for Reducing Nutrient Loads for Reducing Nutrient Loads
nalysis of the causes of hypoxia in the Gulf of Mexico, as noted

in the second chapter of this report, indicates that nitrogen is

the principal nutrient fueling production and deposition of

excess organic matter which, when stratified conditions exist,

results in hypoxia. Thus, the approaches to reduce, mitigate, and

control hypoxia in the Gulf of Mexico focus on reducing nitrogen

flux to the Gulf. The two primary approaches that can be considered are:3

■ reducing nitrogen loads to rivers and streams in the Basin,

including losses of nitrogen from farmlands and loads from

point sources, atmospheric deposition, and other sources; and

■ restoring and enhancing natural denitrification and nitrogenretention processes to reduce nitrogen loads from the

Mississippi and Atchafalaya Rivers to the Gulf.

Analyses of these approaches examined the economic and

other effects of achieving nitrogen load reductions at two levels:

■ reduction of total nitrogen loads to the Gulf of Mexico by 40

percent (approximately 625,000 metric tons/yr), to the average

level that existed during 1955–70; and

■ reduction of nitrogen loads by approximately 20 percent from

the average level of the 1980–96 period (approximately

310,000 metric tons/yr), midway between reduction to historical levels and no reduction.

Macroeconomic models were used to evaluate system-wide

impacts of some broad policy approaches on agricultural production, farm income, consumer prices, and the like. For many

aspects, available models offer only rough approximations.

Model results report the broad economic consequences of

meeting a water quality goal and are useful for comparing broad

policy options. However, the model results cannot be used to

make actual policy recommendations for any particular area in

Chapter 5: Approaches ■ 37

3 A third approach, recently raised, calls for diverting nutrient-laden waters away

from the heart of the hypoxic zone in the central Gulf to areas that are not

currently affected by hypoxia. To do this would require an alteration of the

current congressionally mandated 70/30 flow split at Old River, so that additional

flows would be passed into the Gulf by the main channel of the Mississippi River,

or to the east side of the “Bird Foot Delta,” while reduced flows would enter the

Gulf through the Atchafalaya River distributary. This option would have multiple

consequences and has not been analyzed. 
the Basin. Any program for reducing

nitrogen loading to the Gulf should consider local hydrologic conditions and the

characteristics of agricultural production,

the resource base, and the producers. An

optimal strategy would take appropriate

advantage of the full range of possible

measures to deal with hypoxia in the Gulf

of Mexico.

Reducing Nitrogen Loads to Rivers

and Streams in the Basin

After a period of marked increase in

nitrogen flux to the Gulf between 1970

and 1983, loads have remained variable

but without statistically significant trends,

despite increasing agricultural production,

increasing population, and other pressures

in the Mississippi River Basin. Improved

agricultural practices are probably the

major contributors to keeping nitrogen

loads from significantly increasing since

the 1980s. Innovations by researchers,

industry, and individual farmers have

reduced nitrogen losses, in part to reduce

fertilizer expenditures. 

There are several sources of nutrient

loadings that might be targeted for further improvement, including agricultural

sources, urban point and nonpoint

sources, and atmospheric deposition.

There are also several possible options for

sharing the burden of nutrient load reductions among all sectors. Analyses suggest

that changes in agricultural practices to

achieve modest levels of

nutrient reduction can

provide the leastcost option to society overall while

maintaining high

levels of production and realizing relatively minor impacts

on net farm income

sector-wide. However,

because different agricultural regions and individual

crops are more sensitive to changes in

nutrient levels and

because producers

have different levels of management skills, specific producers might incur severe costs, while others

might benefit from reduced costs due to

improved management practices. 

Ultimately, there are many questions

about how these improved practices

would be motivated and how the costs

would be borne. Differing implementation

strategies could impose costs primarily on

agricultural producers, consumers, or—if

these costs are supported through incentive payments—society at large. How to

pursue effective implementation of a solution to Gulf hypoxia across the different

nutrient sources and a multitude of state

and federal water quality laws and programs will require consideration of a

broader range of factors than was

attempted in this assessment.

