Nitrogen Loss through De-Nitrification: 6 Factors

This article throws light upon the six main factors that affect nitrogen loss through de-nitrification. The factors are: 1. Organic Matter 2. Moisture Content 3. Aeration 4. Soil Reaction 5. Amount of Nitrate Present 6. Temperature.

Factor # 1. Organic Matter:

Because denitrifying bacteria require organic matter as a source of carbon (and nitrogen) for their activity, the nature and amount of organic matter in soils largely determine the de-nitrification at a particular temperature and nitrate content.

The following equations developed by Burford and Bremer (1975 may be used for the calculation of the amount of available carbon required for microbial reduction of nitrate to N₂O or N₂:

4(CH₂O) + 4 NO₃^– + 4H⁺ = 4CO₂ + 2N₂O + 6H₂O

5(CH₂O) + 4 NO₃^– + 4H⁺ = 5CO₂ + 2N₂ + 7H₂O

Based on these equations, 1 ppm of available carbon is required for the production of 1.17 ppm of nitrogen as N₂O or of 0.99 ppm of nitrogen as N₂.

Factor # 2. Moisture Content:

When soils are waterlogged, the rate of de-nitrification is increased by impeding the diffusion of O₂ to sites of microbial activity.

Factor # 3. Aeration:

Aeration affects de-nitrification in two contrasting ways. Formation of NO₃^– and NO₂^–, which are the forms of nitrogen denitrified, is dependent upon the sufficient supply of O₂. Their gentrification, however, proceeds only when the O₂ supply is too low to meet the requirements of denitrifying organisms.

The de-nitrification process can operate in seemingly well-aerated or up land soils, presumably in anaerobic microsites where biological O₂ demand exceeds the supply. Decreased p0₂ will increase de-nitrification losses.

Factor # 4. Soil Reaction:

Acid soils (pH < 5.0) have pronounced effect on the de-nitrification process because many of the bacteria responsible for the process are very sensitive to low pH. As a result, many acid soils contain very small numbers of denitrifiers. The rate of de-nitrification is strongly influenced by soil pH, being very slow in acid soils having pH 3.6 to 4.8 and being very high in soils having pH 8.0 to 8.6.

Soil acidity also controls the sequence and relative abundance of the various nitrogen containing gases formed during the de-nitrification. At pH below 6.0 to 6.5, nitrous oxide predominates. Formation of nitric oxide (NO) is generally confined to low pH (pH < 5.5).

Nitrous oxide may be the first gas detectable at neutral or slightly acid soil conditions, but it is reduced microbiologically so that elemental nitrogen gas (N₂) tends to be the main product above pH 6.0. The occurrence of N₂O under acidic soil conditions is believed to be due to its resistance to further reduction to nitrogen gas.

Factor # 5. Amount of Nitrate Present:

It is obvious that the supply of nitrate and/or nitrite in soil is a prerequisite for carrying out the de-nitrification process. With an increase in nitrate concentration in soil the rate of de-nitrification increases and also exert a strong influence on the ratio of nitrous oxide (N₂O) to elemental nitrogen (N₂) released from the soil through de-nitrification.

In case of submerged soils where nitrogen loss due to de-nitrification involves NO₃^– diffusion, the de-nitrification appears to be a first-order reaction rather than zero-order indicating the need for NO₃—N for the process.

Factor # 6. Temperature:

The rate of de-nitrification in submerged soils increases as the temperature increases from 5 to 45°C. However, the de-nitrification process is very much sensitive to soil temperature and its rate increases very rapidly in the temperature range of 2 to 25°C. De-nitrification process is inhibited by temperatures greater than 60°C.