, reported increases in tissue P and K due to fertilization both in the legumes and bahiagrass. Nutritive values of forages are important to maintain profitable animal operations. Thus, while focusing on P removal from soils, concentrations of nutrients including P in forage tissue should be given proper consideration for sustainable phytoremediation of P-impacted sites.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONPHOSPHORUS ACCUMULATION IN SOILS...FORAGE PRODUCTION AND PHOSPHORUS...CONCLUSIONSREFERENCES |
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ACKNOWLEDGMENTS |
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TOPABSTRACTINTRODUCTIONPHOSPHORUS ACCUMULATION IN SOILS...FORAGE PRODUCTION AND PHOSPHORUS...CONCLUSIONSREFERENCES |
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