August 2007

Nitrogen Dynamics

Introduction

Nitrogen is one of the most important nutrients for crop growth, second only to water. In dryland farming, this is the major nutrient the producer can control. Nitrogen exists in many different chemical forms, which determines whether nitrogen is available to plants or whether nitrogen escapes and is no longer available to plants. These nitrogen losses may result in an economic loss to the producer, environmental repercussions, or both.

The soil nitrogen cycle is somewhat similar to the water cycle (see Figure 1). Nitrogen may be found in air, water or land; nitrogen may exist in several different chemical forms; it will undergo many changes in form and location throughout the cycle due to processes which occur as a result of weather, plants, animals and humans. In some cases nitrogen may be lost to the soil nitrogen cycle - it is important to determine whether human activity has accelerated the rate of natural losses and whether these losses pose an environmental threat.

Chemical Forms of Nitrogen

Nitrogen gas (N₂) - makes up 78 per cent of atmosphere; not directly available for use by plants; used in nitrogen fixation and industrial fertilizer manufacture.

Ammonia (NH₃) – basic form of nitrogen fertilizer; unavailable to plants.

Nitrate (NO₃^-) - most common form available to plants; mobile, leachable form; usually the end product of mineralization.

Ammonium (NH₄⁺) - compared to nitrate, a less-common plant-available form, but preferred nitrogen source; less energy required for uptake, many beneficial side effects and less likely to be lost than other forms.

Nitrite (NO₂^-) - less-common plant-available form; an intermediate in the conversion of ammonium to nitrate; more toxic to plants and more prone to gaseous losses than nitrate.

Nitrous Oxide (N₂O) – greenhouse gas; form lost through denitrification.

Nitric Oxide (NO) – form lost through denitrification; may be harmful to the ozone layer.

{All of the above forms of nitrogen are known as inorganic forms of nitrogen}

Organic Nitrogen Compounds - complex; unavailable to plants; end products of immobilization.

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Figure 1

Nitrogen Conversion Processes

Mineralization - conversion of organic nitrogen to inorganic nitrogen.

Immobilization - conversion of inorganic nitrogen into organic forms.

Leaching - downward movement of dissolved nitrogen through the soil profile carried by infiltrating water.

Volatilization - losses of ammonia from the soil surface to the atmosphere.

Denitrification - a process in which nitrogen is lost from the soil; nitrate and nitrite are converted to gaseous forms of nitrogen under conditions of low oxygen availability in the soil.

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How Plants Use Nitrogen

Plants use nitrogen by absorbing either nitrate or ammonium through the roots. Most of the nitrogen is used by the plant to produce protein (in the form of enzymes) and nucleic acids. Nitrogen is readily transported through the plant from older tissue to younger tissue. Therefore, a plant deficient in nitrogen will show yellowing in the older leaves due to the underdevelopment or destruction of chloroplasts.

Health Concerns of Nitrate

The main concern of high concentration of nitrates in drinking water is the condition of methemoglobinemia and its effect on infants ("blue baby syndrome"). Nitrate can be reduced to nitrite once ingested, and the nitrite reacts with hemoglobin in the bloodstream to produce methemoglobin, which impairs oxygen transport, similar to carbon monoxide poisoning. Infants under three months of age and the unborn are particularly susceptible. Methemoglobin levels in the blood are normally 1-2% of total hemoglobin; detectable symptoms appear at 10% levels; death occurs at levels of 50-75%. Blue baby syndrome is easy to detect and treat. Although adults and livestock have higher tolerance levels for nitrate, Scientific American (November 1996) reports that high nitrates in drinking water can result in humans being more prone to stomach cancer. Nitrates can react with amino acids to form various carcinogens.

The problem is complicated by the fact that nitrate is not readily removed from drinking water; boiling only increases the concentration of nitrate in the water. Therefore, the acceptable tolerance level for nitrate in drinking water has been set at 10 parts per million (ppm) of nitrate-nitrogen, which is equivalent to 45 mg of nitrate/L.

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Nitrate Leaching

Nitrate leaching into groundwater can be an environmental concern because of health problems for infants and the elderly in particular. Nitrate is very soluble and completely mobile when dissolved. Therefore, nitrate will move in soils to the depth of water infiltration, and is difficult to control once this process starts. If the nitrates are carried by water beyond the maximum rooting depth of plants, the nitrate is no longer available to plants and could end up in the groundwater.

