Nutrients

The principal nutrients of concern in the aquatic environment are nitrogen and phosphorus. An understanding of how these nutrients react in the environment is important to understanding the control processes discussed in later sections.

(i) Nitrogen—Nitrogen occurs throughout the environment—in the soil, water, and surrounding air. In fact, 78 percent of the air we breathe is nitrogen. It is also a part of all living organisms. When plants and animals die or when waste products are excreted, nitrogen returns to the environment and is cycled back to the land, water, and air and eventually back to other plants and animals.

Figure 3-2 depicts the nitrogen cycle. It shows the flow from one form of nitrogen to another. The various forms of nitrogen can have different effects on our natural resources—some good and some bad.

The conversion from one form of nitrogen to another is usually the result of bacterial processes. Some conversions require the presence of oxygen (aerobic systems), while others require no oxygen (anaerobic systems). Moisture content of the waste or soil, temperature, and pH speed or impede conversions.

In water quality analyses, total nitrogen (TN) includes the organic (Org-N), total ammonia (NH3 + NH4), nitrite (NO2), and nitrate (NO3) forms. Total Kjeldahl Nitrogen (TKN) includes the total organic and total ammonia nitrogen. The ammonia, nitrite, and nitrate forms of nitrogen may be expressed in terms of the concentration of N (NO3-N or NH4-N) or in terms of the concentration of the particular ion or molecule (NO3 or NH4). Thus, 45 mg/L of NO3 is equivalent to 10 mg/L of NO3-N. (See chapter 4 for conversions and expressions.)

Organic nitrogen—Nitrogen in fresh manure is mostly in the organic form (60-80% of total N). In an anaerobic lagoon, the organic fraction is typically 20 to 30 percent of total N. Organic nitrogen in the solid fraction (feces) of most animal waste is usually in the form of complex molecules associated with digested food, while that in the liquid fraction is in the form of urea.

From 40 to 90 percent of the organic N is converted to ammonia within 4 to 5 months after application to the land. The conversion of organic N to ammonia (called mineralization) is more rapid in warmer climates. Under the right temperature and moisture conditions, mineralization can be essentially complete in 60 days. Conversion to ammonia can occur either under aerobic or anaerobic conditions.

Organic N is not used by crops; however, it is not mobile once applied to the land unless runoff carries away the organic matter or soil particles to which it might be attached.

Ammoniacal nitrogen—This term is often used in a generic sense to refer to two compounds: NH4 (the ammonium ion) and NH3 (un-ionized ammonia). These forms of ammonia exist in equilibrium, with the concentrations of each depending on pH and temperature.

Un-ionized ammonia is toxic to fish and other aquatic life in very small concentrations. In one study, the concentration required to kill 50 percent of a salmonid (for example, trout) population after 96 hours of exposure (the 96-hour LC50) ranged from 0.083 to 1.09 mg/L; for nonsalmonids the range was 0.14 to 4.60 mg/L. Invertebrates are more tolerant of NH3 than fish, and phytoplankton and vascular aquatic plants are more tolerant than either the invertebrates or fish.

To protect aquatic life, the U.S. Environmental Protection Agency (EPA) has established a recommended allowable limit of 0.02 mg/L for un-ionized ammonia. Table 3-2 shows, in abbreviated form, the relationship between NH3 and NH4 as related to pH and water temperature. As water temperatures and pH rise, the amount of total ammonia required to provide a lethal concentration of NH3 becomes smaller.

Table 3-2 Concentrations of total ammonia (NH3 + NH4) in mg/L that contain an un-ionized ammonia concentration of 0.020 mg/L NH3

5

160

51

16

5.1

1.6

0.53

0.18

10

110

34

11

3.4

1.1

0.36

0.13

15

73

23

7.3

2.3

0.75

0.25

0.09

20

50

16

5.1

1.6

0.52

0.18

0.07

25

35

11

3.5

1.1

0.37

0.13

0.06

The concentration of NH3 from an overflowing lagoon or other storage structure with concentrated animal waste can exceed the EPA criterion by as much as 3,000 times. Runoff from a feedlot or overfertilized pasture can also have high levels of total ammonia nitrogen (NH3 + NH4).

Ammonium nitrogen is relatively immobile in the soil. The positively charged NH4 tends to attach to the negatively charged clay particles and generally remains in place until converted to other forms.