Some reduction in nitrogen loads from

municipalities could be achieved by the

elimination of combined sewer overflows

and the installation of biological nutrient

control processes in new sewage treatment plants. While such sources are not

the major contributor to the total nitrogen load from the Basin, they discharge

directly to major streams, with little

opportunity for natural treatment.

Discharges of nitrogen from farms to

streams and rivers could be reduced by

implementing a wide variety of changes 

in management practices, such as:

■ applying nitrogen fertilizer and manure

at not more than agronomically recommended rates;

■ switching from fall to spring application of fertilizer;

■ improving management of livestock

manures, whether stored or applied to

the land;

■ changing from row-cropping to perennial-cropping systems;

■ planting cover crops for fall and winter

nutrient absorption;

■ switching from conventional to ridgetilling or other reduced-tillage practices;

■ ensuring that the lateral spacing of subsurface tile drainage is not less than 15

meters;

■ controlling water tables to promote

denitrification within the soil column;

and

38 ■ HYPOXIA IN THE NORTHERN GULF OF MEXICO

An optimal

strategy would

take appropriate

advantage of

the full range

of possible

measures to

deal with

hypoxia in the

Gulf of Mexico.
U.S. DEPARTMENT OF AGRICULTURE

Chapter 5: Approaches ■ 39

■ routing soil drainage effluent

through wetlands, grass buffer

strips, or riparian forest buffers.

Nitrification inhibitors for fertilizers, amino acid feed supplements, and other measures have

also been suggested. For some of

the above improved management

practices, reduction in nitrogen

losses to streams are on the order

of 10–20 percent of baseline conditions, but others have been

shown to reduce losses by as

much as 90 percent in specific

field studies (see the Topic 4 and

Topic 5 reports for details).

As shown in Table 5.1, of these

measures, the two with the greatest estimated potential to reduce

nitrogen losses from agriculture to

streams and rivers are:

■ Improved nitrogen management

techniques. This is estimated to

offer the greatest potential

reduction of nitrogen loading.

Reducing “insurance” rates of

nitrogen fertilizer,4 improving

management practices for storage and land application of

manure,5 improving management of runoff from feedlots,

applying appropriate credits

for previous crops and manure,5 and using improved soil

nitrogen testing methods could

reduce nitrogen losses at the

edge of fields by about 0.9–1.4

million metric tons per year.6

■ Alternative cropping systems.

If 10 percent of the corn–soybean farms in the Basin were

changed to include alfalfa or

alfalfa-grass mixes as crops,

edge-of-field nitrogen losses

could be decreased by an estimated 0.5 million metric tons

per year.

4 Data on “insurance” application of fertilizer (application at rates above agronomic recommendations) are sparse. “Insurance” application is

an economic wager: the producer is gambling that all other factors influencing crop production will be optimal in a given year and that an

extra 10–20 pounds of nitrogen fertilizer per acre would result in increased yields. In reality, this is a poor wager. Plant nitrogen is obtained

from both soil reserves and applied nitrogen: thus, response is not solely a function of “this year’s application,” but is a function of the total

available nitrogen status. Agronomic recommendations are developed taking these factors into account. Therefore, insurance applications are

in most cases wasted, and the extra nitrogen is lost from the field and may lead to increased loadings to the aquatic environment.

5 In practice, according to the Potash and Phosphate Institute, nitrogen use on corn in Iowa in 1996 was within the range of agronomic

recommendations, when manure was not used but exceeded agronomic recommendations by more than 30 percent, or about 33 kg of

nitrogen/ha (30 lb/acre) when manure was used.

6 Note that these estimates of the impacts of changes in agricultural practices are for reductions at the edge of the field; estimated edge-offield source reductions do not translate to equivalent reductions in nitrogen loadings to the Gulf, because only a portion of nitrogen sources in

the Basin reaches the Gulf.

The spatial pattern of the manure input to the Basin has changed from a highly dispersed to a highly concentrated distribution as confined animal feedlot operations, such as shown here, have become more common. Improving management practices for storage and application can reduce nitrogen losses to streams and rivers. 
40 ■ HYPOXIA IN THE NORTHERN GULF OF MEXICO

Estimates of impact are based

on field tests and case studies

described in the Topic 5 report.