Once leached to the water table, some nitrate may denitrify and escape in gaseous form, but this represents a very small amount since there is little organic matter in the subsoil, and therefore little carbon available to promote microbial activity at this depth. Denitrification only occurs if conditions are favorable (carbon source present and warm temperatures) for microbial activity and if oxygen is limiting.

To minimize the potential of nitrate leaching, farm management guidelines recommend no more than 150 lb/ac of nitrate-nitrogen in the top four feet of the soil profile, and no more than 20 lb/ac in every one-foot increment thereafter.

Factors which favour nitrate leaching:

• coarse-textured soils or soils with large, deep cracks (macropores)
• downward movement of water favored in the landscape (recharge areas)
• significant precipitation as rain or snowmelt
• high concentration of nitrate present in the soil profile
• limited plant root zone to intercept nitrate due to shallow rooted or immature crops.

To prevent nitrate leaching:

• Identify areas in the landscape where nitrate leaching into groundwater is most likely to occur (these are usually areas with coarse textured soils and shallow water tables)
• Test soil every year. Manitoba's Farm Practices Guidelines recommends no more than 150 lb/ac of nitrate-nitrogen in the top four feet of the soil profile and no more than 20 lb/ac of nitrate-nitrogen in each subsequent foot of the profile. Refer to Appendix F, Monitoring Soil Nitrate, in Manitoba's Farm Practices Guidelines.
• Monitor groundwater levels and sample groundwater for nitrate content annually or when possible. Environmental guidelines have set 10 ppm as the maximum nitrate content for drinking water consumed by humans.
• Apply only as much nitrogen fertilizer to the crop that can be used in one growing season. If 150 lb/ac of nitrate-nitrogen (or more) is present to a depth of 4 feet, then no nitrogen fertilizer should be applied.
• Minimize the window of opportunity for nitrate leaching. The time when nitrate is present in the soil to the time when it is utilized by the crop should be as short as possible. Spring applications are less risky than fall applications, and split applications of nitrogen during the growing season are less risky than applying all the nitrogen at the time of planting.
• If high levels of nitrate-nitrogen are found below four feet, check which crops are best suited to your operation for retrieval of deep leached nitrates.

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Volatilization

Volatilization is a concern because it represents an economic loss in fertilizer inputs. In addition, the release of ammonia can contribute to global warming. Reducing volatilization losses increases fertilizer efficiency and prevents environmental damage caused by ammonia.

Factors conducive to volatilization:

• high soil pH (>7.0);
• soils high in calcium carbonate (lime);
• soils with low retention ability for ammonium
  ie. low clay content, low organic matter, low cation exchange capacity;
• high soil or atmospheric temperature;
• liquid fertilizer applied onto dry soil;
• high wind velocity and/or highly aerated soils;
• high rate of fertilizer application;
• depth of incorporation/penetration < 2 cm.

To prevent volatilization:

• Incorporate nitrogen fertilizers as soon as possible after application, especially manures and urea (46-0-0).
• If broadcasting and dribble banding fertilizers, apply immediately before or during cool, wet weather.

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Denitrification

Denitrification, like volatilization, can contribute to global warming and results in another loss of fertilizer nitrogen. The difference between the processes is that volatilization occurs due to exposure to the atmosphere, whereas denitrification is nitrogen loss in the absence of oxygen. The rate of denitrification decreases with depth and increases with temperature because it is dependent on biological activity.

Factors conducive to denitrification:

• soils with high organic matter (5% or greater);
• limited oxygen, due to high water content, rapid respiration, compaction;
• neutral or alkaline pH (7.0 or greater);
• temperatures > 2oC;
• chemodenitrification (denitrification without microbial activity) requires low pH, but may be significant in freezing soils with high salt concentrations and high nitrite content.

To prevent denitrification:

• Avoid high applications of nitrogen to areas in the landscape with high water tables and intermittent ponding.
• Follow the same guidelines to prevent nitrate leaching.

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References

Brady, N. C. 1984. The Nature and Properties of Soils. 9th ed. New York: Macmillan Publishing Company.

National Academy of Sciences, 1972. Accumulation of Nitrate. Washington, D.C.

Tisdale, S. L., Nelson, W. L. and Beaton, J. D. 1985. Soil Fertility and Fertilizers. 4th ed. New York: Macmillan Publishing Company.

REFER TO FARM PRACTICES GUIDELINES FOR PRODUCERS IN MANITOBA:

• HOGS
• BEEF
• POULTRY
• DAIRY

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First internet edition: June 2001