Ammonia can be lost to the atmosphere in gaseous form (volatilization), a process that is not a function of bacterial activity. As much as 25 percent of the ammonia irrigated from an animal waste lagoon can be lost between the sprinkler head and the ground surface. Temperature, wind, and humidity will affect losses.

Ammonia can be converted to nitrite and then to nitrate (nitrified) only under aerobic conditions. For this reason, organic N and ammonia N generally are the only forms of nitrogen in anaerobic lagoons and waste storage ponds. The ammonia begins to nitrify when the waste from these structures is applied to the land where aerobic conditions exist.

Nitrite (NO2)—This is normally a transitory phase in the nitrification and denitrification processes. Very little NO2 is normally detected in the soil or in most natural waters.

Nitrites occasionally occur in significant concentrations in farm ponds and commercial fish ponds during a fall "overturn" or when the mud on the bottom of the pond is disturbed during commercial harvesting. If the bottom material is enriched with nutrients (from excess commercial feed, fish waste, or other sources of animal waste), the concentrations of nitrites in the overlying water can be raised enough to cause nitrite poisoning or brown blood disease in fish when this mud is disturbed. The dead or dying fish have "chocolate" colored blood, which indicates that the hemoglobin has been converted to methemoglobin.

Nitrite concentrations at or below 5 mg/L should be protective of most warmwater fish, and concentrations at or below 0.06 mg/L should suffice for cold-water fish. Concentrations as high as these are unlikely to occur as a result of natural conditions in surface water.

The EPA has not recommended any special limits on nitrites in surface water; however, some States have criteria for nitrite concentrations in finished or treated water (see chapter 1).

Nitrate (NO3)—The nitrate form of nitrogen is the end product of the mineralization process (the conversion of N from the ammonia form to nitrite and then to nitrate under aerobic conditions). The nitrate form of N is soluble in water and is readily used by plants.

Under anaerobic conditions, microbial activity can convert NO3 to a gaseous form of N, a process called denitrification. Nitrogen in animal waste that has been converted to nitrate after land application can leach into the soil profile, encounter a saturated anaerobic zone, and then be denitrified through microbial activity. The gaseous forms of N created in this process can then migrate upward through the soil profile and be lost to the atmosphere.

The principal source of agricultural nitrates in surface water is runoff from feedlots, cropland, and pastures. Table 3-3 illustrates the possible differences in dissolved N concentrations in runoff from fields that had manure surface applied at agronomic rates and those that had no manure applied.

The values in the table represent estimates of dissolved N only and do not represent amounts that could also be transported with sediment. Although these values were obtained from published data, they do not

Table 3-3

Cropping conditions

Estimated concentrations of total dissolved nitrogen in runoff from land with and without livestock and poultry manure surface applied

Dissolved N concentration in runoff With manure Without manure

Grass Small grain Row crop Rough plow

Source: Animal Waste Utilization on Cropland and Pastureland (USDA 1979).

reflect the variability that could result from such factors as differences in rainfall in various geographic regions, slope of land, amount and age of manure on the ground surface, or extent of crop cover. Therefore, the table is presented only to illustrate the extent to which nitrate concentrations can be increased in runoff from land that has received applications of manure.

Elevated nitrate levels have also been observed in the spring runoff from fields where manure had been applied to snow-covered or frozen ground. In addition, the discharge from underground drainage lines in cropland fields can have elevated concentrations of NO3.

Nitrates are toxic to fish only at very high concentra-tions—typically in excess of 1,000 mg/L for most freshwater fish. Such species as largemouth bass and channel catfish, could maintain their normal growth and feeding activities at concentrations up to 400 mg/L without significant side effects. These concentrations would not result from natural causes and are not likely to be associated with normal agricultural activities.

depicts the relationship between the phosphorus forms and illustrates ways that P can be lost from waste application sites.

Organic phosphorus is a part of all living organisms, including microbial tissue and plant residue, and it is the principal form of P in the metabolic byproducts (wastes) of most animals. About 73 percent of the phosphorus in the fresh waste of various types of livestock is in the organic form.

Soluble phosphorus (also called available or dissolved P) is the form used by all plants. It is also the form that is subject to leaching. The soluble form generally accounts for less than 15 percent of the total phosphorus in most soils.