Measures to reduce nutrient

loads would generally have net

costs. Assuming constant levels of

efficiency, large decreases in

nutrient use below agronomic

rates would lead to reduced agricultural productivity in the MARB

and reduced total production as

marginally fertile land is retired.

The agriculture sector’s response

to these alternative management

strategies was simulated with the

U.S. Mathematical Programming

Model for Agriculture (USMPM),

described below. The model predicts both the economic effects

and the changes in nitrogen

loading under the various scenarios examined.

The USMPM modeling analysis provided a range of potential

results that focusing a variety of

nitrogen-reduction strategies in

the Mississippi Basin could

achieve. The analyses showed that

as a result of different approaches

to nitrogen reduction, shifts in

production could be expected

both inside and outside the Basin.

Many producers inside the Basin

would find that reducing crop

acreage would benefit them,

whereas many producers outside

the Basin would find advantage in

expanding their cropped acreage.

For example, reducing edge-offield nitrogen loss by 20 percent

(in the most cost-effective manner) could result in a 5.8 percent

decline in crop acreage in the

Basin with a potential 2.8 percent

increase in cropland outside the

Basin. This same scenario could

also be expected to increase edgeof-field nitrogen losses outside the

Basin by about 8 percent due to

an increase in production. Modeling analyses indicated that the

higher the level of nitrogen loss

reductions from agriculture shown

in the Basin, the more pronounced shifts in regional production

could be expected, and these production shifts could increase

nutrient-related problems for

other regions. Clearly, the degree

and pattern of production shifts

that could be realized in response

to each alternative strategy would

depend on the nature of the strategy selected (the mix of practices

used), the size of the region

affected, and its location (environmental conditions).

The USMPM modeling analysis

predicts that edge-of-field nitrogen losses can be reduced by 20

percent (941,000 metric tons/yr)

using various management practices at a total societal cost under

economically optimum conditions

of $831 million/yr, or $0.88 per kg

reduction in edge-of-field nitrogen

flux. This result assumes all factors

of production are adjusted in the

most efficient manner. Although

meeting the 20 percent reduction

in edge-of-field nitrogen loss

could also be achieved by targeting fertilizer reductions only, it

would require a 45 percent reduction in fertilizer use, resulting in

costs of $2.85 per kilogram of

nitrogen reduction. However,

smaller reductions would be more

attractive on a per kilogram basis:

a 20 percent reduction in fertilizer

would reduce edge-of-field losses

by 503,000 metric tons/yr (about

10 percent) at a total societal cost

of $347 million, or $0.69 per kilogram. As part of the economic

analysis, the impact of raising the

price of fertilizer by increasing fertilizer taxes (an approach designed

to spur farmers to find ways to

reduce fertilizer use) was evaluated. However, the analysis

showed that to achieve a 20 percent reduction in edge-of field

losses, such taxes would have to

reach 500 percent of current retail

price for fertilizer. The resulting

cost to individual farmers and

society as a whole was predicted

to be much greater than other

approaches evaluated (see Table

5.3 on page 45).

U.S. Mathematical Programming Model for 

Agriculture (USMPM)

Both the economic and the environmental effects of agricultural

changes were estimated using the

USMPM, a model developed by the

U.S. Department of Agriculture’s

Economic Research Service for

analysis of government commodity

programs. The model calculates commodity prices and quantities, net

returns to producers, net social benefits, and environmental emissions

recognizing spatial variation and

explicitly representing various management practices used by farmers.

Adjustments to crop production can

be estimated in response to restrictions placed on nitrogen or land use

by altering any or all of the following:

acreage planted, crop mix, rotations

used, tillage practices, and fertilizer

application rates (see the Topic 6

report, p. 25ff., for details).
Chapter 5: Approaches ■ 41

Point sources of nitrogen, as

well as urban nonpoint sources,

also contribute to the total nitrogen load to the Gulf. Nitrate concentrations in urban nonpoint

sources are generally not high,

compared to urban point sources

or Corn Belt cropland. Urban

areas constitute only 0.6 percent

of the Basin; thus, the contribution of urban nonpoint sources is

comparatively small but has not

been precisely estimated. Point

sources constitute less than 2 percent of nitrogen inputs to the

Basin, or about 11 percent of the

nitrogen inputs to the Gulf. Higher unit costs for reductions in

nitrogen from domestic wastewater partly reflect the fact that

these sources are currently regulated and have realized pollution

control costs already. Biological

nitrogen removal from urban

point sources typically costs $40

per kilogram of nitrogen removed.

