Mineralization of Nitrogen in Soil: 10 Factors

This article throws light upon the ten main factors that affect mineralization of nitrogen in soil. The factors are: 1. Nature of Organic Materials 2. Soil Moisture 3. Temperature 4. Alternate Wetting and Drying 5. Amount and Sources of Applied Nitrogen 6. Soil Reaction 7. Amount of Nitrogen in Soil 8. Chemical Inhibitors 9. Aeration 10. Soil-Plant Interactions.

Factor # 1. Nature of Organic Materials:

Organic materials having higher lignin content show a very slow rate of mineralization because of less activity offered by the micro-organisms, whereas the young and succulent plant residues and other easily decomposable organic materials show relatively higher rate of mineralization releasing higher amount of inorganic nitrogen in the soil solution.

Fresh organic materials contains higher amount of carbohydrate materials initially show the lower rate of mineralization. Mineralization and immobilization of nitrogen occur continuously in microbial metabolism and the magnitude and direction of the net effect is greatly determined by the amount and nature of organic materials.

Factor # 2. Soil Moisture:

The amount of soil moisture very often affects the rate of mineralization as soil moisture influences the activity of micro-organisms responsible for carrying out the process.

It has been found that the rate of mineralization of nitrogen (both NH₄⁺ and NO₃^–—N) was found highest at field capacity moisture regime, whereas the same rate decreased with the increased amount of soil moisture as in the case of waterlogged soils where mineralization of nitrogen proceeds up to the formation of NH₄—N stage.

But the net amount mineralized under waterlogged conditions is greater because less nitrogen is immobilized. Between 15 bars and air dryness, nitrogen mineralization goes on decreasing.

Factor # 3. Temperature:

Since mineralization is considered as a biological process, temperature, the most important environmental factor, affects the process by influencing the activity of micro-organisms. The rate of mineralization has been found to be increased when the temperature increased from 15 to 30°C. Below 5 and above 40°C the rate of nitrogen mineralization drastically reduces. Nitrification, although, occurs at all soil temperatures.

However, the optimum temperatures for carrying out ammonification and nitrification are between 40-60°C, and 30°C respectively. This difference results in a characteristic distribution of NO₃^– and NH₄⁺—N in the soil in relation to temperature.

Soil moisture at sub-optimal level and temperature at 30°C show a strong positive interaction effect on the net nitrogen mineralization indicating that they should not be considered independently.

Factor # 4. Alternate Wetting and Drying:

Alternate wetting and drying conditions of the soil enhances the rate of mineralization of nitrogen as compared to continuous wetting and drying conditions. Nitrification proceeds more rapidly in soils subjected to alternate wetting and drying than in soils kept permanently moist, because the alternate wetting and drying causes a more rapid oxidation of soil humus.

Factor # 5. Amount and Sources of Applied Nitrogen:

With an increase in the amount of applied nitrogen, the rate and net mineralization increases. Slow release of N-fertilizers like coated fertilizers indicates a slower rate of nitrification.

Factor # 6. Soil Reaction:

Soil reaction most oftenly determines the activity of micro-organisms responsible for the mineralization. Acidic soil reaction depresses but does not eliminate mineralization altogether.

The nitrification rate falls below a negligible value at the soil pH values lower than 5.0 and this may be due to lack of Ca present in the soil, at this condition increasing pH by applying lime, trace metals etc. will be helpful for the nitrification process as nitrifying bacteria need an adequate amount of Ca, P and some trace metals like Fe, Cu and Mn etc.

By and large, the change in nature of these soil micro-organisms with change of soil pH merely substitutes one group of organisms for a somewhat different group that does the same thing. Therefore, the transformation of organic N-materials to NH₄⁺ and from NH₄⁺ to nitrite and also of nitrite to nitrate is influenced by soil pH.

Factor # 7. Amount of Nitrogen in Soil:

The total nitrogen content in soils provides a measure of the amount of the organic materials undergoing decomposition. The net amount of nitrogen mineralized in soils having conditions of temperature (about 25°C) and water content (matric suction of 0.33 bar) suitable for those concerned microbial activity is approximately proportional to the total amount of nitrogen present in the soil.

If, however, the ratio of carbon to nitrogen (C: N ratio) in the soil is too wide, any NH₄⁺—N released from the organic matter will be utilized by the heterotrophic micro-organisms that is decomposing the organic material.

Factor # 8. Chemical Inhibitors:

Some chemicals for the inhibition of nitrification may be used in increasing the efficiency of a given amount of nitrogen in crop production through an influence of mineralization process.

Factor # 9. Aeration:

Mineralization of N particularly nitrification is affected by soil aeration. Transformation of ammonium to nitrite and nitrate requires oxygen. In the absence of molecular oxygen the oxidation of ammonium decreases. Therefore, soils containing lower amount of O₂, show poor rate of mineralization.

Factor # 10. Soil-Plant Interactions:

Mineral nitrogen is produced in soils as a by-product of microbial metabolism. The net mineralization of nitrogen is not entirely independent or the growth of plants. The presence of crops, however, decreased nitrogen mineralization. The mineralization of nitrogen decreased with an increase in total root weight as well as increase in number of micro-organisms around rhizosphere.