Attached phosphorus includes those compounds that are formed when the anionic (negatively charged) forms of dissolved P become attached to cations, such as iron, aluminum, and calcium. Attached phosphorus includes labile, or loosely bound, forms and those that are "fixed," or tightly adsorbed, on or within individual soil particles.

Although nitrates are not normally toxic to aquatic organisms, NO3 is a source of enrichment for aquatic plants. If an adequate supply of other essential nutrients is available (especially phosphorus), nitrates can help promote algae blooms and the production of other aquatic vegetation.

It should be noted that the P that is loosely bound to the soil particles (labile P) remains in equilibrium with the soluble P. Thus, when the concentration of soluble P is reduced because of the removal by plants, some of the labile P is converted to the soluble form to maintain the equilibrium.

The EPA has not recommended any limiting criteria for nitrates as related to surface water. (See chapter 1, section 651.0108(b), for a discussion of limits related to drinking water as it comes from the tap.)

(ii) Phosphorus—Phosphorus (P) is one of the major nutrients needed for plant growth, whether the plant is terrestrial or aquatic. Because phosphorus is used extensively in agriculture, the potential for pollution from this source is high.

Forms of phosphorus—Water samples are often analyzed for only total phosphorus; however, total phosphorus can include organic, soluble, or "bound" forms. An understanding of the relationship among these forms is important to understanding the extent to which phosphorus can move within the environment and the methods for its control. Figure 3-3

Factors affecting the translocation of phosphorus—A number of factors determine the extent to which phosphorus moves to surface or ground water. Nearly all of these factors relate to the form and chemical nature of the phosphorus compounds. Some of the principal factors affecting P movement to surface and ground waters are noted below.

Degree of contact with the soil. Manure that is surface applied in solid form generally has a higher potential for loss in surface runoff than wastewater applied through irrigation, especially in areas that have frequent, high-intensity storms. This also assumes the irrigation water infiltrates the soil surface. Because phosphorus readily attaches to soil particles, the potential for loss in surface runoff is greatly reduced by incorporating land applied solid wastes into the soil profile.

Chapter 3 Agricultural Wastes and Water, Part 651

Air, and Animal Resources Agricultural Waste Management

Field Handbook

Figure 3-3 Phosphorus inputs and losses at a waste application site and phosphorus transformation within the soil profile (abbreviated phosphorus cycle)

inputs and losses

Figure 3-3 Phosphorus inputs and losses at a waste application site and phosphorus transformation within the soil profile (abbreviated phosphorus cycle)

inputs and losses

Dissolved P

Attached P

P transformations in soil profile

Organic P

r

Temporarily bound in microbial tissue, dead roots, plant residue, and unmineralized waste; competes with attached P for adsorption sites

(soluble, available P) H 2PO4 , HPO4 less than 15% of total P

Inorganic P.

Attached P

Labile

Exchangeable P loosely bound to Al, Fe, Ca. A small fraction of attached P

Fixed

Tightly bound within the soil as

Al & Fe phosphates and as Ca 2HPO4, Ca3(PO4 )2 and other compounds lost through leaching

SoilpH. After animal waste makes contact with the soil, the phosphorus will change from one form to another. Organic P eventually converts to soluble P, which is used by plants or converted to bound P. However, the amount of soluble P is related to the pH of the soil as illustrated in figure 3-4. In acid soils the soluble P occurs primarily as H2PO4, and when the pH increases above 7, the principal soluble form is HPO4.

Figure 3-4 illustrates that most inorganic phosphorus occurs as insoluble compounds of aluminum, iron, calcium, and other minerals typically associated with clay soils. Therefore, these bound forms of P will generally remain in place only so long as the soil particles remain in place.

Soil texture. Phosphorus is more readily retained on soils that have a high clay fraction (fine textured soils) than on sandier soils. As noted in figure 3-4, those soil particles that contain a large fraction of aluminum, iron, and calcium are very reactive with phosphorus. Thus, clay soils have a higher adsorption potential than that of sandy soils.