However, in some cases, costeffective reductions in nitrogen

load could be realized. 

Atmospheric deposition of

nitrogen (wet and dry nitrate, wet

ammonia, and organic nitrogen) is

estimated to be about 10 percent

of the nitrogen input to the Basin.

The costs of reducing atmospheric

deposition of nitrogen are estimated to be $20–100 per kilogram of nitrogen.

Nitrogen trading among all

sectors could offer opportunities

to reduce costs overall.

Restoring and Enhancing

Denitrification and NitrogenRetention Processes

A second general approach to

reducing nitrogen flux to the Gulf

of Mexico is to place or restore

ecosystems that are effective

nitrogen sinks along the paths of

nitrogen flow from sources to the

Gulf. Table 5.2 summarizes the

impacts of dentirification options.

Nitrogen transformations in

wetlands and riparian soils, surface

water, and ground water involve

several microbiological processes,

some of which denitrify—that is,

make the nitrogen effectively

unavailable for plant uptake.

Wetlands also serve as traps for

phosphorus, which limits primary

production within the Basin and,

under some conditions, on the

shelf in the Gulf.

High rates of denitrification

are possible, but wetlands and

riparian zones can also act as

sources of nitrogen under some

conditions. Loading rates (flow

times nitrogen concentration of

inflowing water) dictate the effectiveness of wetlands in reducing

nitrogen. The most effective use

of wetland restoration and creation would be in watersheds that

discharge high amounts of nitrogen. Retention time is also important. Storm events, if significant

enough, can cause nitrogen to

move rapidly through the system,

bypassing effective retention. 

Extensive experience with flowthrough wetland systems suggests

a narrow range centered around

10–25 g N/m2/yr as a reasonable

target for denitrification by wetlands. Assuming denitrification at

15 g N/m2/yr (150 kg N/ha/yr),

2.1 million hectares (5 million

acres, or 0.7 percent of the Mississippi River Basin area) of constructed

or restored wetlands would reduce

nitrogen load to the Gulf of

Mexico by 20 percent. The current national goal is to restore wetlands at a rate of 100,000 acres/yr

(about 40,000 ha/yr) (see the Clean

Table 5.1 Potential Approaches to Reducing Nitrogen Inputs

Approach Potential Nitrogen Reduction

(Thousands of metric tons/yr)

Changing Farm Practices

Nitrogen management 900–1,400a

(Reduction in “insurance” rates of N fertilizer

application, improved management practices for

storage and land application of manure, improved

management of runoff from feedlots, and crediting of nonfertilizer nitrogen)

Alternative cropping systems 500a

(Perennial crops in lieu of corn and soybeans on

10% of acreage)

Reduction in Point Sources

Tertiary treatment of domestic wastewater 20

a Note that these estimates of the impacts of changes in agricultural practices are for reductions at

the edge of the field; estimated edge-of-field source reductions do not translate to equivalent in

nitrogen loadings to the Gulf, because only a portion of nitrogen sources in the Basin reaches the

Gulf.
42 ■ HYPOXIA IN THE NORTHERN GULF OF MEXICO

Water Action Plan, available at

<http://www.cleanwater.gov>).

The total direct costs for permanent easements and restoration of

wetlands to reduce nitrogen load

by 20 percent was estimated to be

$4.7 billion. Annual costs, including net effects on consumers and

producers plus annualized wetland

restoration costs, were estimated to

be $3.1 billion, or about $8.90 per

kilogram reduction in nitrogen flux

per year. There would be ecological and wildlife benefits in addition

to those that might accrue in the

Gulf from using this approach.

Riparian areas are vegetated

areas, generally forested or

grassed, next to water resources.

They have been shown to be

effective in removing nitrates

from ground water as it passes

through the vegetated buffer.

