BIOL/CSES 4684: Nitrogen Cycle, Fate of Nitrate

BIOGEOCHEMICAL CYCLES

SOIL MICROBIOLOGY

BIOL/CSES 4684

The Nitrogen Cycle: Fate of Nitrate

Nitrate (NO₃^-) is removed by biological DENITRIFICATION, chemodenitrification, Nitrate ASSIMILATION (see below), removal by leaching, and erosional losses.

ENVIRONMENTAL IMPACT OF NITRATE
Nitrogen use in an irresponsible manner can have potenetial adverse environmental impacts (see Table N7). Elevated levels of NO₃^-in food and water may constitute a health hazard to both humans and animals. Apart from the fact that excessive N application to soil gives rise to accumulation of NO₃^- in plants, undesirably high levels of NO₃^- in potable water is also possible. Ingestion of large amounts of NO₃^- by infants that suffer from gastrointestinal upsets may cause a condition known as methemoglobinemia. Methemoglobinemia occurs under conditions where ingested NO₃^- is reduced to NO₂^- by gastrointestinal bacteria present in the tract of ruminant animals and in the human infant during the first few months of life. The NO₂^- is rapidly absorbed from the stomach into the blood, where it readily oxidizes the Fe of hemoglobin to the ferric state, forming methemoglobin. Methemoglobin cannot function in molecular oxygen transport and cellular anoxia can result. The effects can be observed when approximately 5% of the hemoglobin is converted to methemoglobin, but death is likely only if the methemoglobin level exceeds 50%.

Table N7. Potential adverse environmental impacts of nitrogen.

Impact	Causative Agents
Human Health
Methemoglobinemia in infants	Excess NO₃^- and NO₂^- in water and food
Cancer	Nitrosamines from NO₂^- and secondary amines
Respiratory illness	Peroxyacyl nitrates, alkyl nitrates, NO₃^- aerosols, NO₂^-, and HNO₃ vapor in urban atmospheres
Animal Health
Loss of livestock	Excess NO₃^- in feed and water
Plant Growth
Stunted growth	High levels of NO₂^- in soil
Excessive growth	Excess available N
Environmental Quality
Eutrophication	Inorganic and organic N in surface waters
Stratospheric ozone depletion	Nitrous oxide from nitrification, denitrification, and stack emissions
Materials and ecosystem damage	HNO₃ aerosols in rainfall

Ingestion of high levels of NO₃^-may also result in the formation of carcinogenic nitrosamines by reaction of ingested secondary aliphatic and aromatic amines with NO₂^- in the human stomach. Elevated levels of NO₃^- may also affect human cardiac function. A relation between high NO₃^- concentrations of drinking water and hypertension has been reported.

Although eutrophication in fresh waters is normally P limited, there are instances where eutrophication is N limited. Oligotrophic Lake Tahoe in California is N limited, as are a number of eutrophic lakes and coastal and estuarine ecosystems.

Depletion of ozone in the stratosphere - N₂O oxide tends to deplete the ozone layer. Nitrous oxide undergoes photolysis by absorbing ultraviolet radiation yielding a singlet oxygen (O) and N₂. The singlet O (also produced by O₃ photolysis) reacts with N₂O to produce NO which serves as a sink for O₃ and the final reaction product is HNO₃.

ASSIMILATORY NITRATE REDUCTION (ANR)

Utilization of nitrate by microorganisms and plants
Microorganisms and plants can take up and use both nitrate and ammonia as sources of nitrogen. Most microbes and plants prefer ammonia if given a choice since ammonia is ready to be combined with Krebs Cycle intermediates to make amino acids. When nitrate is used, the nitrate must first be reduced to ammonia (the reverse of nitrification). This reduction is a three step process that requires an NADH cofactor and energy is consumed at each step. Among microorganisms, some do not have the ANR enzymes and must use ammonia, while others cannot use ammonia and must have nitrate.

First step: reduction of nitrate to nitrite

2 HNO₃-----> 2HNO₂
2 NADH-H ------> 2NAD
enzyme: Assimilatory Nitrate Reductase

Second step: reduction of nitrite to hydroxylamine

2 HNO₂------> 2NH₂OH
NADH-H ----->NAD
enzyme: Assimilatory Nitrite Reductase

Third step: reduction of hydroxylamine to ammonia

2 NH₂OH -----> 2NH₃
NADH-H ----> NAD
enzyme: Assimilatory Hydroxylamine Reductase

Fourth step: protein synthesis

2NH₃ + Organic Acids ---> Amino Acids ---> Proteins

Back to Nitrogen Cycle Overview.

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Last Updated 5/1/98 Created by Katie Corbin