Research has shown that soils with even a modest clay fraction have the potential to adsorb large amounts of P. For example, one study revealed that a Norfolk sandy loam soil receiving swine lagoon effluent at phosphorus application rates of 72, 144, and 288

pounds per year would require 125, 53, and 24 years to saturate the adsorption sites in the soil profile to a depth of 105 cm (41 inches). This does not mean that all of the applied P would be adsorbed within the soil profile. Rather, the soil simply has the potential for such adsorption, assuming none is lost through other means.

Amount of waste applied. Organic P readily adsorbs to soil particles and tends to depress the adsorption of inorganic P, especially where organic P is applied at high rates. Thus, the concentrations of soluble and labile P increase significantly at high application rates of organic P.

When organic P and commercial superphosphate are applied at the same rates, the superphosphate P will be less effective in raising the concentration of soluble P than the P applied in manure or other organic waste. This occurs because the organic P competes for adsorption sites, resulting in more P staying in soluble form rather than becoming attached as labile P.

Long-term applications of organic P at rates that exceed the uptake rate of plants will result in saturation of the adsorption sites near the soil surface. This, in turn, results in greatly increased concentrations of both soluble and labile P. The excess soluble P can either leach downward to a zone that has more attach

Figure 3-4 Phosphorus retention and solubility as related to soil pH

Figure 3-4 Phosphorus retention and solubility as related to soil pH

pH of soil solution

ment sites and then be converted to labile P or fixed P, or it can be carried off the land in runoff water.

If soils that have high labile P concentrations reach surface water as sediment, they will continuously desorb or release P to the soluble form until equilibrium is attained. Therefore, sediment from land receiving animal waste at high rates or over a long period of time will have a high potential to pollute surface water.

Table 3-4 illustrates typical dissolved phosphorus concentrations reported in surface runoff from fields where animal waste was applied at recommended agronomic rates. Although this table is based on research findings, it is provided for illustration only because it does not necessarily represent concentrations that might occur in different regions of the country where the land slopes, soil types, waste application quantities and rates, or amounts of precipitation could be different than those for which the research was conducted.

Waste that is surface applied can produce total P concentrations in surface runoff higher than those shown in table 3-4, especially if the waste is applied at high rates, not incorporated, applied on snow-covered or frozen ground, or applied on fields with inadequate erosion control practices.

Erosion control measures. Although organic matter increases the water holding capacity of soils and generally helps to reduce the potential for erosion, erosion can still occur on land receiving livestock and poultry wastes. If wastes are applied to satisfy the nitrogen requirements of the crops, the phosphorus concentrations in the soil may become extremely high. Because such soils generally have a high concentration of labile P, any loss of soil to surface water poses a serious threat to water quality in the receiving water, especially ponds and lakes. For this reason, good erosion control measures are essential on land receiving animal waste.

Phosphorus entrapment. Providing an adequate buffer zone between the source of organic contaminants (land spreading areas, cattle feedlots) and stream or impoundment helps provide settling and entrapment of soil particles with attached P. Forested riparian zones adjacent to streams form an effective filter for sediment and sediment related phosphorus. In addi tion, water and sediment control basins serve as sinks for sediment-attached phosphorus.

Animal waste lagoons are also very effective for phosphorus storage. Typically 70 to 90 percent of the phosphorus in waste that enters a waste treatment lagoon will settle and be retained in the sludge on the bottom of the lagoon.

Phosphorus retention. Sandy soils do not effectively retain phosphorus. If the ground water table is close to the surface, the application of waste at excessive rates or at nitrogen-based rates will most likely contaminate the ground water beneath those soils. However, ground water that is below deep, clay soils is not likely to be contaminated by phosphorus because of the adsorptive capacity of the clay minerals.

Phosphorus will change forms rapidly once contact is made with the soil. Equilibria can be established between the bound forms and those in solution within just a few hours. However, as time goes on, more of the P is converted to the fixed or tightly bound forms. The conversion to these unavailable forms may take weeks, months, or even years. Therefore, the soil has the potential to retain large amounts of P (to serve as a phosphorus "sink"), especially if given ample time between applications.

Aerobic conditions. Compounds of phosphorus, iron, manganese, and other elements react differently where oxygen is present or absent in the surrounding

Table 3-4

Estimated dissolved phosphorus concentrations in runoff from land with and without animal wastes surface applied

Cropping conditions

- Dissolved phosphorus in runoff -with manure without manure

- mg/L---------

Grass

3.0

0.44

Small grain

4.0

0.40

Row crop

1.7

0.40

Rough plow

1.7

0.20

Source: Animal Waste Utilization on Cropland and Pastureland (USDA 1979).