Typically, denitrification rates are

2-6 g N/m2/yr. Assuming denitrification at 4 g N/m2/yr (40 kg

N/ha/yr), 7.8 million hectares (19

million acres, or 2.7 percent of

the Basin) of additional riparian

buffers would be needed to reduce

nitrogen load to the Gulf by 20

percent. 

A current national goal is to

establish two million miles of

conservation buffers on agricultural lands (also per the Clean

Water Action Plan). Total direct

costs for permanent easements

and restoration of 27 million

acres of riparian buffers were estimated to be $46.3 billion, equating to $18.0 billion annually in

changes to consumer and producer surpluses plus annualized

restoration costs, or about $26

per kilogram reduction in nitrogen flux per year. 

Vegetated buffer areas reduce both surface and subsurface flux of nitrogen to streams and rivers. Riparian buffers can be

particularly effective in capturing nutrients (and providing other protection) during floods. 

Table 5.2 Potential Approaches to Increasing Denitrification

Approach Potential Nitrogen Reduction

(Thousands of metric tons/yr)

Creating and restoring 5–13 million acres 300–800

of new wetlands

Creating and restoring 19–48 million acres 300–800

of riparian bottomland hardwood forest buffers

Diverting rivers in coastal Louisiana 50–100

U.S. DEPARTMENT OF AGRICULTURE
Chapter 5: Approaches ■ 43

Separation of the Mississippi

River from its floodplain and

coastal estuaries may be an

important factor in supplying

nutrients to the Gulf. The river

once spread out over the Delta

during flood periods. Today, however, with the exception of such

locations as the Bonne Carre

Spillway and the Atchafalaya

Basin, the river is mostly shunted

directly to the sea. Through

enhanced water management

strategies, it may be possible to

increase the amount of water

reaching inland and coastal wetlands, and thereby increase

nitrate removal from the water

and reduce coastal land loss.

Using a denitrification rate of 10

g N/m2/yr based on studies at

Caernarvon, Louisiana, and other

locations, diverting 13 percent of

the total river flow over 1.2 million acres (500,000 hectares)

would remove 50,000 metric tons

of nitrate per year. However,

without upstream controls, the

system might become nitrogensaturated, or it might release

nitrogen in a different form, in a

different season, or in a different

location. A recent study (Turner

1999) compared nutrient concentrations entering and leaving the

Atchafalaya River basin, a major

floodway used to divert floodwaters from the Mississippi and Red

Rivers to the Gulf during the

annual high-water season. This

study found that nitrate concentrations remained essentially

unchanged from upstream to

downstream monitoring stations.

However, enhanced water management strategies within the

floodway could improve the

potential to remove nitrate from

waters flowing through this area.

Other studies have shown that

harmful algal blooms may result

when large quantities of

Mississippi River water are

diverted into coastal areas.

The construction and restoration of wetlands and riparian

zones in the Basin to reduce

nutrients would contribute to

several important national goals,

including those for drinkingwater protection, adding to the

nation’s disappearing wetland

habitat, improving river ecosystems, enhancing terrestrial

wildlife in river corridors, and

mitigating the effects of floods.

This restoration of wetlands is in

keeping with recommendations

by the National Research Council’s Committee on the Restoration

of Aquatic Ecosystems (NRC

1992), which called for a national

program of wetland restoration

that would contribute to an overall gain of 10 million acres by the

year 2010. Well-placed wetlands

and riparian buffers generally support larger populations of wildlife

because of the diverse habitats

they provide. The restoration of

riparian vegetation improves the

ecological condition of streams

and rivers and protects the

aquatic communities that depend

on them. Roots of riparian vegetation stabilize the stream bank

and prevent both bank erosion

and downstream sedimentation.

The National Research Council

also called for restoration of about

640,000 kilometers (400,000

miles) of streams, rivers, and

floodplains across the nation

(NRC 1992).

Cost-Effective Strategies

Various potential components

of a program to reduce nitrogen

loads to the Gulf via the MARB

system were analyzed to evaluate

their cost-effectiveness; their

impacts on the economy, farmers,

and consumers; and the reductions in loads (Table 5.3).