Source: Animal Waste Utilization on Cropland and Pastureland (USDA 1979).

environment. This is true in the soil environment as well as in impoundments. Under anaerobic conditions iron changes from the ferric to the ferrous form, thus reducing P retention and increasing P solubility.

Soils receiving frequent applications of wastewater can become saturated and anaerobic. Such soils will not be as effective at removing and retaining phosphorus as well aerated soils.

Harvesting. Soluble phosphorus will be removed from the soil by plants. The amount removed depends on the amount required by the plant and the reserve of P in the soil. If the plants are removed through mechanical harvesting, all of the phosphorus taken up by the plant will be removed except that associated with the roots and unharvestable residue. If the plants are removed be grazing animals, only a part of the plant phosphorus will be removed because a large fraction of the P consumed will be returned to the land in the feces. If plants are not harvested and removed, either mechanically or through animal consumption, they will eventually die, decay, and return the phosphorus to its source. It then becomes available again as a source of plant food or of pollution.

Effects of phosphorus in the aquatic environment—When phosphorus enters the freshwater environment, it can produce nuisance growths of algae and aquatic weeds and can accelerate the aging process in lakes. Direct toxicity to fish and other aquatic organisms is not a major concern. Some algae species are toxic to animals if ingested with drinking water.

In the marine or estuarine environment, however, phosphorus in the elemental form (versus phosphates or other forms of combined P) can be especially toxic and can bioaccumulate in much the same way as mercury. For this reason, EPA has established a criterion of 0.01 |g/L (micrograms per liter) of yellow (elemental) phosphorus for marine and estuarine water. This concentration represents a tenth of the level demonstrated to be lethal to important marine organisms. Other forms of P are virtually nontoxic to aquatic organisms.

Although no national criteria exist for other forms of phosphorus to enhance or protect fresh water, EPA recommends that total phosphate concentrations not exceed 50 |g/L (as P) in any stream at the point where it enters a lake or reservoir (EPA 1986). A desired goal for the prevention of plant nuisances in streams or other flowing water not discharging directly to lakes or impoundments is 100 |ig/L of total phosphorus.

Relatively uncontaminated lakes have from 10 to 30 |ig/L total phosphorus in the surface water. However, a phosphate concentration of 25 |ig/L at the time of spring turnover in a lake or reservoir may occasionally stimulate excessive or nuisance growths of algae and other aquatic plants.

EPA reports these findings regarding phosphorus in natural water (EPA 1984):

• High phosphorus concentrations are associated with accelerated eutrophication of water, when other growth-promoting factors are present.

• Aquatic plant problems develop in reservoirs and other standing water at phosphorus values lower than those critical in flowing streams.

• Reservoirs and lakes collect phosphates from influent streams and store part of them within consolidated sediment, thus serving as a phosphate sink.

• Phosphorus concentrations critical to noxious plant growth vary, and nuisance growths may result from a particular concentration of phosphate in one geographic area, but not in another.

Whether or not phosphorus will be retained in a lake or become a problem is determined by nutrient loading to the lake, the volume of the photic (light-penetrating) zone, the extent of biological activity, the detention time of the lake, and level at which water is withdrawn from the lake. Thus, a shallow lake in a relatively small watershed and with only a surface water discharge is more likely to have eutrophication problems than a deep lake that has a large drainage area-to-lake volume ratio and bottom water withdrawal. This assumes that the same supply of nutrients enters each lake.

Figure 3-5 depicts average inflowing phosphorus concentrations into a lake versus hydraulic residence time, which is the time required for the total volume of water in the lake to be replaced with a "new" volume. The dotted lines represent phosphorus concentrations of 10, 25, and 60 |g/L and roughly delineate the boundaries between oligotrophic, mesotrophic, eutrophic, and hyper-eutrophic conditions. This figure is presented for purposes of illustration only because the delineations between the different trophic states cannot be precisely defined. The model used to develop figure 3-5 is only one of many models used to predict trophic state. Some are more useful in cool, northern climates, while others are best suited to warmwater lakes or lakes in which nitrogen rather than phosphorus is limiting.

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