Direct costs, including changes

in consumer and producer surpluses and easement and restoration costs for wetlands, were

estimated using the USMPM. All

of the nutrient-reduction

approaches will also produce environmental benefits within the

drainage basin. These include the

values associated with restored

wetlands, reduced soil erosion,

reduced nitrogen contamination

of drinking water, reduced vulnerability to floods, enhanced

wildlife habitat and improved

recreational water quality. Other

potential benefits of nutrientreduction activities include more

efficient use of organic and inorganic fertilizers (see Figure 5.1)

and the energy associated with

them, lower overall fertilizer

costs, decreased health risk from

contamination of public and private drinking-water supplies, and

improved aquatic habitat in

streams, lakes, rivers, and estuaries. Good estimates for the economic value of these benefits are

only available for a few categories,

such as wetlands and erosion.

When these estimated benefits are

factored in, the net costs of wetlands and farm practices become

close: 20 percent edge-of-field

nitrogen-loss reduction was estimated to have a net cost of $0.80

per kilogram ($0.36 per pound),

while 5 million acres of wetlands

would have a net cost of $1.00

per kilogram ($0.45 per pound).

An optimal strategy would

take appropriate advantage of the

full range of possible approaches

to deal with hypoxia in the Gulf.

Implementation mechanisms,

including incentive payments,

voluntary stewardship and technical assistance programs, more

stringent requirements for regulated facilities or various combinations of these approaches, have

advantages and disadvantages in

various circumstances and locales;

however locale-specific considerations were beyond the scope of

this assessment. Since, generally,

the costs per pound of nitrogen

kept out of the river system and

Gulf, or removed by biogeochemical processes in restored wetlands or riparian areas, are in the

same order of magnitude, these

regional or site-specific circumstances may drive the choice of

the preferred mix of actions to

address hypoxia—no single

approach is clearly far more economical or costly.
44 ■ HYPOXIA IN THE NORTHERN GULF OF MEXICO

The benefits of a program to

reduce nitrogen loads to the Gulf

are difficult to quantify. Although

there are known impacts to the

Gulf ecosystem, an economic

analysis based on past data did

not detect a direct relationship

between hypoxia and Gulf fisheries. The information to estimate the benefit value for such

actions as restoring the ecological communities in the Gulf or

improving water quality in the

Basin is not available.

Another factor important in

assessing the impact of potential

actions on restoring damaged

resources and on mitigating and

controlling hypoxia in the Gulf is

the uncertainty around and contribution of time lags. It appears

that a substantial quantity of

nutrient storage in soils and

ground water has built up over

many decades. This suggests that

continuing loads may contribute

to elevated discharges to the

Gulf perhaps for decades, even

after reduction programs are

implemented.

Nutrient application can be optimized based on geo-referenced records of soil test values, soil yield potential, and previous

yield and application histories.



FIG. 5.1 High-Tech Tools for Site-Specific Crop Nu trient Management

Source: Phosphate and Potash Institute
Chapter 5: Approaches ■ 45

Table 5.3 Summary of Economic Costs of N-Loss Reduction Actions

Net Cost within Net Cost, incl.

N-Loss Agricultural Environmental

Reduction Sector Benefits

Approach (Thousands of metric tons/yr) ($/kg N loss) ($/kg N loss)

Edge-of-field N-loss reductions, through economically optimum actionsa

20% 941 0.88 0.80

30% 1,412 1.90 1.80

40% 1,882 3.37 3.25

50% 2,352 5.20 5.08

60% 2,822 7.48 7.37

Fertilizer reductionsa

20% 503 0.69 0.67

45% 1,027 2.85 2.81

500% fertilizer taxa 1,027 14.54 14.50

Wetlands

1M acres 67 6.06 - 2.19

5M acres 350 8.90 1.00

10M acres 713 10.57 2.81

18M acres 1,300 11.93 4.27

Riparian buffers (27M acres) 692 26.03

River diversions to coastal wetlandsb 75 ~6

Tertiary treatment of waste waterb 20 ~40

a The nitrogen-loss reduction effects of these approaches are estimated at the edge of the field. b The costs for these approaches include engineering and construction costs only.